advanced practical electronics
DESCRIPTION
The Book that makes Electronics PracticalTRANSCRIPT
ADVANCED PRACTICAL ELECTRONICSADVANCED PRACTICAL ELECTRONICS
FROM POWER SUPPLIES TO INTERFACINGFROM POWER SUPPLIES TO INTERFACING
PHUMZILE MALINDIPHUMZILE MALINDI
Preface
Nowadays, electronics has become part of every one daily life. When you are relaxing you listen to radio, CD player, or watch DVD, VCR or just TV. When you are going away you arm the alarm to protect your house, and then you press the immobilizer to open or start your car. When you go to the bank you use an ATM or the teller use a computer for your transaction. You go to the hospital the physician order the nurse to connect you to a machine to diagnose the cause of your illness. You want to talk to somebody in another town you use a phone.
What is common in the above few examples is that they are all electronic appliances, which proves beyond doubt that electronics will always be part of our lives. Hence there is a dire need for development of expertise in the field. This book is written to help the reader to get into grips with how to put electronics in practice. Its emphasis is on application of electronics in solving our daily problems. So for each device introduced the author will try to show how it is used in real situation. This will help the reader to be able to design circuits for different applications.
The text is divided into 9 chapters and is organised in such a way that the information is built up in a logical way. The first chapter is on power supplies. Chapter 2 is on transducers, chapter 3 is the theory of operational amplifier, chapter 4 is signal conditioning and interfacing, chapter 5 is filtering, chapter 6 is wave generation and shaping, chapter 7 is large signal amplification. Chapter 8 is noise, and chapter 9 is hardware/software interfacing.
Phumzile Malindi, December 2004
1. POWER SUPPLIES
1.1 INTORDUCTION
Almost every electronic circuit needs a dc power source to operate. The main function of the power supply is to take the 220 V ac (mains) voltage and converts it to dc voltage or voltages that are required to power electronic circuits. For most electronic applications the power supply needs to provide dc voltages in the range of 5V for digital electronic circuits to about 80 V (or more) for large signal (or power) amplifiers, with currents from few milliamperes to about 5 or more amperes. In some other applications such as operational amplifier circuits and power amplifier circuits the power supply needs to provide both positive and negative voltages to enable both the positive and negative swings of the output voltages. The latter is referred to as dual, dual-rail or split supply since it is providing both positive and negative dc voltages while the former is called a single-rail supply because it is only providing a single voltage, which can be either positive or negative.
The circuit for converting an ac supply to a dc supply involves three main stages: the stepping-down of ac voltage, ac-to-dc conversion, and voltage regulation as depicted in Figure 1.1.
Figure 1.1 Basic power supply stages
The incoming 220 Vrms mains voltage is stepped-down to a smaller ac voltage, before it is converted into a dc voltage. The dc voltage is regulated to produce a stable dc voltage.AC-to-dc conversion stage involves two processes: changing of ac to pulsating dc and smoothening pulsating dc to produce a steady dc. This has resulted in ac-to-dc stage being divided into two separate stages, namely rectification and filtering as shown in the following diagram.
Figure 1.2 Power supply
1.2 STEPPING DOWN OF AC VOLTAGE
The stepping down of the incoming 220 Vrms mains voltage to a smaller ac voltage involves the use of a transformer as shown in Figure 1.3.
Figure1.3a Figure 1.3b
Where Figure 1.3a is a normal transformer, which gives out a single stepped-down output, whereas Figure 1.3b is a center-tapped transformer that gives out two stepped-down voltages of the same magnitude. For a single-rail power supply the normal transformer is the best candidate but for dual or split supply the best transformer to use is the center-tapped in order to generate the two voltages required.
The transformer has two sets of coils: the primary at the input and the secondary at the output. The primary consists, most of the time, of one coil and the secondary can have one or more coils depending on the outputs required. For example, if you are designing a power supply to provide fixed positive, fixed negative, variable positive, and variable negative voltages, you will need a transformer that will provide four voltages at it secondary; that is, two center-tapped coils on the secondary side of the transformer. The output voltage of the transformer is determined by the number of turns in both primary and the secondary or turns ratio of the transformer. That is,
(1.1a)
(1.1b)
Thus, for the transformers shown in Figure 1.3 the output voltages at the secondary will be 22 Vrms and two 22 Vrms for the two transformers, respectively (since the turns ratio NP:NS = 10:1).1.3 AC-TO-DC CONVERSION
The function of this stage is to convert the output of the transformer, which is a stepped-down ac voltage into a dc voltage. This is accomplished in two stages: rectification and filtering (or smoothening). The first stage called the rectifier converts the ac into a pulsating dc, which is a dc with an ac component, then the second stage called the filter removes the ac component of the pulsating dc to produce a steady dc that has a value given by
Vs VsVpVp
T210TO1CT
T110TO1
=
(1.2)Rectification
A rectifier circuit called a full wave bridge rectifier is shown in Figure 1.4. It consists of four diodes.
D1 D2
D3 D4
Figure 1.4 Full wave bridge rectifier
The way the full-wave bridge rectifier works is that only two parallel diodes are conducting while the other parallel pair is off for each cycle. That is, during the positive half-cycle diodes D2 and D3 are conducting while D1 and D4 are off and for negative half-cycle diodes D1 and D4 are conducting while D2 and D3 are in the off state. This results in a series of positive pulses across the output terminals that has a peak amplitude of
(1.3)
where Vs(pk) is the peak value across the secondary of the transformer and VD is the volt drop across each diode. Since there are two diodes conducting per cycle, these diodes are in series with the input ac voltage hence the volt drop introduced by the rectifier is 2VD, which is 1.4 V for silicon diodes.
Filtering
The output of the rectifier is a series of positive pulses, which is called pulsating dc, not genuine dc as required. In order to get a genuine dc voltage the output of the rectifier is filtered or smoothened by a low pass filter to remove the ac component. The most popular filter circuit used for smoothening is a capacitor that is connected across the output of the rectifier. The action of the capacitor filter depends upon the fact that the capacitor stores energy during the conduction period and delivers it to the load during the non-conduction period. The most commonly used capacitor is a polarized capacitor called electrolytic capacitor and its value is chosen so that
(1.4)
where fr is the ripple frequency, which is twice the mains frequency (i.e. twice 50 Hz in South Africa and Europe while it is twice 60Hz in America). This makes the time constant for discharging to be longer than the time between recharging thus ensuring small ripple. In most instances the value of the load is unknown and it is only the value of the voltage and the current that are available. Using the available information and the Ohms law, the value of the capacitor can be determined using
(1.5)
At this stage let us put together all the different stages of the power supply covered so far into one circuit. This power supply is called the unregulated power supply and is shown in Figure 1.5 for both single and dual-rail supplies, respectively.
+
-
Figure 1.5 Single-rail and dual-rail (split) unregulated power supplies
1.4 VOLTAGE REGULATION
Though the output of the filter is a steady dc voltage it is still having some variations or ripples in it. For general applications this dc voltage is acceptable but for other applications the voltage needs further processing to remove the variations. This is accomplished by using a voltage regulator. These voltage regulators are used in most of the power supplies to provide a dc voltage that almost ripple-free. Regulators can be categorised into linear and switching voltage regulators. Linear regulators are the most commonly used and can be found in almost nearly every regulated power, while switching regulators are used in dc-to-dc converters to step up or to convert the polarity of the input voltage. The first regulators used discrete components such as zener diodes and power transistors to accomplish voltage regulation. However, nowadays complete voltage regulators are available as inexpensive integrated circuits. The availability of
+
C3Vo
Vo
Vo
+C2
+
C1
in
in
ac
ac
T210TO1CT
T110TO1
these IC or monolithic regulators has made the task of designing a power supply simple and the power supply circuit to be more compact.
1.4.1 Linear Voltage Regulators
Linear voltage regulator operates by using a voltage-controlled current source to force a fixed voltage to appear at the output. It has a built-in sense and control feedback circuitry that senses the output voltage and adjust the current source so as to hold the output voltage constant.
The linear IC regulators come as positive and negative, fixed and variable, three-terminal, four-terminal and five-terminal regulators with different voltage and current ratings. The three-terminal regulators are the most commonly used and they can be classified into fixed and variable regulators. For the regulator to work properly the input voltage must be greater that the expected regulated output and for many regulators there is a minimum difference between the two voltages. This minimum difference is called the dropout voltage and for the regulator to operate satisfactory the minimum input to an IC regulator must be greater than or equals to the expected regulated output plus the dropout voltage.
Fixed Three-Terminal Regulators
Three-terminal regulators have three connections: input, output, and ground and is factory-trimmed to provide a fixed output, which can either be positive or negative. A typical example of a positive three-terminal regulator is the 78xx, where the output voltage is specified by the last two digits of the part number and can be any of the following: 05, 06, 08,10, 15, 18, or 24. The typical example of a negative version is 79xx, which is almost the same as the 7800 series except that it works with negative input dc voltage to produce a negative output. Both the 7800 and 7900 series can provide up to 1 amp load current. Low-power versions: 78Lxx and 79Lxx are also available but they can provide currents up to 100 mA to the load. The circuit for a fixed regulator is shown in Figure 1.6 that will provide a 5V at 100 mA to the load. For the regulator to operate satisfactory the minimum input must be 7 V.
Figure 1.6 Fixed 5 V regulator circuit
There are other three-terminal regulator variants, which are not as popular as 7800/7900 series, which have better performance in regards to volt drop from unregulated input to
VoVinIN
COM
OUT
78L05
+
C11uF
regulated output (or dropout voltage). For an example, the LP2950 is a fixed 5V regulator just like a 7805, but it regulates with a dropout voltage of 0.4 V, compared with 2 V dropout for 7805. Except for low-dropout other regulators also can deliver higher load current, for example the LT1085/4/3 series can provide 3 A, 5A, and 7.5 A.
Adjustable Three-Terminal Regulators
Adjustable regulators are regulators that are designed in such a way that the user can set the regulated output. A typical example of an adjustable regulator is a LM317 for positive voltage and LM337 for negative voltage. Unlike the fixed regulators, the adjustable regulator has adjustment (ADJ) terminal instead of the ground terminal. These 317 and 337 can provide currents up to 1.5 A to the load. The magnitude of the output voltage can be set to any value between 1.25 V and 37 V using two additional resistors as shown in Figure 1.7.
Figure 1.7 Adjustable regulator circuit
Resistor R1 is fixed at 240 for 317 and 337 (for other regulators please consult the manufacturer data sheets) and R2 is adjustable to set the required output voltage using the following equation
(1.6)
With VREF equals 1.25 V and adjustment terminal current is in the range of 50 –100 A, which is very small, thus making the product IADJR2 in Equation (1.6) negligible small, the output can be rewritten as
(1.7)
For adjustable three-terminal regulators with higher current ratings, use table 1.1 below.
Table 1.1 High current adjustable three-terminal regulators
R2
VoVin
+
C11uF
IN
ADJ
OUT
317
R1240
Current (A) Positive Adj. Regulator Negative Adj. Regulator3510
LM350LM338LM396
LM333
Lifting Regulator above Ground
The fixed regulators provide fixed voltages at only certain values, which are standard. So if you want a regulated output, which is not standard, for example 9 V, to emulate a battery you can use an adjustable regulator with R2 set to 1.5 k. Alternatively you can still use the standard fixed regulator with its common terminal lifted above ground by means of a zener diode that has zener voltage equals to the required output minus the rated value of the standard regulator. This extends the voltage range by an amount equals zener voltage. That is,
(1.8)
For example to get 9 V from a 5 V regulator you need to use 4 V zener to lift the common of the regulator above ground as shown in the following circuit.
Figure 1.8 Non-standard fixed voltage regulator (with regulator lifted above ground using a zener diode)
The output voltage is the sum of the regulator voltage and the zener voltage, which in this case is 5 V + 4 V = 9 V.
External Capacitors and Protection Diodes
A 0.01 F to 25 F capacitor must be connected across the output, as shown in the above circuits, to eliminate the high frequency noise at the output and to insure stability. For adjustable regulators the adjustment terminal can be bypassed to ground with a 10 F capacitor to improve ripple rejection by 15 dB to obtain a total ripple rejection of 80 dB at the output.A 0.1 F input bypass capacitor is also recommended for adjustable regulators in order to compensate for the problems that may arise due to the devices sensitivity when adjustment or output capacitors are used.
D1
VoVinIN
COM
OUT
78L05
+
C11uF
For the adjustment terminal bypass capacitor and output capacitor it is recommended that you include safety discharge diodes. These diodes provide discharging paths for these capacitors in the event of the input or output being shorted. This is very important especially for higher voltages in order to prevent the capacitors from discharging through the regulator and damaging it in the event of the input or output being short-circuited.
This is shown if Figure 1.9, where capacitor C1 is used to eliminate high frequency noise, diode D1 is used to provide a discharging path for C1, and capacitor C2 is used to improve ripple-rejection and diode D2 is used to provide the discharging path for C2.
Figure 1.9 External capacitors and safety discharge diodes for regulators
Electronic Shutdown
In other applications such as electronic laboratories, which are used by first year electronic learners who know little about short circuits, the protection diodes mentioned above are not enough to protect your power supply circuit. These diodes only provide discharging paths when there is a short and do not prevent the excessive current from damaging the regulator.
Electronic shutdown is a short circuit protection technique that is used to clamp the adjustment terminal to ground when an excessive current (due to short circuit) through the regulator is sensed. This is accomplished by sensing the load current via the volt drop across resistor RS and switching on transistor Q1 when the current exceeds the set limit. Once Q1 is turned on it extends the input voltage Vin to the base of Q2 thus making Q2 to also be on and as Q2 switches on, adjustable terminal resistor R2 is shorted out and the output voltage drops to Vref, which is 1.25 V.
D1
LM317
IN
ADJ
OUTVoVin
R1240
R2
C30.1uF
+
C210uF
+ C11uF
D2
D1
VoVinIN
COM
OUT
78L05
+
C11uF
R4
Rs
Q1
Q2
D2
D1
+
C310uF+
C210uF
C10.1uF R2
Vin VoLM317
IN
ADJ
OUT
R1240
Figure 1.10 Regulator with electronic shutdown
Outboard-Pass Transistor for Current-Boosting
Most of the regulators covered can deliver a limited current to the load. This current is sufficient for most applications. However, if you want a current that is more than what the regulator can deliver, you will need to use an outboard pass transistor, which is an external pass transistor that can be added to your normal regulator circuit to provide the extra current. Figure 1.11 shows a circuit of a regulator with an outboard pass transistor.
Figure 1.11 Regulator with an outboard pass transistor
The circuit uses a 78L05, which is designed to deliver a current of 100 mA to the load. The value of the series resistor R1 is chosen such that for current less than the maximum rated current of the regulator, the volt drop across it is less than 0.7 V in order to keep the outboard transistor Q1 in an off state. This will make the regulator to work normal. For currents greater than the maximum rated current (which is 100 mA in this case), the volt drop across R1 becomes sufficient to turn the outboard transistor on. When this happens, the current through the regulator will be limited to the maximum rated current, while the additional current is delivered to the load through the outboard pass transistor.
In some other applications, which need high load currents, one outboard transistor is not sufficient to provide the required current. In such cases multiple outboard pass transistors are used in parallel to deliver the required high output load currents as shown in Figure 1.12.
Q1
R1VoVin IN
COM
OUT
78L05
+
C11uF
Q3
R4
Q2
R3
R2
R1
+
C11uF
IN
COM
OUT
78L05Vin Vo
Q1
Figure 1.12 Regulator with multiple outboard pass transistors
The operation of the circuit in Figure 1.12 is the same as that of Figure 1.11, except that the additional current is shared amongst the three outboard transistors instead of one. Resistors R2, R3, and R4 are included for stability and to prevent current swamping.
The problem with the circuit in Figure 1.11 is that, instead of just providing an additional path for current to pass through, it can amplify the maximum rated current of the regulator to provide a load current that is equal to beta times the maximum rated current of the regulator. This current can be high enough to destroy both the transistor and the load. In order to avoid this from happening, another transistor Q2 is connected across the base-emitter junction of the outboard transistor as shown in the following Figure 1.12.
Figure 1.12 Regulator with an outboard pass transistor and current-limiting circuit
This Q2 transistor limits the current through the outboard transistor so that it does not exceed the stipulated value. This is accomplished by sensing the load current via the volt drop across resistor Rsc and shorting out R1 when the current causes the volt drop across base-emitter junction of Q2 to be 0.7 V. When R1 is shorted, the voltage that drives the outboard will be cut off thereby turning the outboard pass transistor off. The values of the two resistors are given by
(1.9)
(1.10)
Q2
RscQ1
R1VoVin IN
COM
OUT
78L05
+
C11uF
Paralleling Regulators for Higher Current
As an alternative to the use of multiple outboard pass transistor for higher current, two similar regulators can be connected in parallel to double the load current or three similar regulators can be connected in parallel to triple the load current. Figure 1.13 shows a circuit with two LM338 (10A) regulators, which are connected in parallel. The load current is the sum of the currents delivered by the two regulators, and since LM338 used is a 5 A regulator the total current that can be delivered by the circuit to the load is 10 A, which is twice the current of each regulator. The Opamp ensures that the current is divided correctly between the two regulator ICs.
Figure 1.13 10 A Regulator
1.4.2 Switching Regulators, DC Voltage Converters and Isolated Power Supplies
The regulators covered so far are called linear regulators. Though they are easy to use their efficiency is not that good. This has resulted in a shift in industry towards the use of switching regulators in power supplies more than the linear regulators. This shift can be attributed to the switching regulator’s efficient transfer of power to the load. Switching regulators are used in switch-mode power supplies (which are used by computers), dc-to-dc converters, and in isolated power supplies.
Unlike linear regulators, switching regulators converts the input dc voltage to pulses, which are then stepped up, rectified and filtered to provide a smooth dc. A basic switching regulator consists of a switching regulator element (transistor), a reference voltage regulator, a voltage divider circuit, an error amplifier and a control circuit (an oscillator with a pulse-width modulator). Figure 1.14 shows a block diagram of a switching regulator and Figure 1.14 shows a block diagram of a MAX631, which is a
R52.7k+
R40.1
R30.1
IN
ADJ
OUT
LM338
R1120
IN
ADJ
OUT
LM338
1.2V to 30 V
Vo
Vin
2.7kR2
C10.1uF
+C3
10uF
commercial-available switching regulator. Some of the commercially available switching regulators include MAX632, MAX633, MAX634, MAX635, MAX636, MAX637, MAX638, MAX639, MAX640, MAX742, etc.
Figure 1.14 Block diagram of a switching regulator
Most of the dc-to-dc converters use switching principle with building blocks such as the oscillator, the switching transistor (power MOSFET), the energy-storing device (capacitor or inductor), feedback and smoothening capacitor, hence they are referred to as switching regulators. The oscillator generates pulses that switch the transistor on and off. As the transistor goes into saturation it applies the input dc voltage across the energy-
storing device for a short interval. During this period the energy ( or ) is stored
in the energy-storing device and then the stored energy is transferred to the capacitor at the output. The output capacitor smoothes the output and carry the load between charging pulses. The feedback controls the output by changing the oscillator’s pulse width or switching frequency in order to regulate the output. Figure 1.13 shows a block diagram of a basic switching regulator and Figure 1.14 shows the configurations for both step-up and polarity inversion. However, complete voltage converters are available as integrated circuits.
(a)
Vin Vo
Q1
L1
Vref
D1
+
R2
R1Osc
(b)
Figure 1.14 Configurations for (a) Step-up and (b) Polarity inversion switching regulator
The dc converter is a circuit that changes dc to another dc voltage. The output dc of the converter can be either higher or lower than the input, and can also be either of the same or opposite polarity to the input voltage. Since all the regulator circuits covered so far can provide the stepping down of voltage, voltage converters are commonly used for stepping up, polarity inversion and provision of isolated supply voltages.
Isolated Power Supply
Galvanic isolation is required for many circuits that are found in medical systems since there are transducers that are attached to the patient. Some of these circuits also need to be powered by isolated power supplies.
In order to get an isolated power supply you need to use an isolated output voltage converter. Isolation eliminates any connections between the input and the output ground and allows the output to float. Unlike ordinary dc-to-dc voltage converter, an isolated output usually has a transformer to provide isolation for the switching currents and an optical isolator to isolate the feedback sensing voltage.
Figure 1.15 Simplified isolated dc-to-dc converter
1.5 POWER SUPPLY CIRCUITS
Vin
Vo
Q1
L1
D1
+OscR1
Figure 1.16 Variable dual supply with electronic shut down for short circuit protection
Figure 1.17 12 V power supply (load current > regulator’s max rated output current)
Figure 1.18 Isolated 15 V split power supply
1.6 INVERTORS
Though this topic is not going to be covered in much detail it is still believed that the coverage of power supplies would not be complete without the mentioning of inverters.
D3 D25V
Q1
IN
COM
OUT
78L05
+
C21uF
12VT1
10TO1
acin +
C1
R21k
R1
Assume that you are having a 12 V car battery and you want to play a TV that can only use 220 V ac. An inverter, which changes dc to ac, can be a solution. This process of dc to ac voltage inversion is accomplished by first changing dc to ac using an oscillator and then stepping up the generated ac signal to 220 V ac voltage. In order to meet the power requirements of the inverter the stepping up is usually preceded by current boosting.
Inverters can be categorized into three basic types: square wave, modified sine wave and pure sine wave dc to ac inverters. Square wave inverters are the simplest, cheapest and low power quality of the three, followed by modified sine wave inverters. Both the square wave and the modified square wave inverters provide square wave outputs; however, the difference between them is that the former provide a normal square wave while the modified sine wave inverter provides a square wave with some dead spots between the half cycles. The sine wave inverter, on the other hand, provides a sine wave with low total harmonic distortion and they are more expensive than the other two inverter types. Figure 1.19
The dc-to-ac inversion process is depicted in the following diagram, where the low-frequency oscillator is responsible for generating the required line frequency of 50 Hz (or 60 Hz) which is usually a square wave. The output of the oscillator is amplified in order to get higher currents before it is applied to a step-up transformer for stepping up the generated low ac voltage to 220 V.
Figure 1.20 Block diagram of a DC to AC inverter
For generating the ac from a dc source an astable multivibrator is normally used and the most commonly used astable multivibrator circuits used include the circuits shown in Figure 1.21
1.8 Notes
The transformer used must be able to deliver the required current and voltage The rectifier diodes (or bridge rectifier) must be capable of passing the high peak
current Adequate heatsinking must be provided for regulators and outboard pass transistors
and if your high current supply is built into a case it is recommended that an extractor fan must be used for adequate cooling.
The polarity and the voltage rating of the capacitors must be correct since wrong polarity or exceeded voltage rating can result in the capacitor blowing.
For the regulator to operate satisfactory the minimum input voltage to an IC regulator must be greater than or equals to the expected regulated output plus the dropout voltage. If an outboard pass transistor is used, the input voltage must exceed the output voltage by the dropout voltage of the regulator plus a VBE drop.
Use a positive regulator with a positive input and a negative regulator with a negative input.
Make sure that the protection diodes are connected correctly (that is, they are reverse biased), to avoid shorting out the device to which they are connected across.
References
Boylestad, R. and L. Nashelsky. Electronic Devices and Circuit Theory, 5th edition. Prentice Hall, New Jersey, 1992.Horrowitz, P. and W. Hill. The Art of Electronics, 2nd edition. Cambridge, New York, 1994.Ibrahim, K.F. Electronic Systems and Techniques, 2nd edition. Longman Scientific and Technical, Essex, 1994. Mathews, T. Switching Regulators Demystified. National Semiconductors.Millman, J. and A. Grabel. Micro-electronics, 2nd edition. McGraw-Hill, New York, 1987.Newport Components. Product data disk.SGS-Thomson Microelectronics. LM138/238/338 Three-Terminal 5-A Adjustable Voltage Regulators Datasheet, 1994.Simpson, S. Linear and Switching Voltage Regulator Fundamentals. National Semiconductors.http://www.mitedu.freeserve.co.uk/Ciruits/Power/boosting.htmhttp://www.mitedu.freeserve.co.uk/Ciruits/Power/1230psu.htm
2. TRANSDUCERS
2.1 INTRODUCTION
A transducer can be defined as a device that can be used to convert energy from one form to another form. In electronics a transducer enables the electronic system to communicate with the outside world by converting physical quantity such as pressure, motion, temperature, or light into an electrical quantity, or vice versa (i.e. converts electrical quantity into physical quantity). This enables the use of electronics to control and log process.
Transducers that are responsible for converting from physical to electrical are referred to as sensors while those that convert from electrical to physical are called actuators. Some few transducers can be used interchangeable as sensors and as actuator. One such transducer is an antenna (aerial) that is used in transceiver systems such as two-way radio and mobile (cellular) phones. A transceiver system has one antenna, which is used as an actuator to convert modulated carrier into waves when transmitting and as a sensor to intercept waves and covert them to electrical signals when receiving.
Transducers can be further classified as passive and active transducers. Active transducers are those that need to be powered in order to work, whereas the passive transducers operate without being powered.
2.2 SENSORS
Sensors are input transducers that transform energy from one form, which is not electric, into electric energy. They are used in electronics to acquire data about their surroundings and convert that into an electrical quantity for further processing. Sensors are classified according to the major forms of energy they detect, hence there are acoustic sensors, temperature sensors, light sensors, pressure sensors, etc. and they convert energy from one form into electrical quantity that can be voltage, resistance, capacitance, current, or inductance.
2.2.1 ACOUSTIC SENSORS
Acoustic sensor converts sound or acoustic energy into electrical signal. A microphone is a typical example of an acoustic sensor that changes the sound waves into electrical energy, which may then be amplified, transmitted, or recorded. Microphones come in different types, which include moving coil (or dynamic), carbon, electret, piezoelectric (or crystal), and capacitor (or condenser) type. Carbon is a very low quality microphone, moving coil, capacitor and electret types are good quality microphones, and crystal type has a performance that is somewhere between carbon and moving coil type.
2.2.2 TEMPERATURE SENSORS
Temperature sensors convert temperature changes into electrical quantity. There are four main types of temperature sensors: thermocouples, resistive temperature detectors (RTDs), thermistors and IC temperature sensors.
Thermocouples
Thermocouple consists of two dissimilar metal conductors, which are connected at one end to form a joint. Due to Seebeck effect, the two wires would produce a small open-circuited voltage, called Seebeck voltage when the junction is heated. This voltage, which is in millivolts, is proportional to the temperature of the junction. Thermocouple can operate over a wide range of temperatures.
Resistive Temperature Detectors (RTDs)
A resistance temperature detector (RTD) is a device whose resistance increases with temperature. They are more stable and more accurate than other temperature sensors. RTDs are available in materials such as platinum, nickel, copper, and tungsten, which have different temperature range and resistance coefficient, . Table 2.1 shows the linear temperature range and resistance coefficient for these RTDs. Since RTD is a resistive device, you must pass current through it to produce voltage that can be conditioned by an amplifier circuit. The disadvantage of RTDs is that they are more expensive and self-heating.
Table 2.1 linear temperature range and resistance coefficient of RTDs
RTD type Temperature range (0C) Resistance coefficient, Platinum -184 to 815 0.0039 /oCCopper -51 to 149 0.0042 /oCTungsten -73 to 276 0.0045 /oCNickel -73 to 149 0.0067 /oC
The temperature coefficient shown in Table2.1 is the amount of resistance change that can be expected for each degree Celsius of change in temperature. The amount of resistance change per oC (r) can be obtained by multiplying the temperature coefficient () by the nominal resistance of the RTD at 00C (R0). That is,
(2.1)
If the temperature changes to Tx, the new resistance would change by a value equals to the product of r and new temperature, hence the new resistance of the RTD at temperature Tx will be
(2.2)
For platinum RTDs, which have nominal resistance of 100 at 00C and resistance coefficient of 0.0039 /oC the amount of resistance change per oC is
If the temperature changes to 1000C, the new resistance would change by a value equals to the product of r and new temperature, hence the new resistance of the RTD at temperature 1000C will be
The temperature ranges shown in Table 2.1 can be extended, but at the expense of linearity in the extended range.
The most popular type of RTD is made of platinum, which has a nominal resistance of 100 at 00C. The reason for this is that platinum is stable, it resists corrosion and oxidation, has a high melting point and a high degree of resistivity. Thermistors
Thermistors is a contracted name for thermally sensitive resistors, which have a very high negative temperature coefficient; that is, their resistances decrease as temperature increases. Thermistors are more sensitive and faster reacting to temperature changes than thermocouples and RTDs but they are not linear and they are self-heating like RTDs. The characteristic temperature-resistance relationship is in the form
(2.3)
where T is temperature in Kelvin’s, R0 is resistance at 00C and b is a constant for a specific thermistor.
IC Temperature Detectors
IC temperature sensors convert temperature into voltage or current. They are very small, which allows them to be placed in PCB. They are the most linear of the temperature sensors and they are inexpensive. However, their disadvantages include their temperature range, which is less than 2000C, they are slow to react to changes in temperature, and they are self-heating. Typical examples of IC temperature sensors include LM35, which provides voltage, and AD590, which produces current.
2.2.3 LIGHT/OPTICAL SENSORS
Light sensors convert light into an electrical quantity. Light or optical sensors include devices such as light dependant resistor (LDR), photodiode, phototransistor, and photovoltaic cell.
Light Dependant Resistor (LDR)
Light dependant resistor is a device that has a resistance that decreases with an increase in the amount of light. That is, it converts light into resistance, and it is also known as photoconductive cell or photoresistive device.
Photodiode and Phototransistor
Photodiodes and phototransistors are devices that are used to convert light into current. Photodiodes are designed for high speed, high efficiency and low noise than the phototransistor. However, phototransistors have more output current than photodiode, but at the expense of speed.
Photovoltaic Cell
A photovoltaic or solar cell converts light into electrical energy
2.2.4 PRESSURE SENSORS
Pressure sensors convert pressure into electrical quantity. Since pressure and force are interrelated, pressure sensors can also used to detect other various form of energy such as weight, and force. Pressure can be defined as the amount of force applied to an area. Force, on the other hand, may be defined as a push or a pull. Weight is a force exerted on a mass of an object by a gravitational field. Other terms related to force and pressure are strain and stress, where strain is a force to make a change on an object and stress is pressure or tension exerted on an object. One of the most commonly used pressure sensors is a strain gauge.
Strain Gauge
The most common strain gauge consists of a grid of very fine foil or wire whose resistance varies linearly with the strain applied to the device. When using a strain gauge, you bond the strain gauge to the device under test, apply force, and then measure the strain by detecting the changes in resistance. Strain gauges are also used in sensor that detect force or other derived parameters, such as acceleration, weight and vibration.
Load Cell
Load cell is a sensor, which uses strain gauges that are mounted in specific patterns to provide a meaningful value of change in pressure or weight. Since load cell is made by combining a number of strain gauges in a common sensor, it will give out resistance that will depend on the amount of pressure applied.
Capacitance Pressure Sensors
Capacitance pressure sensors use capacitance and reluctance to convert pressure to voltage. One plate is kept stationary and the other one is connected to a diaphragm so that when the diaphragm moves due to applied pressure, the plate will move and the amount of capacitance will change. In most cases the dielectric between the two plates is a silicone oil filing
2.2.5 LEVEL SENSORS
Level sensors are used to convert the amount of a product in a container to an electrical quantity. Level sensors can be classified into point-contact and continuous level sensors.
Point-Contact Level Sensors
Point-contact level sensor determines the level at a single set point. Most of them include a switch that is activated when the level reaches a specific point. Point-contact level sensors includes float-level sensor, multiple float-level sensor, displacer-level sensor, paddlewheel-level sensor, beam-breaker level sensor, two-wire conductance-level sensor, and thermistor-level sensor.
Continuous Level Sensors
Continuous level sensors provide continuous level readings from the minimum to the maximum level. They are more expensive than point-contact level sensors and they provide analogue current or voltage. They include RF admittance (capacitance), the sonic, and the conductive type.
2.2.6 POSITION SENSORS
Position or displacement sensors are used to convert position or distance moved into electrical quantity.
Linear Potentiometer and Rotary Potentiometer
Linear potentiometer and rotary potentiometer sensors are basically variable resistors. As you move the slider or the wiper (the adjustable terminal) the value of the resistance is changed and this change is directly proportional to the movement of the slide. Linear potentiometers are used for linear measurements to convert linear motion into resistance, and rotary potentiometers are used to convert rotary motion into resistance.
Linear Variable Differential Transformers (LVDTs)
Linear variable differential transformer position sensor uses a primary transformer winding and two identical secondary windings that are wound around a hollow tube, which provides a cavity for a movable core that is attached to the system whose position is being measured. When ac voltage is applied to the primary of the transformer, there will be an induced voltage at the secondary, which is determined by the turns ratio and the core. As the core move in and out the amount of the induced voltage will increase and decrease.
2.2.7 BIOMEDICAL SENSORS
Our nervous system uses the flow of ions to communicate. The communication activity of the nervous system can be measured on the surface of the skin using electromechanical sensors, which are called microelectrodes or just electrodes. The signals that are detected are referred to as electrophysiological signals and they include gross electrical activities of the brain nerve cells, gross electrical activities of the heart, muscle movement, and summation of receptor potentials in the retina due to light stimulation. Some of these signals are listed in Table 2.2 below. Most of them have frequencies between dc (0 Hz) and 3 000 Hz and amplitudes ranging between 10 microvolts and 10 milli-volts (Plonsey, 1996).
Table 2.2 Electrophysiological signals and sensors
Signal Biological source Average amplitude Frequency
Electrocardiogram (ECG) Heart 1-5 mV 0.05-100 HzElectroencephalogram (EEG) Brain 10-50 V 0-150 Hz
Electromyogram (EMG) Muscles 0.1-1 mV 40-3000 HzElectroretinography (ERG) Retina Under 1 mV 0 – 30 HzElectroocculogram (EOG) Eyeball 0.05-3.5 mV 0-125 Hz
The name of the sensor used is derived from the signal or type of bioelectric phenomenon, for example, to detect heart signals ECG electrodes are used, for brain signals EEG electrodes are used and, so on. The most commonly used type of electrodes in biomedical signals is the Ag-AgCl electrode, which has an electrolyte containing Cl-. There are two properties that make Ag-AgCl a good choice for an electrode. It is non-polarizable, which means that current flows freely across the electrode junction, and secondly, it generates less than 10 V of noise.In other medical applications, especially in the incubators, it is also required to measure the temperature and the amount of oxygen. For temperature one RTD or IC temperature sensor can be used, and for oxygen a fuel cell, which converts the percentage of oxygen into voltage, can be used.
2.3 ACTUATORS
Actuators are output transducers that transform energy from electric into another form, which can be temperature, light, sound, or movement. Like sensors, actuator can be classified according to the major forms of energy they produce.
2.3.1 ACOUSTIC ACTUATORS
Acoustic sensor converts electrical energy into sound. Loudspeaker, headphones, and earpieces are acoustic actuators. A microphone is a typical example of an acoustic sensor that changes the sound waves into electrical energy, which may then be amplified, transmitted, or recorded.
2.3.2 TEMPERATURE ACTUATORS
Temperature actuator converts electrical energy into temperature. An element is a typical example of a temperature actuator. It is fitted in geyser, kettles, irons and cooking appliances to provide heat when current is passed through.
2.3.3 LIGHT ACTUATORS
Light actuators are used to convert electricity into light. Examples of light actuators include a light bulb, light emitting diode (LED), laser diode, liquid crystal display (LCD), and LED display, cathode ray tube (CRT). LCD and CRT are used in computer monitors, video monitors, oscilloscopes and television.
2.3.4 MOTION ACTUATORS
Motion actuator converts electrical energy into motion. A typical example is an electric motor, which converts electrical energy into motion and a relay converts electrical into movement.
2.4 Notes
When choosing a sensor make sure you know its output, its range and other characteristics such as accuracy, sensitivity, response time, and linearity. This is important since most of the sensors will need additional electronics to condition their raw outputs
For sensors that are giving out resistance a Wheatstone bridge is recommended for converting resistance into voltage.
For Sensors that are giving out current a current to voltage conversion is required (check chapter 4).
For sensors that are attached to human beings (biomedical sensors) include some form of isolation in conditioning circuits.
To drive actuators some form of interfacing is needed. Interfacing will be covered later in the text.
References
Barney, G.C. Intelligent Instrumentation. Prentice Hall, New Jersey, 1988.Boylestad, R. and L. Nashelsky. Electronic Devices and Circuit Theory, 5th edition. Prentice Hall, New Jersey, 1992.Duncan, T. Electronics for today and tomorrow. John Murray, London, 1985.Horrowitz, P. and W. Hill. The Art of Electronics, 2nd edition. Cambridge, New York, 1994.Kissell, T.E. Industrial Electronics. Prentice Hall, New Jersey, 1997Mancini, R. (editor in chief). Op Amps For Everyone: Design Reference. Texas Instruments, Advanced Analog Products (SLOD006B), August 2002.National Instruments. Signal Conditioning Tutorial. www.natinst.comPlonsey, R. Electronic Engineer Handbook, 4th edition - Electrocardiography and Biopotentials. McGraw-Hill, 1996.Temperature Controls (Pty) Ltd. Thermocouple & Resistance Temperature Detectors.Tompkins, W.J., editor. Biomedical Digital Signal Processing. Prentice Hall, 1995.
Tompkins, W.J. and J.G. Webster, editors. Interfacing sensors to the IBM PC. Prentice-Hall, 1988.Webster, J.G. Medical Instrumentation - application and design. Houghton Mifflin, 1978.
3. OPERATIONAL AMPLIFIERS (OPAMPS)
3.1 INTRODUCTION
Operational amplifier (Opamp) is monolithic amplifying device with a differential input stage, which consists of many [BJT or FET] transistors, resistors and capacitors connected as multistage transistor amplifiers that are integrated together into one package. These multistage amplifiers are direct coupled to allow the operational amplifier to be used for both dc and ac signals. Operational amplifiers come in packages that are having one, two (dual), or four (quad) identical Opamp units on a single chip.
An Opamp is basically a differential amplifier with the following properties:
(a) High open-loop gain Ao of about 105 at 0 Hz (dc) and low frequencies, which decreases as the frequency increases
(b) Very high input impedance, which is greater or equal to 1 M(c) Very low output impedance(d) The output is the amplified version of the difference of the two input signals
Due to the first property, any input to the Opamp will result in a very high output. However, the output can not be greater than the supply voltage, hence the output will be limited to the supply voltage (or a value that is about supply voltage minus 2 V for most of the Opamps). Due to the second property, the current into the input of the Opamp is assumed to be zero. This assumption is almost true in FET Opamps where the current is about 1 pA or less, but not always true in BJT Opamps where the current can be as high as tens of A.
Since an Opamp is a differential amplifier it has two inputs: the inverting input and the non-inverting input as shown in Figure 3.1 below. Most of the time the supply voltage terminals are omitted as shown on the right.
Figure 3.1 Basic operational amplifiers
The inverting input is marked – and the non-inverting input is denoted by a + mark. If an input is applied to the non-inverting (+) input the output will be in-phase with the input, but if the input is applied to the inverting (-) input the output will be 1800 out-of-phase with respect to the input (i.e. the output will be inverted).3.2 NEGATIVE FEEDBACK AND CLOSED LOOP OPERATION
To control the gain of the Opamp so that the output does not go into saturation, a negative feedback is used. This is accomplished by including a negative feeding back, where some of the output is fed back to the inverting input. This makes the gain of the Opamp to be controllable, and it also reduces the output voltage. The inclusion of the negative feedback changes the gain from being open-loop Ao to close-loop gain Ac.
When the Opamp is having a negative feedback, it is said to be operated in a close-loop mode. The advantage of operating the Opamp in this mode is that, unlike in the open-loop operation, the gain of the Opamp is predictable, and controllable so as to avoid distortion, which is caused by Opamp operating in the saturation region that is not linear.
3.3 BASIC OPAMP CIRCUITS
3.3.1 CLOSED-LOOP OPERATION
Amplifier
One of the most commonly used Opamp circuit is a constant-gain amplifier. A constant-gain multiplier is used to amplify the incoming signal, which is fed to one of the input of the Opamp. There are two versions of this circuit: the inverting and the non-inverting amplifier. For the inverting amplifier, the input is applied to the inverting input via an input resistor Ri, and there is also a feedback resistor Rf to control the gain of the amplifier as shown in Figure 3.2.
Figure 3.2 Inverting amplifier
The gain and the output voltage are given by
(3.1)
and
(3.2)
where the minus (-) indicates that the output is inverted.
For the non-inverting amplifier, the input is applied to the non-inverting input. It also has a feedback resistor Rf, and resistor Ri connected between the inverting input and ground to control the gain as shown in Figure 3.3. Unlike the inverting amplifier, the non-inverting amplifier has an output that is in-phase with the input.
Vi
Rf
RiVo
Figure 3.3 Non-inverting amplifier
The gain and the output voltage of the non-inverting amplifier are given by
(3.3)
and
(3.4)
Voltage-follower
If the value of resistor Ri in Figure 3.3 is increased towards infinity and the feedback resistor Rf towards zero, then Ri will be like an open circuit and equations (3.3) and (3.4) will become
(3.5)
and
(3.6)
The circuit will also change to be like the one in Figure 3.4, which is referred to as a voltage-follower. It is used as a buffer amplifier to provide isolation properties (high input impedance, low output impedance), hence it is also known as unity-gain buffer or just as a buffer.
Figure 3.4 Voltage-follower
Summing (Adding) Amplifier
Vi
Rf
Ri
Vo
ViVo
A summing amplifier provides algebraically addition to the voltages that are fed to one of the inputs, with each voltage multiplied by a constant-gain factor, which is determined by the respective input resistances and feedback resistor. Figure 3.5 shows a circuit of a summing amplifier with four input voltages and the output is given by
(3.7)
Figure 3.5 Summing amplifier
Where voltages V1 through V4 are input voltages that are going to be added and resistors R1 through R4 are input resistors. If we make the input resistors to be of the same value, that is, R1 = R2 = R3 = R4 = Ri, then equation (3.7) becomes
(3.8)
Integrator
The integrator circuit is almost the same as the inverting amplifier, but the feedback resistor is replaced by a capacitor as shown in Figure 3.6.
Figure 3.6 Integrator
Since the Opamp has high input impedance, it draws negligible current, which results in the current flowing through the input resistor to be the same as the current flowing through the feedback capacitor. That is,
(3.9)
R1V1
R2V2
R3V3
V4Vo
Rf
R4
C
ViVo
Ri
Integrating and re-arranging gives an output voltage
(3.10)
Differential (Subtractor) Amplifier
The differential amplifier has two input signals: one to the inverting input and another to the non-inverting input. These input voltage signals are fed to the Opamp inputs via input resistors Ris and there is also a feedback resistor Rf to control the gain and another resistor Rg (equal to Rf) between non-inverting input and ground to improve common-mode rejection ratio, as shown in Figure 3.7.
Figure 3.7 Differential amplifier
The output is equal to the amplified difference of the two input signals. That is,
(3.11)
ac Amplifier
The amplifier circuits shown in Figures 3.2 and 3.3 are general-purpose amplifiers and they can amplify both ac and dc signals. However, by adding a capacitor between resistor Ri and ground to the non-inverting amplifier in Figure 3.3 the operation of the circuit can be completely changed for dc inputs. At dc the capacitor will have a very high impedance, which will be infinite, and it will act as an open circuit. This will make the gain of the amplifier at dc to be
As the frequency starts increasing above 0 Hz (dc), the impedance of the capacitor will decrease, thus resulting in the gain increasing. At a certain frequency, which is determined by the capacitor and resistor Ri, the gain will be 70.7% of the maximum gain of the amplifier. This point is known as the cut-off, half-power, or low-frequency 3dB point. The gain and the frequency at this point is given by
Rg
RiV2
V1Vo
Rf
Ri
(3.12)And
(3.13)
At high frequencies the capacitor’s impedance becomes very small and can be neglected. Thus making the gain to be
which is the maximum gain Av(MAX) of the ac amplifier. The ac amplifier circuits are shown in Figure 8 for dual-rail and a single-rail supply.
(a) (b)
Figure 8 ac amplifier circuits (a) with a split supply, and (b) with a single supply
For the ac amplifier in Figure 8 (b), which uses a single-rail supply the reference voltage of the input has to be raised to a value that is above zero so as to allow both positive and negative swings of the output voltage. This is achieved by using two resistors, R1 and R2,
which set the reference to a value that is equal to . During amplification this
reference voltage is not amplified since it is dc but ac is amplified and since the reference is above zero both the positive and negative swings will appear at the output of the amplifier. Coupling capacitor Cc at the input and at the output of the amplifier are used to confine the introduced reference voltage to the ac amplifier so that it does not interfere with the circuits that are connected to the input or output of the amplifier. Their values are chosen such that they offer little impedance to the signal of interest.3.3.2 OPEN-LOOP OPERATION
Comparator
The Opamp circuits covered so far operate in a closed loop mode; hence their gain is predictable and controllable. The comparator, on the other hand, operates in an open-loop
Vi Vo+
20V
Cc
Ci
Cc
Ri 1.527k
Rf270k
R2100k
R1100k
-10V
Vi Vo+10V
Ci
Ri 1.527k
Rf270k
mode; that is, there is no negative feedback to control the gain. The absence of gain- control drives the output into [negative or positive] saturation. If the inverting input is greater than the non-inverting input, the output will go to negative saturation, but when the non-inverting input is greater than the inverting input, the output will go to positive saturation. In most comparator applications one input is held constant at value called Vref, which can be zero, greater than zero or less than zero, and the input voltage is only applied to one of the comparator input. Like in amplifiers, comparator circuits come in both inverting and non-inverting versions, where the inverting comparator introduces a phase-shift of 1800 with respect to the input signal. Some comparator circuits are shown in Figure 3.9, where (a) and (b) depict the non-inverting and inverting comparators with a reference voltage of zero (zero-crossing detectors), respectively, and (c) and (d) depict non-inverting comparators with positive (Vref > 0) and negative (Vref < 0) reference voltages, respectively.
(a)
(b) (c) (d)
Figure 3.9 Comparators: (a) non-inverting, (b) inverting, (c) Vref > 0, and (d) Vref < 0
There are special IC such as LM311, LM339, LM306, LM393, NE521, NE527, and TLC372, which are specifically designed for use as comparators. Though an ordinary Opamp can be used in many comparator circuits, the above-mentioned comparator ICs have some advantages over ordinary Opamps. Firstly, they have a fast response than ordinary Opamps, and secondly a pull-up resistor connected between output and supply voltage can enable the output to swing from the supply voltage to ground, whereas in ordinary Opamps the output can swing between the supply voltages minus 2 V.
The configuration of a comparator using a comparator IC is the same as that of ordinary Opamps (Figure 3.9), but it also includes a pull-up resistor, which is not present when using ordinary Opamp. This is shown in Figure 3.10 below, where resistor R1 is the pull-up resistor.
Vo
Vi
5V
R21k
R1
1kVo
Vi
-5V
R21k
R1
1kVo
Vi
VoVi
R11k
5V
VoVi
Figure 3.10 A comparator circuit using a comparator IC
Schmitt trigger
A Schmitt trigger is a comparator with a positive feedback resistor. The inclusion of the positive feedback resistor ensures a rapid output transition, regardless of the speed of the input waveform. It also widens the threshold by making the circuit to have two trip points (TPs): upper threshold point (UTP) and lower threshold point (LTP). Figure 3.11 shows the threshold for an ordinary comparator and a comparator with a positive feedback (Schmitt trigger), where the Schmitt trigger has a wider threshold than the ordinary comparator.
Vsat Vsat
UTP TP LTP
-Vsat -Vsat
(a) (b)
Figure 3.11 Threshold for an ordinary comparator and a Schmitt trigger
For noisy input signals the thin threshold in ordinary comparator will result in an erratic output that jumps back and forth between its low and high states when the input is near the trip point (TP). The widened threshold in the Schmitt trigger minimises multiple triggering, which results when a noisy input is near the threshold as shown in Figure 3.12.
Figure 3.12 Output waveforms of a comparator and a Schmitt trigger for a noisy input.Some basic Schmitt trigger circuits of Schmitt triggers are shown in Figure 3.13, below.
Vin Vo
-15V
15V
+
R2
R1 +Vin Vo
-15V
15V
R2R1
R3+Vin
Vo
5V
R2R1
VSAT VSAT
TP
-VSAT-VSAT
UTP
LTP
InputInput
Output Output
(a) (b) (c)
Figure 3.13 Schmitt trigger circuits: (a) Non-inverting, (b) & (c) inverting
The output voltage of the Schmitt trigger depends both on the input voltage and on its recent history. That is, the output voltage will remain in a given state until the input exceeds the reference voltage for that state.
For non-inverting Schmitt trigger:
(3.14)
and
(3.15)
Assume the output is negatively saturated. The feedback will result in a LTP at the non-inverting input. Since LTP is less than the voltage at the inverting input (which is zero), the output will remain negatively saturated until the input is positive enough to make the error voltage positive. When this happens, the output switches to positive saturation and the voltage at the non-inverting input switches to UTP. The output remains until the input is negative enough to make an error voltage negative, and when it does the output changes to negative saturation and the non-inverting input changes to LTP. This phenomenon can be illustrated by a graph of output voltage versus input voltage as shown in Figure 3.14.
Vo
LTP UTP Vin
Figure 3.14 Graph of output voltage versus input voltage
For inverting Schmitt trigger:
(3.16)
and
(3.17)
Assume the output is positively saturated. The feedback will result in a UTP at the non-inverting input. The output will remain positively saturated until the input is more than UTP. When this happens, the output switches to negative saturation and the voltage at the non-inverting input switches to LTP. The output remains low until the input is more negative than LTP, and when it does the output changes to positive saturation and the non-inverting input changes to UTP. This phenomenon can be illustrated by a graph of output voltage versus input voltage as shown in Figure 3.15.
Vo
LTP UTP Vin
Figure 3.15 Graph of output voltage versus input voltage
The output voltage versus input voltage graphs shown in Figures 3.14 and 3.15 show that the positive feedback causes the hysteresis. This hysteresis is equal to the difference between the trip points. That is,
(3.18)
The hysteresis makes Schmitt trigger to be more immune to false triggering than ordinary comparator. That is, it prevents noisy inputs from causing false triggering when the input is near the threshold. However, for hysteresis to be effective the peak-to-peak noise voltage must be less than hysteresis.
Window-Comparator
A window comparator is an electronic circuit that consists of two comparators in parallel. Each of the comparators has its own reference, which is completely different from the other. Unlike the ordinary comparator covered previously, a window comparator operates as a window detector. The circuit for a window comparator is shown in Figure 3.16.
Vo2
Vo1
Vin
R1 390
R2 1k
R3 4k
R4 1k
A2Vref2
A1Vref1
5V
5V
VoVin
A2Vref2
A1Vref1
5V
5V
R1 390
R2 1k
R3 4k
R4 1k
Figure 3.16 Window comparators
3.3.3 OTHER OPAMP CIRCUITS
Constant current source
An external transistor can be connected at the output of an Opamp to make a current source, as shown in Figure 3.17.
Figure 3.17 Current source
Resistors R1 and R2 set the input voltage to . The feedback forces a
voltage equals supply voltage minus input voltage across resistor R3, thus giving a collector current that is equal to
(3.19)
which is almost the same to the emitter or load current.
As an improvement to the current source circuit shown above, resistor R2 is replaced by a zener diode to produce a stable reference voltage that is used as input voltage Vin. Another output power transistor, mounted on a heatsink, is also added to the existing external
Load
Vcc
R3
R2
R1
Q1
Q2
Q1
D1
Load
Vcc
R3
R1
transistor to form a Darlington pair that will provide more current to the load, as shown in Figure 3.18.
Figure 3.18 Improved current source suitable for high currents
The feedback forces a voltage equals reference (zener or input) voltage across resistor R3, thus giving an emitter current that is equal to
(3.20)
which is almost the same to the collector or load current.
Power booster
Opamps amplify the voltage not the current of the input signal. However, the current can be boosted by connecting a power transistor at the output of an Opamp, as shown in Figure 3.19. The output voltage is given by
(3.21)
and
(3.22)
Vo
Vin
Q1
Vcc
R3R1
R2
Figure 3.19 Power booster
Single-to-dual power supply
Opamp can also be used with other components to convert a single rail supply to a dual or a split supply. The circuits for doing that are shown in Figure 3.20.
Figure 3.120 Single-to-dual (split) supply circuits
The reference voltage set by the resistors (or by a zener diode, D1) is fed to the non-inverting input of a unity-gain buffer. The output of the buffer, which is the same voltage as the input, is used as the reference point for the output voltages. One output voltage, which is positive, is taken across capacitor C1, and another one, which is negative, is taken across capacitor C2. The value of these output voltages depends on the supply voltage and the reference voltage. The positive voltage is given by
(3.23)
and
(3.24)
Peak detector
A peak detector is an Opamp circuit that is used to determine the peak value of a waveform. The circuit of a peak detector is shown in Figure 3.21 below.
Figure 3.21 Active peak detector
When a positive input is fed to the non-inverting input of the first Opamp, the output will be positive. This positive output forward biases the diode and charges up the capacitor. This charging will last until the inverting and the non-inverting inputs of the first Opamp are at the same voltage, which is equal to the input voltage, and when the input increases, the capacitor will charge to the new peak value. This process will repeat for each increase so as to keep the voltage at the inverting input and at the capacitor equals to the highest input voltage that is applied to the non-inverting input. The capacitor stores this peak voltage for minutes depending on the load connected to the circuit. After some few minutes the capacitor will start discharging towards zero. The unity-gain buffer at the output is used to isolate the peak detector from the effects of the load so as to minimize the rate of discharging.
3.4 CHARACTERISTICS OF AN OPARATIONAL AMPLIFIER
Common-mode input voltage range, VICR: Range within which a differential amplifier remains linear.
Common-mode input voltage, VIC: The average voltage at the input pins, .
Common-mode rejection ratio, CMRR: The ratio of the differential voltage gain, AVD, to common-mode voltage gain, AVC.
Common-mode voltage gain, AVC: The ratio of the output voltage to common-mode input voltage signal.
Continuous total power dissipation, PD: The power that can be dissipated by the Opamp, including the load power
Differential input voltage range, VIDR: Maximum difference signal that can be applied safely to the inputs of an Opamp.
Differential input voltage, VID: The voltage at the non-inverting input with respect to the inverting input.
Differential voltage gain, AVD: The ratio of the output voltage to differential input voltage signal.
Gain-Bandwidth or cut-off frequency, fc: The frequency at which the open-loop gain drops by 3 dB.
Gain bandwidth product, GBW: The product of open-loop voltage gain and the frequency at which it is measured.
Input bias current: Half the sum of the separate currents that are entering the two input terminals of a balanced amplifier.
Input current, II: The amount of current that can be sourced or sinked by the Opamp input.
Input offset current, IIO: The difference between the separate currents that are entering the inputs of a balanced amplifier
Input offset voltage, VIO: the voltage that must be applied to input terminals to balance the amplifier
Input resistance, ri: The dc resistance between the input terminals of an Opamp with either input grounded.
Input voltage range, VI: The range of input voltages that may be applied to either input of the Opamp.
Maximum peak output voltage swing, VOM: The maximum peak-to-peak output voltage that can be obtained without clipping when the Opamp is operated from a dual (split) supply.
Maximum peak-to-peak output voltage swing, VO(PP): The maximum peak-to-peak output voltage that can be obtained without clipping when the dc output voltage is zero.
Noise Figure, NF: The ratio of the total noise power at the output of an amplifier to the noise power at the input.
Open-loop voltage gain, AOL: The ratio of change in output voltage to the change in voltage across the input terminals.
Operating temperature, TA: The temperature over which the Opamp may be operated.
Output current, IO: The amount of current that can be drawn from the Opamp output.
Output Impedance, Zo: The frequency dependant small-signal impedance that is placed in series with an ideal amplifier and the output terminal.
Output resistance, ro: The dc resistance that is placed in series with an ideal amplifier and the output terminal.
Short-circuit output current, IOS: The maximum continuous output current available from the amplifier with output short-circuited to ground, to supply, or to specified point.
Slew rate: Rate of change of the output voltage of the Opamp when driven by a large step-input signal.
Supply current, ICC/IDD: The current into the supply voltage terminal of the Opamp while it is operating.
Supply Voltage, VCC/VDD: The bias voltage applied to the Opamp power supply pins (terminals). Usually specified as a value.
Total power dissipation, PD: the total dc power supplied to the device less any power delivered from the device to a load.
Unity-gain bandwidth, B1, or frequency, f1: The frequency at which the open-loop gain drops to unity. The unity-gain frequency f1 (or bandwidth B1) and cut-off frequency fc are related by .
3.5 DECIBEL (dB)
A decibel, dB, is an important unit that is used in telecommunication, as well as in electronics, to express gain or loss in a system or component. For example, an amplifier is used to increase the voltage, current, or power of a signal while the attenuator is used to decrease the voltage, current or power of a signal. The basic equations for decibel are
(3.25)
(3.26)
(3.27)
Notice that the voltage and current are 20 times the logarithmic ratio whereas power is 10 times the logarithmic ratio. To differentiate between the decibel for gain and that for loss, a negative unit is used for expressing loss, for example, –10 dB expresses a loss of 10 decibels, while the positive unit is used for gain.
Sometimes the decibel is referenced to voltage or power to express the absolute value, for example, dBm meaning decibel relative to a milliwatt, or dBV meaning decibels relative to a microvolt. Another unit that is sometimes used is dB, which is decibel referenced to 1microwatt instead of 1 mW. That is,
(3.28)
(3.29)
(3.30)
References
Boylestad, R. and L. Nashelsky. Electronic Devices and Circuit Theory, 5 th edition. Prentice Hall, New Jersey, 1992.Duncan, T. Electronics for today and tomorrow. John Murray, London, 1985.Horrowitz, P. and W. Hill. The Art of Electronics, 2nd edition. Cambridge, New York, 1994.Ibrahim, K.F. Electronic Systems and Techniques, 2nd edition. Longman Scientific and Technical, Essex, 1994. Mancini, R. (editor in chief). Op Amps For Everyone: Design Reference. Texas Instruments, Advanced Analog Products (SLOD006B), August 2002.Millman, J. and A. Grabel. Micro-electronics, 2nd edition. McGraw-Hill, New York, 1987.Texas Instruments. TL071, TL071A, TL071B, TL072, TL072A, TL072B, TL074, TL074A,TL074B Low-noise JFET-Input Operational Amplifiers (SLOS080D). August 1996. www.amalnet.k12.il/projects/compforum/docs/forum/Pdf/Tl702.pdfvan Roon, T. 741 Op-Amp tutorial. July 2004.www.uoguelph.ca/~antoon/gadgets/741/741.html
4. CONDITIONING AND INTERFACING
4.1 INTRODUCTION
Signal conditioners are ancillary circuits, which are intended to condition the raw signals that are produced by the sensors to usable signals for further processing. The signal conditioning may involve conversion form one form of electrical quantity to another, filtering, amplifying, attenuation, common-mode rejection, and/or scaling. The
conditioning circuit that are going to be covered in this chapter are those that are used for conversion, amplifying and scaling. Filtering is going to be covered in the next chapter.
Interfacing is about putting two processes, or circuits together. The emphasis will be on devices and circuits that are used to drive the actuators and other loads. It will also include ways of providing isolation between different stages of an electronic system.
4.2 SIGNAL CONDITIONING
It was mentioned in chapter 2 that sensors are used in electronics to acquire data about their surroundings and convert that into an electrical quantity for further processing. This electrical quantity can be voltage, resistance, capacitance, or current. Since processing is done using voltage, quantities such as resistance, capacitance and current need to be converted into voltage first before processing can take place. Signal conditioning circuits are used to improve the quality of the signals generated by the sensors
4.2.1 RESISTANCE –TO-VOLTAGE CONVERSION
Sensors such as RTDs, thermistors, LDR, strain gauges, carbon microphone, and others exhibit a change resistance in response to quantity they are measuring or detecting. Some have their resistance increasing as the quantity being measured is increasing, and others have their resistance decreasing as the quantity being measured is increasing. For example, the resistance of an RTD increases as the temperature is increasing, whereas the resistance of an LDR decreases with the increase in the light intensity. In both cases, the change in resistance that is produced by the sensor must be converted to change in voltage before it can be processed further.
There are two most commonly used methods for converting resistance into voltage: the use of a voltage divider network and the use of a bridge. The use of a voltage divider network is the simplest method than using the bridge. However, it is not as efficient as the bridge method since it amplifies the entire voltage measured by the sensor, whereas the bridge only amplifies the change in the resistance of the sensor.
Voltage divider network
Voltage divider network comprises of a reference resistor Rr and the sensor Rs as shown in Figure 4.1.
Vs
Vcc
Rr
Rs
Vo
Vcc
Rr
Rs
Vs
Figure 4.1 Resistance-to-voltage using divider network
Voltage VS represents the voltage that is developed across the sensor. Depending on the application, this voltage can only need buffering as shown above, or it can need further amplification, which can be accomplished by replacing the voltage-follower with an amplifier that is a having a gain that is greater unity. The output of the divider network is given by
(4.1)
This voltage is fed into an amplifier circuit to produce an output, which is a product of the sensor voltage and the gain of the amplifier. For example, the output voltage Vo for the circuit shown above is
Bridge
The bridge only amplifies the voltage due to the change in the resistance of the sensor. The most commonly used bridge is a Wheatstone bridge, which is formed by the sensor and three other resistors as shown in Figure 4.2.
Figure 4.2 Wheatstone bridge and instrumentation circuit
The bridge is ratiometric, so the null does not shift with variations in supply voltage, and its output is given by the difference between the voltage across R3 and that across the sensor. That is,
(4.2)
To amplify the output of the bridge a differential amplifier is used as shown in Figure 4.2. The advantage of using a differential amplifier is that it has an ability to eliminate any noise that can be present in its input (common-mode rejection). The output of the differential amplifier is given by
(4.3)
If the sensor is a distance away from the conditioning circuit, the sensor connection shown in Figure 4.2 is not recommended since the resistance of the connecting wires will add to that of the sensor, thereby compromising the whole accuracy of the instrumentation circuit.
To cancel the effects of the resistance of the connecting wires a third connecting wire, similar to the other two connecting wires, is used. The two wires connect the sensor to the bridge, and since they are on the either side of the bridge, they effectively cancel, while the third wire functions as the extended supply lead to configure the bridge as shown in Figure 4.3 below.
Figure 4.3 Instrumentation circuit with a bridge that is using a third lead
It is evident in the above figure that a resistance equals to twice the lead resistance r will be added to both the sensor resistance and to resistor R3, thus effectively cancelling the effect of each other.
Self-heating
In order to convert resistance of the sensor to voltage an electric current from a voltage source must be passed though the sensor to produce a voltage that is equal to the product of the current passing through the sensor and the resistance of the sensor. This passing of
r
Rs
rR1 R2
R3
Vo
Vcc
r
Ri
RfRi
Rg
the current through the sensor produces a heating effect, which can affect the operation of the sensor, especially temperature sensors.
To minimize the effects of self-heating, the energising current must be reduced to a minimum. This can be done by either reducing the value of the voltage source VCC, or by increasing the value of the series resistance R1 (Rr in the divider network).
4.2.2 CURRENT-TO-VOLTAGE CONVERSION
Photodiodes, phototransistors and other sensors exhibit a change current in response to quantity they are measuring or detecting. This current need to be converted to voltage before the signal can be processed further. In order to convert current output of the sensor to a voltage a circuit called current-controlled voltage source is used as shown in Figure 4.4 below.
Figure 4.4 Current-controlled voltage source
The output voltage of the circuit is given by
(4.4)
To ensure more efficiency in conversion use a FET Opamp, because the current into the input of the Opamp is negligible small.
4.2.3 CAPACITANCE-TO-VOLTAGE CONVERSION
Other sensors, such as piezoelectric sensor, capacitor microphones and other sensors are based on the property of capacitance; that is, a property which varies directly proportional to the distance separating the two plates. To convert capacitance to voltage the same two most commonly used methods for converting resistance into voltage can be used; that is the voltage divider network and the use of a bridge. However, instead of using resistors and dc voltage source, capacitors and ac voltage source are used.
4.2.4 VOLTAGE-TO-VOLTAGE CONVERSION
Some other sensors exhibit a change voltage in response to quantity they are measuring or detecting. In most cases the raw output voltage of the sensor is not appropriate for other applications, hence a circuit, which converts the output voltage of the sensor into a more useful voltage rage, must be used. This circuit is known as the voltage-controlled
voltage source VCVS and its function is either to increase or decrease the amplitude of the sensor output voltage. Figure 4.5 depicts some of the voltage-controlled voltage source circuits.
(a) Inverting amplifier (b) Non-inverting amplifier (c) Voltage follower
(e) Instrumentation amplifier
Figure 4.5 Voltage-controlled voltage source circuits
Inverting amplifier The inverting amplifier has a gain that is determined by the ratio of feedback to input
resistor (i.e. ). The values of the resistors can be chosen such that the incoming
signal is amplified, attenuated, or just inverted.
If Rf > Ri the input signal will be amplified If Rf < Ri the input signal will be attenuated If Rf = Ri the input signal will be just inverted and the amplitude will remain
unchanged. This is sometimes referred to as an inverting buffer.
Non-inverting amplifier
The non-inventing amplifier can only amplify the input voltage by a gain of , and it
cannot be used to attenuate, or to invert the input signal.
Vi
Rf
RiVo
Vi
Rf
Ri
Vo ViVo
Rc
R1
R1
A2
RiRf
RiRg
A3
Vo
V1
V2
A1
Voltage-follower
The unity-gain amplifier is used for buffering; that is, it provides isolation properties (high input impedance, low output impedance).
Instrumentation amplifier
Instrumentation amplifier is one of the most important VCVS circuit for amplifying sensor outputs. The instrumentation amplifier is formed by placing a non-inverting amplifier at each of the inputs of the differential amplifier, and instead grounding the Ri
resistors, the two Ri resistors are connected together to create one common resistor Rc. The advantages of instrumentation amplifier include very high input impedance, very high gain, and very high common-mode rejection ratio (CMRR). The high CMRR ensures that any interference that is common to both inputs is eliminated.
The two non-inverting amplifiers are designed to have equal gains given by
(4.5)
After amplification the two signals are fed to a differential amplifier with a gain that is determined by Rf and Ri. That is,
This differential amplifier will produce an output that is equal to the difference of the two input signals amplified by the ratio of the feedback to the input resistor. It will also try to cancel the noise that is common on its input terminals. The overall gain of the instrumentation amplifier will be the product of the gain of the non-inverting amplifier and differential amplifier. That is,
(4.6)
This implies that the output voltage of an instrumentation amplifier is
(4.7)
Note: For high CMRR in instrumentation and differential amplifiers it is important that the input resistors Ris, feedback resistors R1s of the non-inverting amplifiers (instrumentation amplifier), and Rf and Rg are closely matched using precision resistors. To satisfy this demand, monolithic IC instrumentation amplifiers such as 1NA111AP, INA114AP, and INA131AP were developed. Each package includes all the [active and passive] components; that is, the complete instrumentation amplifier circuit, within a single package. The components remain matched over temperature, thus ensuring excellent performance over a wide range of temperature.
Level-shifting
Most of analogue-to-digital converters (ADCs) are designed to work with unipolar signal, which most of the time are ranging from zero to about positive 5 volts. Level shifter is a type of VCVS, which can be used to shift a bipolar signal that is having a zero average to a signal that is having an average that is greater than zero. For example, Figure 4.6 shows an input voltage of 20 V peak-to-peak that needs to be conditioned for an ADC0804, which works with voltages between zero and 5 volts.
Figure 4.6 ADC conditioning circuit (using level-shifting)
First the input signal to be shifted is added to a dc voltage with a value equals to the negative peak using a summer. This will change the average value of the input signal from zero to another value, but since the summer is using the inverting input the output would be inverted. To reverse the inversion introduced by the summer an inverting amplifier is placed at the output of the summer to invert the signal, thus restoring the 1800
phase shift back to zero. To reduce the peak to peak value of the input voltage from 20 Vp-p to 5 Vp-p the feedback resistors are chosen to be less than the input resistance. Note that the attenuation of the signal can be also done in one stage only; that is, at the summer, or at the inverting stage.
10V
S1
150pF
5V
+A2
+12V
-12V
CAB
-12V
+12V
10kHz
Vin-10/10V
+A1
1112131415161718
6
19
4
3
5
1 2 7 8 10
20
U3
ADC0804Ri1
10k
10k
Ri2
10k
RF25k
RF15k
Ri1
10kC
AB
Digital outputs
4.3 INTERFACING
Interfacing involves connecting two items of hardware, software or hardware and software. In this section the discussion is going to be limited to the interfacing between hardware and hardware. Software-hardware interfacing is going to be covered on another chapter later in the text.
Most electronic and digital ICs are not able to deliver voltages and currents that are sufficient to drive the actuators and other loads. To enhance the output current, a buffer or external transistor between the IC output and the load is used, and to enhance the output voltage, a relay or an opto-isolator can be used.
4.3.1 CURRENT ENHANCEMENT
Buffers
Buffers are circuits that prevent loading of an input or output by providing isolation properties (high input impedance, low output impedance). They can be built using ordinary Opamps (Figure 4.5c), or they can come in the form of digital ICs. The one built from Opamps can work with both digital and analogue signals, while those in the form of digital ICs can only work with digital signals. Both analogue and digital buffers can come as inverting or non-inverting buffers as shown in Figure 4.7.
Digital buffers come in packages that are having four, six or eight identical buffers within a single package. The digital IC that is having four buffers is called a quad buffer. The typical examples of quad buffer include 74125 and 74126 tri-state buffers. The digital IC that is having six buffers is called a Hex buffer. The typical examples of Hex buffer include 7404/05/06/16, 4049, 4069 (inverting buffers) and 7407, 7417, 4050, 74365/7 tri-state (non-inverting buffers). The digital IC that is having eight buffers is called an octal buffer. The typical examples of octal buffer include 74240, 40240, 74540 (tri-state inverting buffers) and 74241, 74244, 74541 (tri-state non-inverting buffers).
Tri-state buffer has an extra terminal, called control line, which is used to control the state of the output as shown in Figure 4.8. This control line is used to enable the buffer, and when it is enabled, the buffer functions as a normal buffer; that is, for non-inverting buffer: if the input is high the output is also high, and if the input is low the output will also be low. For inverting buffer if the input is high the output will be low, and if the input is low the output will be high. If the tri-state buffer is not enabled, the output will be disconnected from the input and the buffer will act as a high-impedance or as an open circuit between the input and output. Other tri-state buffers use active-high output enable as shown in Figure 4.8 (a), while others use active-low output enable as shown in Figure 4.8 (b).
(a) (b)
Figure 4.8 Tri-state non-inverting buffers
Output transistors
A single transistor or Darlington pair (for more current) can also be used to enhance the output current. The output of the controlling circuit is applied to the base of the transistor and if it is high enough it will switch on the transistor, and once the transistor is on it will provide a path for current from supply voltage through the load to ground. The load can either be placed between the transistor and supply voltage or between the transistor and ground. The two configurations are referred to as current sink and current source, respectively.
(a) (b) (c) (d)
Figure 4.9 (a) & (b) Current sinks, and (c) & (d) current sources
Other output current enhancing circuits for driving loads are shown in Figure 4.10
control
outinout
control
in
VccVccVccVcc
ViVi
ViVi
Q4
Q3
Q2
Q1
LoadLoad
LoadLoad
Load
Vcc
Q3
uP,
uC,
etc.
U2
uP,
uC,
etc.
U1
Q2
Load
Vcc
Load
Vcc
Q1
uP,
uC,
etc.
U3
Load
Vcc
uP,
uC,
etc.Load
uP,
uC,
etc.
Figure 4.10 Output current enhancing circuits for driving loads
4.3.2 VOLTAGE ENHANCEMENT
To drive ac loads, remote loads, loads requiring high voltage, or loads with an independent ground system a relay or preferable an opto-isolator or an opto-coupler can be used. These devices offer some electrical isolation between the driving circuit and the load.
Relays
A relay is an electrical controlled switch, which consists of a coil, armature and contacts. When a voltage source is connected across the coil it causes current to flow in the coil, and when this current is sufficient the coil pulls the armature and the contacts change their states. That is, if the contact was closed it will be open, and vice versa. Relays come in many varieties, including latching and stepping relays. The most common coil voltages are from 5 V up to about 100 V or more. The relay can have one set of contacts or more, which come in different power ratings with others even capable of handling ac loads. Figure 4.11 shows the symbols for the coils, contacts, and the complete relay.
Figure 4.11 Symbols for the coils, contacts, and the complete relay
Opto-coupler and opto-isolator
The opto-coupler and opto-isolator are opto-devices that are having an LED on the driver side that illuminates a photodetector on the load side. These devices are available in various speeds and with different output configurations ranging from logic output, saturated transistor or Darlington pair output, SCR to triac output. They provide an isolation that is greater or equals to 2500Vrms, 1012-ohm insulation and less than a picofarad coupling between input and output. Those that are having saturated transistor or Darlington pair output are usually referred to as opto-couplers.
OPTO-COUPLEROPTO-ISOLATOR
RLY35VCOIL
RLY25VCOIL
RLY1SPDTRELAY
NOCNC
CONTACTS
NOCNC
CONTACTS
Figure 4.12 Opto-coupler and opto-isolator
To use the relays and the opto-devices you use the same configurations as in Figure 4.10 above but in the place of the load you put a relay or an opto-device. For opto-devices you also include a current limiting resistor in series with the LED (of the opto-device) and for relays you connect a diode across the coil of the relay to protect the transistor from inductive kick. Some of the circuits for voltage enhancement are depicted in Figure 4.13.
R MOC3020R4180
Q2
Live
Neutral
Loadac
F21A
uP,
uC,
etc.
Vcc
4N25R
Load
Vcc3
uP,
uC,
etc.
Vcc
F11A
Loadac
Neutral
Live
Q1
R1180
R
uP,
uC,
etc.
MOC3020
Vcc2
Load
R
4N25uP,
uC,
etc.
Loadac
Live
Neutral
F11A
RLY1
uP,
uC,
etc.
U1
Q2
VccVcc Vcc1
RLY1
uP,
uC,
etc.
U3
Q1
Load
Figure 4.13 Output voltage enhancing circuits for driving ac and dc loads
4.4 MONITORING, CONTROLLING AND REGULATING SYSTEMS
Having discussed sensors, signal conditioning and interfacing circuits, let us look at systems that apply them.
Figure 4.14 Automatic light switch (turns on the light bulb when it is dark)
Figure 4.15 Disco light (light flashes to the beat of music)
Figure 4.16 Basic optical fiber receivers
10kVR2
10kVR1
10k 40%
MOC3020
Q1
Live
Neutral
F11A
1uF
Ci
LM311
5V
5V
TL071
L1R1k
R6180
Rf270k
Ri 1.527k
R51k
R1100k
R2100k
R3100k
R4100k
Mic
RLDR(light) < R1 < RLDR(dark)
Figure 4.17 Temperature regulating system (switches on the heater when cold and a fan when hot)
The last circuit, shown in Figure 4.18, is an over-and-under [mains] voltage monitoring system. It monitors the mains voltage and disconnects the load when the voltage drops below or rises above a set value. The circuit uses a window comparator, just like the temperature regulating system shown in Figure 4.17 above.
PT100Rt
5V
U7
5V
5V
D3COLD
D2HOT
D1NORMAL
U4A
U3B
U3A
U1
U2
NEUTRAL
CSR1
F12.5A
LIVE
MOC3040
Fan
NEUTRAL
CSR2
F22.5A
LIVE
MOC3040
Heater
R12270k
R11270k
R10100
R93.8kR83.8k
R647k
R747k
R161k
R154k
R141k
R13390
R3330
R2330
R1330
R4
150
R5
150
LOAD
AC source
R11
A
B
3B
3A
2A
LED2Vin
10V
10V
LED1
LED3
1A
+ C14700uF
T6
40:1
L
N
E
N
LF1
2.5A
CSR1
5V
+-
Buzzer
RLY1
MOC3020
R10
R91k
R81.181k
R61k
R4512.402
R5
R7
R13
270
R12
270
Figure 4.18 Over-and-under voltage monitoring system.
4.5 ISOLATION
Galvanic isolation is required for many circuits that are found in medical systems since there are sensors that are attached to the patient. This has been traditionally accomplished by means of transformers and opto-couplers, with transformers used to couple ac signals and opto-couplers used primarily for dc and digital signal coupling. The advantage of using opto-couplers is that they are small and cheaper than isolation transformer. However, their non-linearity made them not to be suitable for ac signals.
However, developments in technology have resulted in the production of a linear opto-coupler that can be used for both ac and dc signals as a cheaper technology to replace the expensive and bulky cumbersome transformers and non-linear opto-couplers. A typical example of a linear-opto-coupler is a LOC110. This device is a linear opto-coupler that features an infra-red LED, which is optically coupled with two phototransistors as shown in Figure 4.19. One phototransistor is typically used in a servo feedback mechanism to control the LED drive current thus compensating for the LED’s nonlinear time and temperature characteristics. The other (output) phototransistor provides output current that is linear with respect to the servo LED current.
Figure 4.19 LOC110 linear opto-coupler
This device provides up to 5 kV-peak isolation and it can couple both dc and ac signals. Figures 4.20 and 4.21 show the implementation of LOC110 in providing isolation for dc and ac signals, respectively.
Figure 4.20 Isolation Amplifier (Photovoltaic Operation) for dc signals
TheLOC110 devices are used in a photovoltaic mode to achieve best linearity, lowest noise drift performance. In this mode the phototransistors within the LOC110 act as current generators. These phototransistors are connected across the Opamp inputs, and as the input voltage, Vin, increases, the current through the LED increases and so does the optical flux. The LED flux is incident on the servo phototransistor, which starts current Iin
to flow from the Opamp (IC1A) inverting input through the phototransistor. This servo photo current is linearly proportional to input voltage, Iin = Vin/Rin and keeps the voltage on the inverting input equal to zero. The flux from the LED is also incident on the output phototransistor. The output phototransistor generates an output current Io that is proportional to the LED flux and LED current. This current Io is fed into the output Opamp (IC1B), which is configured as a current-controlled voltage source, to convert the output current to voltage, Vo, which is a product of Io and resistor Ro. Since the same LED flux is incident on both phototransistors, the currents generated by the phototransistors will be the same. Therefore, the output voltage can be expressed in terms of input current, Iin, or input voltage, Vin, as follows
IC3A
IC3B
=
(4.8)
Rf is chosen such that the current If through the diode is less than or equal to 15 mA, thus
(4.9)
A capacitor is connected across the output resistor Ro, forming a low pass filter, to eliminate high frequency interference.
For ac signals, the circuit in Figure 4.20 is modified as shown in Figure 4.21. Resistor R2
from 5 volts pre-biases the input amplifier such that a quiescent forward current in the LED is established. This is done to accommodate bipolar ac signals. And at the output, a 10-k variable resistor R4 is connected in series with R3 so that the gain can be varied from unity to about 1.5.
Figure 4.21 Isolation Amplifier (Photovoltaic Operation with pre-biased input) for ac signals
In this photovoltaic mode, it is possible to preserve accurately the linearity compatible with a 14-bit DAC.
References
Barney, G. C. Intelligent Instrumentation, Prentice Hall, London, 1988.Boylestad, R. and L. Nashelsky. Electronic Devices and Circuit Theory, 5th edition. Prentice Hall, New Jersey, 1992.de Vaux-Balbirnie, T.R. Mini Disco light. Everyday Electronics, Vol. 16, No. 6, June 1987.
de Vaux-Balbirnie, T. Disco lights flasher. Everyday Practical Electronics, Vol. 27, No. 1, January 1998.Floyd, T.L. Digital Fundamentals, 7th edition. Prentice-Hall, New Jersey, 2000.Horrowitz, P. and W. Hill. The Art of Electronics, 2nd edition. Cambridge, New York, 1994.Mancini, R. (editor in chief). Op Amps For Everyone: Design Reference. Texas Instruments, Advanced Analog Products (SLOD006B), August 2002.Millman, J. and A. Grabel. Micro-electronics, 2nd edition. McGraw-Hill, New York, 1987.Rakes, C.D. Looking at Optoelectronic Devices. Popular electronics, Vol. 14, No. 9, September 1997.Sterling, D.J. Jr. Technician’s guide to fiber optics, 2nd edition. Delmar Publishers Inc., New York, 1993.Stulens, P. Linear Opto-coupler - Application newsletter and notes. CP Clare Corporation, 1996.Texas Instruments. TL071, TL071A, TL071B, TL072, TL072A, TL072B, TL074, TL074A,TL074B Low-noise JFET-Input Operational Amplifiers (SLOS080D). August 1996. www.amalnet.k12.il/projects/compforum/docs/forum/Pdf/Tl702.pdfTemperature Controls (Pty) Ltd. Thermocouple & Resistance Temperature Detectors.Tompkins, W. J., editor. Biomedical Digital Signal Processing, Prentice Hall, New Jersey 1995.Tompkins, W. J. and J.G. Webster, editors. Interfacing sensors to the IBM PC, Prentice-Hall, Englewood Cliffs, New Jersey, 1988.van Roon, T. 741 Op-Amp tutorial. July 2004. www.uoguelph.ca/~antoon/gadgets/741.htmlWebster J. G. Medical instrumentation (Application and Design), Houghton Mifflin Company, Boston, 1978.
