e&i _unit5 ppt

8/10/2019 E&I _Unit5 ppt

1/79

UNIT-5

Primary sensing elements andsignal conditioning

8/10/2019 E&I _Unit5 ppt

2/79

The General Measurement System (GMS)

8/10/2019 E&I _Unit5 ppt

3/79

3

ExamplePressure Gauge

Measured Medium / Quantity:

Primary Element:

Variable Conversion Elements:

Variable Manipulation (Gain)

Data Transmission:

Data Presentation:

Air Pressure

Piston

Spring (F x)

Piston Rod

Pointer/Scale

8/10/2019 E&I _Unit5 ppt

4/79

4

The primary sensing element

which first receives energy from the measured medium andproduces an output depending on the way of measured quantity(measurand").

8/10/2019 E&I _Unit5 ppt

5/79

5

Signal conditioning

Signal Conditioning is the manipulation of theoutput of a sensor, probe, or transducer toperform one or more of these functions:

Signal level change - amplification orreduction

Filtering

Impedance matching A/D conversion

8/10/2019 E&I _Unit5 ppt

6/79

Transducers Transducer

a device that converts a primary form of energy into a corresponding

signal with a different energy form

Primary Energy Forms: mechanical, thermal, electromagnetic, optical,

chemical, etc.

Sensor(e.g., thermometer) a device that detects/measures a signal or stimulus

acquires information from the real world

Actuator(e.g., heater)

a device that generates a signal or stimulus

real

world

sensor

actuator

intelligent

feedback

system

8/10/2019 E&I _Unit5 ppt

7/79

Advantages of electrical transducers

Amplification & attenuation

Mass-inertiaeffects are minimized

Effects of frictionare minimized

Controlled using small power level

Output can be easily used ,transmittedandprocessed .

Telemetry & remote controlExplosive development in field of electronic

components and devices

8/10/2019 E&I _Unit5 ppt

8/79

Classification of transducers

On the basis of transduction form used.

Primary & secondary transducers

Active & passive transducers

Analog & Digital transducers

Transducers & inverse transducers

8/10/2019 E&I _Unit5 ppt

9/79

On the basis of transduction

Variation of resistance

Variation of inductance

Variation of capacitance Piezo-electric effect

Magnetostrictive effect

Elastic effect

Hall effect

Thermo electric effect

Piezo-resistive effect

8/10/2019 E&I _Unit5 ppt

10/79

8/10/2019 E&I _Unit5 ppt

11/79


Resistance transducers

-change in resistance due to change in physical quantity.

Example

Potentiometer

RTD

Strain gauges Photoconductive cells

Thermistor

8/10/2019 E&I _Unit5 ppt

12/79


Inductive transducers

-any of these quantity changes L=f(N,fr,A,L) theinductance changes.

ExampleLVDT

Synchro

Reluctance pickupEddy-current pickup

Velocity transducer

8/10/2019 E&I _Unit5 ppt

13/79


Capacitive Transducers

C =orA/d

Any one of these quantity changes the capacitancealso changes

Example

Variable capacitance pressure gaugeCapacitor microphone

Dielectric gauge

8/10/2019 E&I _Unit5 ppt

14/79

Active and Passive transducers

Active transducers

-no need of any external power source

Ex:photovolatic,thermoelectric,piezoelectric

Passive transducers

Do not generate energy for conversion

Need of external power source

Types

Variable resistance

Opto electronic

Variable reactance

8/10/2019 E&I _Unit5 ppt

15/79

Variable resistance

Photo conductors

Strain gauge

Thermistor

Opto electronic

Photo- emissive cellPhoto junctions

8/10/2019 E&I _Unit5 ppt

16/79

Variable reactance

Inductive

Variable reluctance

Variable permeability

LvdtEddy current

Capacitive

Variable area

Variable separation

Variable permittivity

8/10/2019 E&I _Unit5 ppt

17/79

Analog & digital transducers

Analog-output continuous function of time

Example

Thermocouple

ThermistorLVDT

Digitaloutput in pulses-discrete function of time

Digital tachometer

Push button switch

Television set tuner

8/10/2019 E&I _Unit5 ppt

18/79

Direct and Inverse Transducers

Direct-one form to electrical

Example :microphone [sound-electrical]

Inverse-electrical into non-electrical

Example:loudspeaker[electrical-sound]

8/10/2019 E&I _Unit5 ppt

19/79

Characteristics of transducers

Input characteristics

Type of input & operating range

Loading effects

Transfer characteristics

Transfer function Error

response of transducer to environmental influences

Output characteristics

Type of electrical output Output impedence

Useful range

8/10/2019 E&I _Unit5 ppt

20/79

Factors influencing the choice of

transducers Operating principle

Sensitivity

Operating range

Accuracy

Error

Transient & frequency response

Loading effects

Environmental compatibility

Insensitive to unwanted signals

Usage & ruggedness Electrical aspects

Stability & reliability

Static characteristics

8/10/2019 E&I _Unit5 ppt

21/79

Operational Amplifiers

An amplifier which not only perform amplification of signal

but also Some mathematical functions like,

adding signals

subtracting signals

integrating signals,

dttx )(The applications of operational amplifiers ( shortened

to op amp ) have grown beyond those listed above.

8/10/2019 E&I _Unit5 ppt

22/79

Operational Amplifiers

inverting input

noninverting input

output

V-

V+

The basic op amp with supply voltage included is shown

in the diagram below.

Figure : Basic op am diagram with supply voltage and IC configuration

+

1

2

3

4

8

7

6

5

OFFSET

NULL

-IN

+IN

V

N.C.

V+

OUTPUT

OFFSET

NULL

DIP-741

8/10/2019 E&I _Unit5 ppt

23/79

OPERATIONAL AMPLIFIER

Vd

+

Vo

Rin~inf Rout

Input 1

Input 2

+Vcc

-Vcc

8/10/2019 E&I _Unit5 ppt

24/79

Operational Amplifier

Definition

Operational amplifier is basically a differentialamplifier whose basic function is to amplify the

difference between two input signals.op-ampalso called as difference amplifier.

Terminal ais known as inverting input terminal.

The signal which is applied at the inverting

terminal(v1) is inverted at the output.Thenegative sign indicates the polarity change atoutput

8/10/2019 E&I _Unit5 ppt

25/79

OPERATIONAL AMPLIFIER

Terminal bis known as non-inverting input

terminal.

The signal which is applied at the non-

inverting terminal(v2) is not- inverted at the

output.The positive sign indicates the no

polarity change at output

The output voltage V out directly proportional

to the difference of the input volteges(V1~V2)

8/10/2019 E&I _Unit5 ppt

26/79

Operational Amplifier

Operational amplifier is a direct coupled high gaindifferential input amplifier.

They are used in voltage regulators, activefilters,Instrumentation,A/D ,D/A converters

The performance of the op-amp is well controlled anddetermined by the application of negative feedback.

Usually the feedback elements are passive. So theoperation of the ckt can be made very stable.

The advantage of using differential amplifier in op-ampis due to its rejection capability of unwanted signals.

8/10/2019 E&I _Unit5 ppt

27/79

Ref:080114HKN Operational Amplifier 27

Common-Mode and Differential

Mode Operation

+

Vo

Vi ~

Same voltage source is applied

at both terminals

Ideally, two input are equally

amplified

Output voltage is ideally zero

due to differential voltage is

zero

Practically, a small output

signal can still be measured

Note for differential circuits:

Opposite inputs : highly amplified

Common inputs : slightly amplified

Common-Mode Rejection

8/10/2019 E&I _Unit5 ppt

28/79

Ref:080114HKN Operational Amplifier 28

Common-Mode Rejection Ratio (CMRR)

Differential voltage input :

VVVd

Common voltage input :

)(21 VVVc

Output voltage :

ccddo VGVGV Gd: Differential gain

Gc: Common mode gain

)dB(log20CMRR 10c

d

c

d

G

G

G

G

Common-mode rejection ratio:

Note:

When Gd>> Gc or CMRR

Vo= GdVd

+

NoninvertingInput

InvertingInput

Output

8/10/2019 E&I _Unit5 ppt

29/79

Operational Amplifier 29

CMRR ExampleWhat is the CMRR?

Solution :

dBCMRRand

V(2)From

V(1)From

VV

VV

40)10/1000log(20101000

607007060

806006080

702

4010060

2

20100

60401008020100

21

21

cd

cdo

cdo

cc

dd

GG

GGV

GGV

VV

VV

+

100V

20V

80600V

+

100V

40V

60700V

(1) (2)

8/10/2019 E&I _Unit5 ppt

30/79


Op-Amp Properties(1) Infinite Open Loop gain

- The gain without feedback

- Equal to differential gain

- Zero common-mode gain

- Pratically, Gd= 20,000 to 200,000

(2) Infinite Input impedance

- Input current ii

~0A

- T-in high-grade op-amp

- m-A input current in low-grade op-

amp

(3) Zero Output Impedance

- act as perfect internal voltage source

- No internal resistance

- Output impedance in series with load

- Reducing output voltage to the load

- Practically, Rout~ 20-100

4. Bandwidth infinite

+

V1

V2 Vo

+

Vo

i1~0

i2~0

+

Rout

Vo'Rload

outload

loadoload

RR

RVV

8/10/2019 E&I _Unit5 ppt

31/79


Ideal Vs Practical Op-Amp

Ideal Practical

Open Loop gainA 105

BandwidthBW 10-100Hz

Input ImpedanceZin >1M

Output ImpedanceZout 0 10-100

Output Voltage Vout Depends onlyon Vd= (V+V)

Differential

mode signal

Depends slightly

on average input

Vc= (V++V)/2

Common-Mode

signal

CMRR 10-100dB

+

~

AVin

Vin Vout

Zout=0

I deal op-amp

+

AVinVin Vout

Zout

~

Zin

Practical op-amp

8/10/2019 E&I _Unit5 ppt

32/79

Op-amp Amplications Voltage Comparator

digitize input

Voltage Follower

buffer

Non-Inverting Amp Inverting Amp

M O C fi i

8/10/2019 E&I _Unit5 ppt

33/79

More Op-amp Configurations

Summing Amp

Differential Amp

Integrating Amp

Differentiating Amp

8/10/2019 E&I _Unit5 ppt

34/79

Converting Configuration

Current-to-Voltage

Voltage-to-Current

8/10/2019 E&I _Unit5 ppt

35/79

FILTERS

Need of filters

-eliminate unwanted signals

- to improve Sample/Noise ratio.

Purpose of filters in circuits

-to passthe signals of wanted frequencies

-to reject the signals of unwanted frequencies

8/10/2019 E&I _Unit5 ppt

36/79

TYPES OF FILTERS

Any physical form

-mechanical,electrical,pneumatic,hydraulic

The most commonly used types are electricaltype.They are

Passive filters

Active filters

Passive filters

Filters use only passive circuit elements(R,L,C)Active filters

PE+Op-amp

8/10/2019 E&I _Unit5 ppt

37/79

8/10/2019 E&I _Unit5 ppt

38/79

8/10/2019 E&I _Unit5 ppt

39/79

ATTENUATORS

Attenuators are devices used in bringing down

the voltage conducted between the circuits

that are connected to its input and output

Types of attenuators

resistance attenuators

symmetrical attenuators( T shape)

L type attenuators

pi type attenuators

8/10/2019 E&I _Unit5 ppt

40/79

8/10/2019 E&I _Unit5 ppt

41/79

MODULATION

Introduction

modulation means modify or to change

The transduced signal is super-imposedon a

high frequency waveform (called carrier) .Sothat the original signal can be recovered anddisplayed.

The high frequency waveform is then said tobe modulated by the transduced signal. Theprocess of recovery is called demodulation

8/10/2019 E&I _Unit5 ppt

42/79

MODULATION

The process of changing some characteristics(amplitude,frequency,phase) of a carrier wavein accordance with the intensity of the signal

is known as modulation.The resultant wave iscalled modulated wave.

Types of modulation

AM-Radio broadcasting

FM-television sound signal

PM

8/10/2019 E&I _Unit5 ppt

43/79

AM

When the amplitude of high frequency carrier

waveis changed in accordance with the

intensity of the signal, it is called amplitude

modulation

Three signals

audio signal

carrier wave

modulated signal

8/10/2019 E&I _Unit5 ppt

44/79

8/10/2019 E&I _Unit5 ppt

45/79

Modulation Factor

determines strength and quality of thetransmitted signal

It is the ratio of the amplitude change of carrierwave to normal carrier wave.

DEMODULATION

The process required for recovery of originalsignal from modulated waveform is calleddemodulation

It involves rectificationof the modulated signalfollowed by elimination of carrier frequency.

8/10/2019 E&I _Unit5 ppt

46/79

8/10/2019 E&I _Unit5 ppt

47/79

Analog to Digital Converters

8/10/2019 E&I _Unit5 ppt

48/79

AnalogSignals

Analog signalsdirectly measurable quantitiesin terms of some other quantity

Examples:

Thermometermercury height rises astemperature rises

Car SpeedometerNeedle moves farther

right as you accelerate StereoVolume increases as you turn the

knob.

8/10/2019 E&I _Unit5 ppt

49/79

Digital Signals

Digital Signalshave only two states. For digital

computers, we refer to binary states, 0 and 1.

1 can be on, 0 can be off.

Examples:

Light switch can be either on or off

Door to a room is either open or closed

8/10/2019 E&I _Unit5 ppt

50/79

Examples of A/D Applications

Microphones - take your voice varying pressure waves in the airand convert them into varying electrical signals

Strain Gages - determines the amount of strain (change indimensions) when a stress is applied

Thermocoupletemperature measuring device convertsthermal energy to electric energy

Voltmeters

Digital Multimeters

J t h t d

8/10/2019 E&I _Unit5 ppt

51/79

Just what does an

A/D converter DO?

Converts analog signals into binary words

8/10/2019 E&I _Unit5 ppt

52/79

AnalogDigital Conversion

2-Step Process:

Quantizing - breaking down analog value is a

set of finite states

Encoding - assigning a digital word or number

to each state and matching it to the input

signal

8/10/2019 E&I _Unit5 ppt

53/79

Step 1: Quantizing

Example:

You have 0-10V signals.

Separate them into a set

of discrete states with1.25V increments. (How

did we get 1.25V? See

next slide)

Output

States

Discrete Voltage

Ranges (V)

0 0.00-1.25

1 1.25-2.50

2 2.50-3.75

3 3.75-5.00

4 5.00-6.25

5 6.25-7.50

6 7.50-8.75

7 8.75-10.0

8/10/2019 E&I _Unit5 ppt

54/79

Quantizing

The number of possible states that the converter canoutput is:

N=2n

where n is the number of bits in the AD converter

Example: For a 3 bit A/D converter, N=23=8.

Analog quantization size:Q=(Vmax-Vmin)/N = (10V0V)/8 = 1.25V

8/10/2019 E&I _Unit5 ppt

55/79

Encoding

Here we assign the

digital value (binary

number) to each state

for the computer toread.

Output

States

Output Binary Equivalent

0 000

1 001

2 010

3 011

4 100

5 101

6 110

7 111

8/10/2019 E&I _Unit5 ppt

56/79

Accuracy of A/D Conversion

There are two ways to best improve accuracy of A/Dconversion:

increasing the resolution which improves theaccuracy in measuring the amplitude of the analogsignal.

increasing the sampling rate which increases themaximum frequency that can be measured.

8/10/2019 E&I _Unit5 ppt

57/79

Resolution

Resolution (number of discrete values the converter can

produce) = Analog Quantization size (Q)

(Q) = Vrange / 2^n, where Vrange is the range of analog

voltages which can be represented

limited by signal-to-noise ratio (should be around 6dB)

In our previous example: Q = 1.25V, this is a high resolution. A

lower resolution would be if we used a 2-bit converter, then

the resolution would be 10/2^2 = 2.50V.

8/10/2019 E&I _Unit5 ppt

58/79

Sampling Rate

Frequency at which ADC evaluates analog signal. As we see in

the second picture, evaluating the signal more often more

accurately depicts the ADC signal.

8/10/2019 E&I _Unit5 ppt

59/79

Aliasing

Occurs when the input signal is changing much fasterthan the sample rate.

For example, a 2 kHz sine wave being sampled at 1.5kHz would be reconstructed as a 500 Hz (the aliasedsignal) sine wave.

Nyquist Rule: Use a sampling frequency at least twice as high as

the maximum frequency in the signal to avoidaliasing.

8/10/2019 E&I _Unit5 ppt

60/79

Overall Better Accuracy

Increasing both the sampling rate and the resolution you

can obtain better accuracy in your AD signals.

8/10/2019 E&I _Unit5 ppt

61/79

A/D Converter Types

Converters

Flash ADC

Delta-Sigma ADC

Dual Slope (integrating) ADC

Successive Approximation ADC

8/10/2019 E&I _Unit5 ppt

62/79

Flash ADC

Consists of a series of comparators, each one

comparing the input signal to a unique

reference voltage.

The comparator outputs connect to the inputs

of a priority encoder circuit, which produces a

binary output

8/10/2019 E&I _Unit5 ppt

63/79

Flash ADC Circuit

8/10/2019 E&I _Unit5 ppt

64/79

How Flash Works

As the analog input voltage exceeds the

reference voltage at each comparator, the

comparator outputs will sequentially saturate

to a high state. The priority encoder generates a binary

number based on the highest-order active

input, ignoring all other active inputs.

Flash

8/10/2019 E&I _Unit5 ppt

65/79

Flash

Advantages

Simplest in terms ofoperational theory

Most efficient in terms ofspeed, very fast

limited only in terms ofcomparator and gatepropagation delays

Disadvantages

Lower resolution

Expensive

For each additionaloutput bit, the number ofcomparators is doubled

i.e. for 8 bits, 256

comparators needed

8/10/2019 E&I _Unit5 ppt

66/79

Delta Sigma ADC

Over sampled input signal

goes to the integrator

Output of integration is

compared to GND Integraters to produce a

serial bit stream

Output is serial bit stream

with # of 1s proportionalto Vin

Outputs of Delta Sigma

8/10/2019 E&I _Unit5 ppt

67/79

p g

Sigma Delta

8/10/2019 E&I _Unit5 ppt

68/79

Sigma-Delta

Advantages

High resolution

No precision external

components needed

Disadvantages

Slow due to oversampling

8/10/2019 E&I _Unit5 ppt

69/79

Dual Slope Converter

The sampled signal charges a capacitor for a fixedamount of time

By integrating over time, noise integrates out of the

conversion Then the ADC discharges the capacitor at a fixed rate

with the counter counts the ADCs output bits. A longerdischarge time results in a higher count

t

VintFIX tmeas

8/10/2019 E&I _Unit5 ppt

70/79

Dual Slope Converter

Advantages

Input signal is averaged

Greater noise immunity

than other ADC types High accuracy

Disadvantages

Slow

High precision external

components required toachieve accuracy

8/10/2019 E&I _Unit5 ppt

71/79

Successive Approximation ADC

A Successive Approximation Register (SAR) isadded to the circuit

Instead of counting up in binary sequence,

this register counts by trying all values of bitsstarting with the MSB and finishing at the LSB.

The register monitors the comparators outputto see if the binary count is greater or lessthan the analog signal input and adjusts thebits accordingly

8/10/2019 E&I _Unit5 ppt

72/79

Successive Approximation ADC Circuit

8/10/2019 E&I _Unit5 ppt

73/79

Output

8/10/2019 E&I _Unit5 ppt

74/79

Successive Approximation

Advantages

Capable of high speed andreliable

Medium accuracy compared

to other ADC types Good tradeoff between speed

and cost

Capable of outputting the

binary number in serial (onebit at a time) format.

Disadvantages

Higher resolution successiveapproximation ADCs will beslower

Speed limited to ~5Msps

ADC Types Comparison

8/10/2019 E&I _Unit5 ppt

75/79

ADC Resolution Comparison

0 5 10 15 20 25

Sigma-Delta

Successive Approx

Flash

Dual Slope

Resolution (Bits)

Type Speed (relative) Cost (relative)

Dual Slope Slow Med

Flash Very Fast High

Successive Appox Medium Fast Low

Sigma-Delta Slow Low

ADC Types Comparison

8/10/2019 E&I _Unit5 ppt

76/79

8/10/2019 E&I _Unit5 ppt

77/79

8/10/2019 E&I _Unit5 ppt

78/79

8/10/2019 E&I _Unit5 ppt

79/79

e&i _unit5 ppt

Documents