circuits for sensors
DESCRIPTION
Circuits for sensors. Ideal OP Amps Basic OP Amp Circuit Blocks Analog Computation Nonlinear OP Amp Applications OP Amp Considerations Guarding Passive Filters Active Filters VCO(Voltage Controlled Oscillator). Function of Amplifiers. Amplifiers provides GAIN - PowerPoint PPT PresentationTRANSCRIPT
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Circuits for sensors
Ideal OP AmpsBasic OP Amp Circuit BlocksAnalog ComputationNonlinear OP Amp ApplicationsOP Amp ConsiderationsGuardingPassive FiltersActive FiltersVCO(Voltage Controlled Oscillator)
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Chap 0 2
Function of Amplifiers
Amplifiers provides GAIN Filtering, Signal processing, Correction for
Nonlinearities
TemperaturePressure
FlowMotion
….
SensorSignal
ConditioningCircuitry
DigitalComputer
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Chap 0 3
Ideal OP Amps
Transfer Function = Output / Input Voltage Amp TF (Gain):
Usually Av 1 OP Amp is preferred
Easy to use in circuit designed compared to discrete Transistor circuits
ov
i
vA
v
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Chap 0 4
Ideal OP Amps (Cont.)
Assumptions Open loop Gain = Infinity Input Impedance Rd = Infinity Output Impedance Ro = 0 Bandwidth = Infinity
Infinite Frequency Response vo=0 when v1 = v2
No Offset Voltage
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Chap 0 5
Ideal OP Amps (Cont.)
Note v0 = A(v2 – v1)
If v0 = , A = (Typically 100,000)• Then v2 – v1 = 0 v2 = v1
Since v2 = v1 and Rd = • We can neglect the current in Rd
Rule 1 When the OP Amp is in linear range the two inputs are
at the same voltage Rule 2
No Current flows into either terminal of the OP Amp
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Chap 0 6
Basic OP Amp Circuit Blocks
Inverting Amplifier Noninverting Amplifier Unity-Gain Amplifier Differential Amplifier Instrumental Amplifier The Electrocardiogram Amplifier
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Chap 0 7
Inverting Amplifier
From Rule 1 v- = v+ = 0
From Rule 2 & KCL ii + if = 0 ii = -if From Ohm’s law
ii = vi / Ri , , if = vo / Rf
vi / Ri = - vo / Rf vo / vi = -Rf / Ri
Inverting Amp Gain -Rf / Ri
Inverting Amp with Gain = - Rf / Ri
Virtual Ground
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Chap 08
Inverting Amplifier (Cont.)
Linear Range By Power Supply Voltage
Input Impedance Low (Ri) Increasing Ri Decreasing Gain
Increasing Gain by increasing Rf• But there is practical limit
Saturation
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Chap 0 9
Noninverting Amplifiers
Noninverting Amp Gain = (Rf + Ri) / Rf
By Rule 2 Vo = If (Rf + Ri) Vi = If Ri Vo = Vi (Rf + Ri)/Ri
Gain: Vo/Vi = 1 + Rf / Ri Gain 1, Always Input Impedance
Very Large (Infinite)
By Rule 1Vi
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Chap 0 10
Unity-Gain Amplifier
Verify that the Gain of Unity-Gain Amp is 1
Vo = Vi Applications
Buffer amplifier Isolate one circuit
from the loading effects of a following stage
Impedance converter Data conversion
System (ADC or DAC) where constant impedance or high impedance is required
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Chap 0 11
Differential Amplifiers
Combination of Inverting and Noninverting Amp Can reject 60Hz interference Electrocardiogram amplifier
Differential Noninverting
Instrumentation
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Chap 0 12
Differential Amplifiers (Cont.)
Gain of Differential Amp By Rule 2
V5 = I2 * R2 V2 = I2 * R1 + V5 = V5 *
R1 /R2 + V5 V5 = R2 * V2 / (R1 + R2)
By Rule 1 V1 = R1 * I1 + V5 V5 = R2 * I1 + V6V6 = (V2 – V1) * R2 / R1
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Chap 0 13
Differential Amplifiers (Cont.)
CMV (Common Mode Voltage) If V1 = V2, then V6 = 0
CMG (Common Mode Gain) = 0 DG(Differential voltage Gain)
If V1 V2, then V6 = (V2-V1)*(R2/R1) In practice, CMG 0 CMRR (Common Mode Rejection Ratio)
Measure of the ability to reject CMV CMRR = DG / CMG
The Higher CMRR, the better quality Typically, 100 ~ 10,000 60Hz noise common to V1 and V2 can be rejected
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Chap 0 14
Instrumentation Amplifiers
One OP Amp Differential Amplifier Input Impedance is not so High
Good for Low impedance source• Strain gage Bridge
Bad for High impedance source Instrumentation Amplifier
Differential Amp with High Input Impedance and Low Output Impedance
Two Noninvering Amp + One Differential Amp
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Chap 0 15
Instrumentation Amplifiers (Cont.)
Instrumentation Amp = Noninverting Amp + Differential Amp
We have: DG = (V1-V2) / (V3-V4) = (2*R4 + R3) / R3 V6 = (V3-V4)*DG*R2 / R1
First Stage CMRR CMRR = DG / CMG =
DG Overall CMG = 0
High CMRR High Input Impedance Gain is adjustable by changing
R3
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Chap 0 16
The Electrocardiogram Amplifier
< 0.2 VGain = 40
Maximize CMRRHigh Pass Filter
>0.05Hz
Low Pass Filter< 100Hz
Gain = 32
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Chap 0 17
Analog Computation Circuit
Digital Signal Processing is preferred Flexibility Easy to Change Elimination of hardware
However, Analog Signal Processing Is preferred when DSP consumes too much
time
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Chap 0 18
Inverter and Scale Changer
Inverting Amp with Gain = - Rf / Ri Inverter Rf / Ri = 1
Inverter and Scale Changer Proper choice of Rf / Ri
Application Use of inverter to scale the output of DAC
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Chap 0 19
Adders (Summing Amplifiers)
Adder Inverter with Several inputs
Vo = -Rf(V1/R1 + V2/R2 +… + Vn/Rn) If = I1 + I2 + In I1 = V1/R1, … Vo = -If * Rf
Rf determines overall Gain Ri determines weighting factor and input
impedance
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Chap 0 20
Integrator
Drawbacks Vo will reach saturation voltage, if Vi is left connected
indefinitely Integrator operates as an open-loop amplifier for
DC inputs
1
0 0
1 t
i icv v dt vRC
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Chap 0 21
Practical Integrator
Reset S1 Closed, S0
Open Inverter C is initialized to Vr
Integrate S1 Open, S0
Closed Hold
S1 Open, S0 Open Keeps Vo constant
Read and Process
Controlled ByRelay or Solid State Switch orAnalog Switch
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Chap 0 22
Differentiators
Drawbacks Instability at High frequencies
Practical Differentiator To Stable0
idvv RCdt
0 0i
RRA C
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Chap 0 23
Comparators
Compare Two Inputs Vi > Vr
Vo = -Vs Vi < Vr
Vo = Vs
Drawbacks If Vi = Vr + small noise
Rapid fluctuation between Vs
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Chap 0 24
Comparators with Hysteresis
Positive Feedback Hysteresis loop Can remove the effect of Small Noise
Reduce Fluctuation1
1 2
( )S rr r r
V V RV V V
R R
1
1 2
( )S rr r r
V V RV V V
R R
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Chap 0 25
Rectifiers
Precision Half Wave Rectifier
Precision Full Wave Rectifier
Limiters
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Chap 0 26
OP Amp Considerations
Effects of Nonlinear characteristics Compensation
Undesirable Oscillation at High frequency• Add external Capacitance according to Spec sheet
GBW (Gain Bandwidth Product) Gain Bandwidth = Constant (Typically 1MHz)
• For Noninverting Amp: Bandwidth = GBW / Gain Input Offset Voltage
Practical OP Amp• Zero input Does NOT give Zero output
Input Offset Voltage• Applied input voltage to obtain Zero output
Nulling the offset Voltage• Adding External Resister according to Spec sheet
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Chap 0 27
OP Amp Considerations (Cont.)
Input Bias Current Practical OP amp
• Current flowing into the terminal is NOT Zero• To keep the input Tr of OP amp turned on• Causes errors proportional to feedback network R
To minimize errors• feedback R should be low (<10K)
Slew Rate Maximal rate of change of amplifier output voltage
• Ex: Slew rate of 741 = 0.5 V / s– Time to output change from –5V to 5V = 20 s
To Minimize slew rate problem• Use OP amp with smaller external compensating C
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Chap 0 28
OP Amp Considerations (Cont.)
Power Supply Usually 15V
• Linear Range 13V Reducing power supply voltage
• Results reduced linear range• Device does not work < 4V
Different OP Amps Bipolar Op Amps
• Good input offset stability• Moderate input bias current and Input resistances
FET• Very Low input bias current and Very High Input resistances• Poor Input offset voltage stability
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Chap 0 29
Guarding
Elimination of Surface Leakage Currents Elimination of Common Mode Signals
Very important in practice But skip in this course
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Chap 0 30
Passive Filters
Passive Circuits Contains only passive elements
Registers, Capacitors and Inductors Examples
Bridge Circuit Voltage Divider Filters
Filters Eliminate unwanted signal from the loop Low Pass, High Pass, Band Pass, Notch, …
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Chap 0 31
Passive first-order Low pass Filter
Pass desired Audio signal and reject undesired RF
Order of Filter Number of C and L
Plot Magnitude and Phase plot (Bode plot)
Meaning of C
1 ,1
o
i
VRC
V j
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Chap 0 32
Passive first-order High pass Filter
Pass desired High frequency signal and reject undesired low frequency signal
Plot Magnitude and Phase plot (Bode plot)
Meaning of C
,1
o
i
V j RCV j
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Chap 0 33
Passive second-order Low pass Filter To increase the
attenuation of transfer function
Order of Filter Number of C and L
Meaning of Quality factor
2
1( / ) (2 / ) 1
1 ,2
o
i c c
c
VV j j
R CLC L
1
1
1
1, 3
2cQ dB BW
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Chap 0 34
Passive second-order High pass Filter To increase the
attenuation of transfer function
Order of Filter Number of C and L
2
2( / ) (2 / ) 1
1 ,2
o
i c c
c
VV j j
R CLC L
1 1
1
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Chap 0 35
Active First-order Low Pass Filter
Inverting Amp + Feedback Capacitor Identical frequency response with Passive filter Very Low Output impedance
Negligible Loading Effect
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Chap 0 36
Active First-order High Pass Filter
Inverting Amp + Input Capacitor
Identical frequency response with Passive filter
Very Low Output impedance Negligible Loading
Effect
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Chap 0 37
Active High-order Filters
Low Pass Filters
High Pass Filters
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Chap 0 38
Bandpass and Band-reject Filters
Butterworth Filters Maximally Flat Magnitude response in pass band High Attenuation Rate
Chebyshev Filters Maximum Attenuation Rate Ripple in pass band
Bessel Filters Maximally flat time delay in response to step input Attenuation Rate is very gradual
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Chap 0 39
Filter Design Table
C when 0 = R0 = 1
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Chap 0 40
Filter Design Example Low pass five-pole Butterworth filter with a corner
frequency of 200Hz and input resistance of 50K Economic Solution = 3rd order + 2nd order Desired R and C ?
C1A = (0 R0 C0 ) / ( R) = 1x1x1.753 / 2x200x50K = 27.9 nF C2A = 21.6 nF, C3A = 6.7 nF, C1B = 51.5 nF, C2B = 4.9 nF
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Chap 0 41
VCO(Voltage Controlled Oscillator)
VCO = Voltage to Frequency(V/F) Converter VCO converts an input voltage to a series of output digital pulses whose frequency is proportional to the input voltage
Applications ADC Digital Transmission Telemetry Digital Voltmeter
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Chap 0 42
VCO (Cont.)
Module form Better linearity, Lower Gain drift, Higher full-scale
frequencies than IC Monolithic IC form
Less expensive, Small size Lower drift, Better flexibility of frequency range
Examples LM331
Low cost VCO from National Semiconductor Maximum nonlinearity 0.01% over 1 ~ 100KHz
CD4046B PLL contains VCO Maximum nonlinearity 1.0% over 1 ~ 400MHz
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Chap 0 43
PLL(Phase Locked Loop) VCO is commonly used
in PLL Applications
Communications Radar Time and frequency
control Instrumentation system
Control loop Goal
Minimize z(t) s(t) = r(t)
Change r(t) until z(t)=0 s(t) can be
obtained By reading r(t)
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Chap 0 44
VCO Interfacing
Output of VCO Digital pulses whose frequency is proportional to input
voltage
# of pulse / Duration Duration
Controlled by Sampling Gate # of Pulse
Counted in Counter
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Chap 0 45
Divider Circuit
Convert Register Variations to Voltage Variations Output Voltage Vo = {R2 / (R1 + R2)} Vs
R1
R2
Vs
Vo
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Chap 0 46
Divider Circuit: Drawbacks Vo is not linearly changed
Ex: Vs = 5V, R1 = 1K, R2 = 0 ~ 1K(Sensor)
Output Impedance(R1 || R2) is not so High Large Power Consumption
R2
Vo
1K500
Vs/2Vs/3
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Chap 0 47
Divider Circuit: Example
R1 = 10K, R2 = (4K ~ 12K), Vs = 5V Maximum Vo = 5 {12 / (10+12)} = 2.73V Minimum Vo = 5 { 4 / (10 + 4)} = 1.43V Maximum Z = (10K || 12K) = 120/22 K Minimum Z = (10K || 4K) = 40/14 K Maximum Power = (Vo)2/R2 = (2.73)2/12K = 0.62mW Minimum Power = (1.43)2/4K = 0.51mW