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Prof. Dr. Qiuting HuangIntegrated Systems Laboratory
Electronic Circuits
5. Instrumentation Amplifier
Precise amplification of weak sensor signals in the presence of distortion and noise, typically at microvolt level
High input impedance Internal feedback to achieve desired functionality Selectable gain, typically ðºðº = 10, 100, 1000 Quality of InAmps determined by Common mode rejection ratio (CMRR) Voltage offset Noise
Application example: ECG
ETH 2Integrated Systems Laboratory
Characteristics of Instrumentation Amplifiers
and sensor front-end
Voltage difference amplifier Amplifies the difference signal with a precise gain Suppresses distortion (common mode signals) Presents the same impedance at both input terminals
Differential gain must be equal for both input branches
Set ð ð 1 = ð ð 3, ð ð 2 = ð ð 4 to equally load both input branches
ETH 3Integrated Systems Laboratory
Basic Instrumentation Amplifier
with ðði+ = 0,
with ððiâ = 0,
ððo = âð ð 4ð ð 3
ððiâ
ððo =ð ð 2
ð ð 1 + ð ð 2ï¿œð ð 3 + ð ð 4ð ð 3
ðði+
ððo =ð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
ðði+ âð ð 4ð ð 3
ððiâ =ð ð 2ð ð 1
1 + ð ð 4ð ð 3
1 + ð ð 2ð ð 1
ðði+ âð ð 4ð ð 3
ððiâ
superposition principle:
ððo = ðºðº ï¿œ ðði+ â ðºðº ï¿œ ððiâ ðºðº =ð ð 2ð ð 1
=ð ð 4ð ð 3
Try to obtain ideally high input impedance by input buffering
ETH 4Integrated Systems Laboratory
Buffered Instrumentation Amplifier
ððid = ðði+ â ððiâ ððicm =ðði+ + ððiâ
2
ððiâ = Vicm âVid2
ðði+ = Vicm +Vid2
ððo = ð ð 2 ð ð 3+ð ð 4ð ð 3(ð ð 1+ð ð 2)
(Vicm+ Vid2
) â ð ð 4ð ð 3
(Vicm â Vid2
)
ððo = ððicmð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
âð ð 4ð ð 3
+Vid2
ð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
+ð ð 4ð ð 3
ðŽðŽcm =ððoððicm
=ð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
âð ð 4ð ð 3
ðŽðŽd =ððoððid
=12
ð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
+ð ð 4ð ð 3
ð¶ð¶ð¶ð¶ð ð ð ð =ðŽðŽdðŽðŽcm
=12ð ð 3 + ð ð 4 ð ð 2 + ð ð 1 + ð ð 2 ð ð 4
ð ð 2ð ð 3 â ð ð 4ð ð 1
ETH 5Integrated Systems Laboratory
InAmp - Common Mode Rejection and Precision
Resistor manufacturing tolerance ð ð 1 = ð ð 1 + ðð
Example:
ð ð 2 = ð ð 3 = ð ð 4 = ð ð
ð¶ð¶ð¶ð¶ð ð ð ð =12ð ð 2
ð ð 2(3 + 1 + ðð)(1 â 1 â ðð)
â2ðð
ðð = 1% â ð¶ð¶ð¶ð¶ð ð ð ð = 200 = 46 dB
to achieve ð¶ð¶ð¶ð¶ð ð ð ð ⥠100ðððð: ðð †0.002%
ð¶ð¶ð¶ð¶ð ð ð ð =12ð ð 3 + ð ð 4 ð ð 2 + ð ð 1 + ð ð 2 ð ð 4
ð ð 2ð ð 3 â ð ð 4ð ð 1
ð ð 2ð ð 1
=ð ð 4ð ð 3
leads to infinite CMRR, but can not be realized with real-world resistorsdue to manufacturing deviations
ðð ⪠1
ETH 6Integrated Systems Laboratory
Instrumentation Amplifier â Input Stage Gain
Differential gain of input stage:
ðŽðŽB =ððBdððid
=ððB+ â ððBâðði+ â ððiâ
= 1 +ð ð 5 + ð ð 6ð ð 7
ð ð 6 = ð ð 5 â ðŽðŽB = 1 +2ð ð 5ð ð 7
CMRR increased by factor of ðŽðŽB
ð¶ð¶ð¶ð¶ð ð ð ð =ðŽðŽðŽdðŽðŽcm
= ðŽðŽBðŽðŽdðŽðŽcm
ðŽðŽðŽd =ððoððid
=ðŽðŽB2
ð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
+ð ð 4ð ð 3
ð ð 1 = ð ð 3, ð ð 2= ð ð 4
ðŽðŽðŽd =ð ð 2ð ð 1ðŽðŽB
Total differential gain:
ððid = ðði+ â ððiâ
ððBd = ððB+ â ððBâ
Differential input
Differential output of
the input stage
Common mode gain of input stage:
ðði+ = ððiâ = ððCM â ideal op-amp ððd = 0â no current flowing through ð ð 7, northrough ð ð 5 and ð ð 6 â ððB+ = ððCM = ððBâ
ðŽðŽcm,B =ððB+ + ððBâðði+ + ððiâ
= 1
Total common mode gain:
ðŽðŽcm = ðŽðŽcm,Bð ð 2 ð ð 3 + ð ð 4ð ð 3(ð ð 1+ð ð 2)
âð ð 4ð ð 3
ð ð 7 placed externally or programmable internally Called gain resistor ð ð G Gain accuracy depends on tolerance on ð ð G Selectable-gain pins in some models
ETH 7Integrated Systems Laboratory
Instrumentation Amplifier - Architecture
Offset has its origin in small, production-related deviations of integrated devices from their nominal values
ETH 8Integrated Systems Laboratory
Voltage Offsets in Differential Amplifiers
For symmetry ðŒðŒD1 = ðŒðŒD2 â ððTH1 =ððTH2 is necessary
But: ððTH1 = ððTH,ððTH2 = ððTH + ððϵ ðððð caused by production deviation An offset voltage ððos = ðððð can be
added externally in order to restore symmetry
Then ððTH1 = ððTH = ððTH2 + ððϵ â ððos
Small signal compared to a relatively large offset High gain desirable
ETH 9Integrated Systems Laboratory
Trouble with Voltage Offsets in Amplifiers
ðði ð¡ð¡small input signal, 10ððV
ðði + ððos
ð¡ð¡Relatively large voltage offset, 10mV
The voltage offset needs to be compensated, otherwise, the output voltage ððo reaches the saturation level even for small values of ðði
At saturation level, the amplified signal is distorted
ððo = ðºðº(ðði + ððos)
ð¡ð¡Amplification by gain ðºðº = 100
max. ððoof amplifier
distorted signal
ETH 10Integrated Systems Laboratory
Inverting Amplifier with DC Offset Voltage
If ðŽðŽo â â then ððd â 0, ððdâ² = ððos
KCL: ððoâððosð ð 2
= âððððâððosð ð 1
â ððo = âð ð 2ð ð 1ðði + 1 + ð ð 2
ð ð 1ððos
Offset term can become problematic if |ðði| â |ððos|
sensor applications
offset term
Offsets ððos1, ððos2 can be seen as part of the input signals ðði+ and ððiâ ððos1, ððos2 see the same gain as input signal
ððoð ð = ððos2 â ððos1 1 + ð ð 5+ð ð 6ð ð 7
Offsets can be modeled as uncorrelated random variables. This means that ððos2 â ððos1 is also a random variable, with twice the variance
ETH 11Integrated Systems Laboratory
Input Stage Voltage Offset
Offset trimming (ânullingâ) by external components in op-amp
If such compensation methods are not appropriate, amplifiers based on signal chopping can be used chopper amplifiers
ETH 12Integrated Systems Laboratory
Voltage Offset Compensation in Integrated Amplifiers
741 op-amp
(as used in thelaboratory)
ETH 13Integrated Systems Laboratory
Chopping Principle
ððs
ððsððcððc + ððsððc â ððs
ðði+
ððiâðð
ð¹ð¹
ð¹ð¹ðði+ =ðŽðŽ2ð¿ð¿(ðð â ððs)
ðði+ = ðŽðŽcos 2ððð¡ð¡ððs Fourier transform,
negative frequencies
neglected for simplicity
ðði+ = ðŽðŽcos 2ððð¡ð¡ððs + ððos
ð¹ð¹ðði+ =ðŽðŽ2ð¿ð¿ ðð â ððs + ððosð¿ð¿(ðð)
ðði+ = cos 2ððð¡ð¡ððc ðŽðŽcos 2ððð¡ð¡ððs + ððos
ð¹ð¹ðði+ =ðŽðŽ4ð¿ð¿ ðð â (ððc + ððs) +
ðŽðŽ4ð¿ð¿ ðð â (ððc â ððs) + ððosð¿ð¿(ðð)
ð¹ð¹c =12ð¿ð¿ ðð â ððcð ð c = cos 2ððð¡ð¡ððc
ððo+
ððoâ
ððs ðð
ð¹ð¹
ðð
ð¹ð¹
ðºðº
ððosðði+ ððo++ â
ðºðº
ððosð ð c
ððo++ âðði+
ðºðº
ðði+ =ðŽðŽ2
[cos 2ððð¡ð¡(ððsâððc) + cos 2ððð¡ð¡(ððs+ððc) ] + ððos
The output of the chopper amplifier is finally low pass filtered in order to suppress out of band frequencies
ETH 14Integrated Systems Laboratory
Chopping Principle
ð¹ð¹o+ =ðºðºðŽðŽ4ð¿ð¿ ðð â ððs
ð ð c
ððo++ âðði+
ððos
ðºðº LPF
ð ð cðððŽi+
ððo+ =ðºðºðŽðŽ2
cos 2ððð¡ð¡ððs + ððos ðºðºcos 2ððððc
+ ðºðºðºðº4
cos(2ððð¡ð¡(ððs + 2ððc)) + ðºðºðºðº4
cos(2ððð¡ð¡(ððs â 2ððc))
ð¹ð¹o+ =ðºðºðŽðŽ4ð¿ð¿ ðð â ððs +
ðºðºððos2
ð¿ð¿ ðð â ððc +ðºðºðŽðŽ8ð¿ð¿(ðð â ððs â 2ððc) +
ðºðºðŽðŽ8ð¿ð¿(ðð + ððs â 2ððc)
ð ð cððo+
+ âðði+
ððos
ðºðº
ð ð c
ððððs ððc
ð¹ð¹
2ððc
Summary:1. The chopper amplifier modulates
the input signal ðði+â² to a high frequency ððc
2. The amplifier adds offset and amplifies the modulated (chopped) input signal
3. In the second modulation step the amplified signal is demodulated to ððs, while the offset is modulated at ððc
4. The demodulated amplified signal is low-pass filtered
ETH 15Integrated Systems Laboratory
Chopping Amplifiers
ðð
ð¹ð¹
Note: The chopping principle can also be applied to suppress other low-frequency interferences, such as flicker noise
ððc
ðð
ð¹ð¹
ððc
ðð
ð¹ð¹
ððs ððc
ðð
ð¹ð¹
ððs