Prof. Dr. Qiuting Huang Integrated Systems Laboratory Electronic Circuits 5. Instrumentation Amplifier
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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 πΊπΊ = 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 π π 1 = π π 3, π π 2 = π π 4 to equally load both input branches
ETH 3Integrated Systems Laboratory
Basic Instrumentation Amplifier
with ππi+ = 0,
with ππiβ = 0,
ππo = βπ π 4π π 3
ππiβ
ππo =π π 2
π π 1 + π π 2οΏ½π π 3 + π π 4π π 3
ππi+
ππo =π π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
ππi+ βπ π 4π π 3
ππiβ =π π 2π π 1
1 + π π 4π π 3
1 + π π 2π π 1
ππi+ βπ π 4π π 3
ππiβ
superposition principle:
ππo = πΊπΊ οΏ½ ππi+ β πΊπΊ οΏ½ ππiβ πΊπΊ =π π 2π π 1
=π π 4π π 3
Try to obtain ideally high input impedance by input buffering
ETH 4Integrated Systems Laboratory
Buffered Instrumentation Amplifier
ππid = ππi+ β ππiβ ππicm =ππi+ + ππiβ
2
ππiβ = Vicm βVid2
ππi+ = Vicm +Vid2
ππo = π π 2 π π 3+π π 4π π 3(π π 1+π π 2)
(Vicm+ Vid2
) β π π 4π π 3
(Vicm β Vid2
)
ππo = ππicmπ π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
βπ π 4π π 3
+Vid2
π π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
+π π 4π π 3
π΄π΄cm =ππoππicm
=π π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
βπ π 4π π 3
π΄π΄d =ππoππid
=12
π π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
+π π 4π π 3
πΆπΆπΆπΆπ π π π =π΄π΄dπ΄π΄cm
=12π π 3 + π π 4 π π 2 + π π 1 + π π 2 π π 4
π π 2π π 3 β π π 4π π 1
ETH 5Integrated Systems Laboratory
InAmp - Common Mode Rejection and Precision
Resistor manufacturing tolerance π π 1 = π π 1 + ππ
Example:
π π 2 = π π 3 = π π 4 = π π
πΆπΆπΆπΆπ π π π =12π π 2
π π 2(3 + 1 + ππ)(1 β 1 β ππ)
β2ππ
ππ = 1% β πΆπΆπΆπΆπ π π π = 200 = 46 dB
to achieve πΆπΆπΆπΆπ π π π β₯ 100ππππ: ππ β€ 0.002%
πΆπΆπΆπΆπ π π π =12π π 3 + π π 4 π π 2 + π π 1 + π π 2 π π 4
π π 2π π 3 β π π 4π π 1
π π 2π π 1
=π π 4π π 3
leads to infinite CMRR, but can not be realized with real-world resistorsdue to manufacturing deviations
ππ βͺ 1
ETH 6Integrated Systems Laboratory
Instrumentation Amplifier β Input Stage Gain
Differential gain of input stage:
π΄π΄B =ππBdππid
=ππB+ β ππBβππi+ β ππiβ
= 1 +π π 5 + π π 6π π 7
π π 6 = π π 5 β π΄π΄B = 1 +2π π 5π π 7
CMRR increased by factor of π΄π΄B
πΆπΆπΆπΆπ π π π =π΄π΄π΄dπ΄π΄cm
= π΄π΄Bπ΄π΄dπ΄π΄cm
π΄π΄π΄d =ππoππid
=π΄π΄B2
π π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
+π π 4π π 3
π π 1 = π π 3, π π 2= π π 4
π΄π΄π΄d =π π 2π π 1π΄π΄B
Total differential gain:
ππid = ππi+ β ππiβ
ππBd = ππB+ β ππBβ
Differential input
Differential output of
the input stage
Common mode gain of input stage:
ππi+ = ππiβ = ππCM β ideal op-amp ππd = 0β no current flowing through π π 7, northrough π π 5 and π π 6 β ππB+ = ππCM = ππBβ
π΄π΄cm,B =ππB+ + ππBβππi+ + ππiβ
= 1
Total common mode gain:
π΄π΄cm = π΄π΄cm,Bπ π 2 π π 3 + π π 4π π 3(π π 1+π π 2)
βπ π 4π π 3
π π 7 placed externally or programmable internally Called gain resistor π π G Gain accuracy depends on tolerance on π π 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 πΌπΌD1 = πΌπΌD2 β ππTH1 =ππTH2 is necessary
But: ππTH1 = ππTH,ππTH2 = ππTH + ππΟ΅ ππππ caused by production deviation An offset voltage ππos = ππππ can be
added externally in order to restore symmetry
Then ππTH1 = ππTH = ππTH2 + ππΟ΅ β ππos
Small signal compared to a relatively large offset High gain desirable
ETH 9Integrated Systems Laboratory
Trouble with Voltage Offsets in Amplifiers
ππi π‘π‘small input signal, 10ππV
ππi + ππos
π‘π‘Relatively large voltage offset, 10mV
The voltage offset needs to be compensated, otherwise, the output voltage ππo reaches the saturation level even for small values of ππi
At saturation level, the amplified signal is distorted
ππo = πΊπΊ(ππi + ππos)
π‘π‘Amplification by gain πΊπΊ = 100
max. ππoof amplifier
distorted signal
ETH 10Integrated Systems Laboratory
Inverting Amplifier with DC Offset Voltage
If π΄π΄o β β then ππd β 0, ππdβ² = ππos
KCL: ππoβππosπ π 2
= βππππβππosπ π 1
β ππo = βπ π 2π π 1ππi + 1 + π π 2
π π 1ππos
Offset term can become problematic if |ππi| β |ππos|
sensor applications
offset term
Offsets ππos1, ππos2 can be seen as part of the input signals ππi+ and ππiβ ππos1, ππos2 see the same gain as input signal
ππoπ π = ππos2 β ππos1 1 + π π 5+π π 6π π 7
Offsets can be modeled as uncorrelated random variables. This means that ππos2 β ππos1 is also a random variable, with twice the variance
ETH 11Integrated Systems Laboratory
Input Stage Voltage Offset
Offset trimming (βnullingβ) 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
ππs
ππsππcππc + ππsππc β ππs
ππi+
ππiβππ
πΉπΉ
πΉπΉππi+ =π΄π΄2πΏπΏ(ππ β ππs)
ππi+ = π΄π΄cos 2πππ‘π‘ππs Fourier transform,
negative frequencies
neglected for simplicity
ππi+ = π΄π΄cos 2πππ‘π‘ππs + ππos
πΉπΉππi+ =π΄π΄2πΏπΏ ππ β ππs + ππosπΏπΏ(ππ)
ππi+ = cos 2πππ‘π‘ππc π΄π΄cos 2πππ‘π‘ππs + ππos
πΉπΉππi+ =π΄π΄4πΏπΏ ππ β (ππc + ππs) +
π΄π΄4πΏπΏ ππ β (ππc β ππs) + ππosπΏπΏ(ππ)
πΉπΉc =12πΏπΏ ππ β ππcπ π c = cos 2πππ‘π‘ππc
ππo+
ππoβ
ππs ππ
πΉπΉ
ππ
πΉπΉ
πΊπΊ
ππosππi+ ππo++ β
πΊπΊ
ππosπ π c
ππo++ βππi+
πΊπΊ
ππi+ =π΄π΄2
[cos 2πππ‘π‘(ππsβππc) + cos 2πππ‘π‘(ππs+ππc) ] + ππos
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
πΉπΉo+ =πΊπΊπ΄π΄4πΏπΏ ππ β ππs
π π c
ππo++ βππi+
ππos
πΊπΊ LPF
π π cπππ΄i+
ππo+ =πΊπΊπ΄π΄2
cos 2πππ‘π‘ππs + ππos πΊπΊcos 2ππππc
+ πΊπΊπΊπΊ4
cos(2πππ‘π‘(ππs + 2ππc)) + πΊπΊπΊπΊ4
cos(2πππ‘π‘(ππs β 2ππc))
πΉπΉo+ =πΊπΊπ΄π΄4πΏπΏ ππ β ππs +
πΊπΊππos2
πΏπΏ ππ β ππc +πΊπΊπ΄π΄8πΏπΏ(ππ β ππs β 2ππc) +
πΊπΊπ΄π΄8πΏπΏ(ππ + ππs β 2ππc)
π π cππo+
+ βππi+
ππos
πΊπΊ
π π c
ππππs ππc
πΉπΉ
2ππc
Summary:1. The chopper amplifier modulates
the input signal ππi+β² to a high frequency ππc
2. The amplifier adds offset and amplifies the modulated (chopped) input signal
3. In the second modulation step the amplified signal is demodulated to ππs, while the offset is modulated at ππc
4. The demodulated amplified signal is low-pass filtered
ETH 15Integrated Systems Laboratory
Chopping Amplifiers
ππ
πΉπΉ
Note: The chopping principle can also be applied to suppress other low-frequency interferences, such as flicker noise
ππc
ππ
πΉπΉ
ππc
ππ
πΉπΉ
ππs ππc
ππ
πΉπΉ
ππs
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