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5962-8688701CA View Datasheet(PDF) - Analog Devices

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5962-8688701CA Datasheet PDF : 16 Pages
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OP227
The differential gain Ad is 1 + 2R1/R0 and the common-mode
input VCM is rejected.
While output error due to input offsets and noise are easily
determined, the effects of finite gain and common-mode rejec-
tion are more subtle. CMR of the complete instrumentation
amplifier is directly proportioned to the match in CMR of the
input op amps. This match varies from 97 dB to 110 dB mini-
mum for the OP227. Using 100 dB, then the output response to
a common-mode input VCM would be:
[ ] VO CM = Ad VCM ¥ 10–5
CMRR of the instrumentation amplifier, which is defined as
20 log10Ad/ACM, is simply equal to the CMRR of the OP227.
While this CMRR is already high, overall CMRR of the
complete amplifier can be raised by trimming the output stage
resistor network.
Finite gain of the input op amps causes a scale factor error and a
small degradation in CMR. Designating the open-loop gain of
op amp A1 as AO1, and op amp A2 as AO2, then the following
equation approximates output:
V
O
ف
1+
R1 Ê
R0
Á
Ë
1
1
A
O1
+
1ˆ
A
O2
˜
¯
Ê
ËÁÁ
A
d
V
d
+
2R1 Ê
R0
Á
Ë
1
A
O1
1ˆ ˆ
A
O2
˜
¯
V
CM
¯˜˜
This can be simplified by defining AO as the nominal open-loop
gain and A0 as the differential open-loop gain. Then:
VO
ف
1+
1
R1
R0
1
AO
Ê
Á Ad
Ë
Vd
+
2R1
R0
DAO
AO 2
ˆ
VCM ˜
¯
The high open-loop gain of each amplifier within the OP227
(700,000 minimum at 25C in RL 2 kW) assures good gain
accuracy even at high values of Ad. The effect of finite open-
loop gain on CMR can be approximated by:
CMRR
ف
AO 2
DAO
If AO/AO were 6% and AO were 600,000, then the CMRR due to
finite gain of the input op amps would be approximately 140 dB.
VCM – 1/2Vd
R1
1/2
OP227
VO
=
(1
+
2R1
R0
)
Vd
R2
R2
A1
V1
VCM + 1/2Vd
R0
R1
OP27
A3
VO
1/2
OP227
A2
R2
V2
R2
Figure 4. Three Op Amp Instrumentation Amplifier Using
OP227 and OP27
The unity-gain output stage contributes negligible error to the
overall amplifier. However, matching of the four resistor R2
network is critical to achieving high CMR. Consider a worst-
case situation where each R2 resistor had an error of ± R2. If
the resistor ratio is high on one side and low on the other, then
the common-mode gain will be 2R2/2R2. Since the output
stage gain is unity, CMRR will then be R2/2R2. It is common
practice to maximize overall CMRR for the total instrumenta-
tion amplifier circuit.
–12–
REV. A
 

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