op227e Analog Devices, Inc., op227e Datasheet - Page 12

• Manufacturer Part Number: op227e
• Description: Dual, Low Noise, Low Offset Instrumentation Operational Amplifier
• Manufacturer: Analog Devices, Inc.

OP227
The differential gain A
input V
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 V
CMRR of the instrumentation amplifier, which is defined as
20 log10A
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 A
equation approximates output:
V
This can be simplified by defining A
gain and A0 as the differential open-loop gain. Then:
The high open-loop gain of each amplifier within the OP227
(700,000 minimum at 25HC in R
accuracy even at high values of A
loop gain on CMR can be approximated by:
O
1
CM
+
V
1
d
R
O
R
is rejected.
/A
as A
0
1
CM
Ê
Á
Ë
A
1
O1
, is simply equal to the CMRR of the OP227.
1
1
+
O
, and op amp A
1
R
R
[ ]
V
+
1
1
0
O
A
d
A
1
is 1 + 2R1/R0 and the common-mode
O
CM
1
O
CM
2
ˆ
˜
¯
Ê
Á
Ë
=
would be:
Ê
Á
Á
Ë
A V
A V
A V
d
d
d
L
d
2
. The effect of finite open-
d
K 2 kW) assures good gain
as A
CM
d
O
+
+
as the nominal open-loop
2 1
R
¥ 10
O2
2 1
R
R
0
R
, then the following
0
–
D
Ê
Á
Ë
A
5
A
O
A
O
1
2
O
1
V
–
CM
A
1
O
ˆ
˜
¯
2
ˆ
˜
¯
V
CM
ˆ
˜
˜
¯
–12–
If A
finite gain of the input op amps would be approximately 140 dB.
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 2 R2/2 R2. Since the output
stage gain is unity, CMRR will then be R2/2 R2. It is common
practice to maximize overall CMRR for the total instrumenta-
tion amplifier circuit.
V
V
O
CM
CM
/A
– 1/2V
+ 1/2V
O
were 6% and A
d
d
R0
CMRR
A1
A2
O
R1
R1
OP227
OP227
were 600,000, then the CMRR due to
1/2
1/2
V2
V1
D
A
R2
R2
A
O
O
V
2
O
= (1 +
R2
A3
2R1
R2
R0
OP27
) Vd
REV. A
V
O