OP227 datasheet(12/16 Pages) AD

12 / 16 page

REV. A

OP227

–12–

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:

VA V

dCM

[] =¥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:

ÁÁ

˜˜

–

This can be simplified by defining AO as the nominal open-loop

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 25

∞C 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

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

VCM + 1/2Vd

1/2

OP227

OP27

VO = (1 + 2R1 ) Vd

1/2

OP227

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.

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