AD627ARZ-R7 datasheet(14/24 Pages) AD

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AD627

Rev. D | Page 14 of 24

THEORY OF OPERATION

The AD627 is a true instrumentation amplifier, built using two

feedback loops. Its general properties are similar to those of the

classic two-op-amp instrumentation amplifier configuration but

internally the details are somewhat different. The AD627 uses a

modified current feedback scheme, which, coupled with interstage

feedforward frequency compensation, results in a much better

common-mode rejection ratio (CMRR) at frequencies above

dc (notably the line frequency of 50 Hz to 60 Hz) than might

otherwise be expected of a low power instrumentation amplifier.

In Figure 35, A1 completes a feedback loop that, in conjunction

with V1 and R5, forces a constant collector current in Q1. Assume

that the gain-setting resistor (RG) is not present. Resistors R2

and R1 complete the loop and force the output of A1 to be equal

to the voltage on the inverting terminal with a gain of nearly

1.25. A2 completes a nearly identical feedback loop that forces

a current in Q2 that is nearly identical to that in Q1; A2 also

provides the output voltage. When both loops are balanced, the

gain from the noninverting terminal to VOUT is equal to 5,

whereas the gain from the output of A1 to VOUT is equal to −4.

The inverting terminal gain of A1 (1.25) times the gain of A2

(−4) makes the gain from the inverting and noninverting

terminals equal.

The differential mode gain is equal to 1 + R4/R3, nominally 5,

and is factory trimmed to 0.01% final accuracy. Adding an

external gain setting resistor (RG) increases the gain by an

amount equal to (R4 + R1)/RG. The output voltage of the

AD627 is given by

VOUT = [VIN(+) – VIN(−)] × (5 + 200 kΩ/RG) + VREF

(1)

Laser trims are performed on R1 through R4 to ensure that

their values are as close as possible to the absolute values in the

gain equation. This ensures low gain error and high common-

mode rejection at all practical gains.

EXTERNAL GAIN RESISTOR

REF

–IN

+IN

+VS

–VS

+VS

100kΩ

25kΩ

200kΩ

25kΩ

2kΩ

0.1V

OUTPUT

100kΩ

Figure 35. Simplified Schematic

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