AD629

Rev. B | Page 9 of 16

THEORY OF OPERATION

The AD629 is a unity gain, differential-to-single-ended

amplifier (diff amp) that can reject extremely high common-

mode signals (in excess of 270 V with 15 V supplies). It consists

of an operational amplifier (op amp) and a resistor network.

To achieve high common-mode voltage range, an internal

resistor divider (Pin 3 or Pin 5) attenuates the noninverting

signal by a factor of 20. Other internal resistors (Pin 1, Pin 2,

and the feedback resistor) restore the gain to provide a differential

gain of unity. The complete transfer function equals

Laser wafer trimming provides resistor matching so that

common-mode signals are rejected while differential input

signals are amplified.

To reduce output drift, the op amp uses super beta transistors

in its input stage. The input offset current and its associated

temperature coefficient contribute no appreciable output

voltage offset or drift, which has the added benefit of reducing

voltage noise because the corner where 1/f noise becomes

dominant is below 5 Hz. To reduce the dependence of gain

accuracy on the op amp, the open-loop voltage gain of the op

amp exceeds 20 million, and the PSRR exceeds 140 dB.

1

2

3

4

8

7

6

5

21.1kΩ

380kΩ

380kΩ

380kΩ

20kΩ

REF(–)

–IN

+IN

NC

OUTPUT

REF(+)

AD629

NC = NO CONNECT

Figure 29. Functional Block Diagram