AD628

Rev. C | Page 14 of 20

THEORY OF OPERATION

The AD628 is a high common-mode voltage difference

amplifier, combined with a user configurable output amplifier

(see Figure 28 and Figure 29). Differential mode voltages in

excess of 120 V are accurately scaled by a precision 11:1 voltage

divider at the input. A reference voltage input is available to the

difference amplifier is the same as the voltage applied to the

reference pin. If the uncommitted amplifier is configured for

gain, connecting Pin 3 to one end of the external gain resistor

establishes the output common-mode voltage at Pin 5 (OUT).

The output of the difference amplifier is internally connected

to a 10 kΩ resistor trimmed to better than ±0.1% absolute

accuracy. The resistor is connected to the noninverting input of

A capacitor may be connected to implement a low-pass filter, a

resistor may be connected to further reduce the output voltage,

or a clamp circuit may be connected to limit the output swing.

The uncommitted amplifier is a high open-loop gain, low offset,

low drift op amp, with its noninverting input connected to the

internal 10 kΩ resistor. Both inputs are accessible to the user.

Careful layout design has resulted in exceptional common-

mode rejection at higher frequencies. The inputs are connected

to Pin 1 (+IN) and Pin 8 (−IN), which are adjacent to the power

ground, input impedance balance and, therefore, common-

mode rejection, are preserved at higher frequencies.

+IN

–IN

+IN

–IN

A2

A1

+IN

–IN

100k

Ω

100k

Ω

10k

Ω

10k

Ω

10k

Ω

OUT

G = +0.1

Figure 28. Simplified Schematic

+IN

–IN

A2

+IN

–IN

100k

Ω

100k

Ω

10k

Ω

10k

Ω

REFERENCE

VOLTAGE

10k

Ω

AD628

OUT

G = +0.1

+IN

–IN

A1

Figure 29. Circuit Connections