AD621

REV. B

–12–

AD548

TO

REF

C

R

TO

–

+

–

+

10

AD621

DAC

0 TO

10V

–

+

0.5V

INPUT A:

10V CM

10V–

+

–

+

INPUT B:

1V

OFFSET

(1.25V + 1V)

G = 10

–

+

10

AD621

TOTAL GAIN = 100

10k

10k

OPTIONAL

USE THIS IN PLACE OF THE DAC FOR ZERO SUPPRESSION FUNCTION.

Figure 6. Suppressing a Large Common-Mode or Offset Voltage in Order to Measure a Small Differential Signal

±15 V)

The AD621, as well as many other monolithic instrumentation

amplifiers, is based on the “three op amp” in amp circuit (Fig-

ure 7) amplifier. Since the input amplifiers (A1 and A2) have a

common-mode gain of unity and a differential gain equal to the

set gain of the overall in amp, the voltages V1 and V2 are defined

by the equations

× V

× V

The common-mode voltage will drive the outputs of amplifiers

A1 and A2 to the differential-signal voltage, multiplied by the

gain, spreads them apart. For a 10 V common-mode 0.1 V

differential input, V1 would be at 10.5 V and V2 at 9.5 V.

A1

A3

–

+

10k

10k

10k

10k

A2

–

+

20k

20k

+

–

V1

V2

INPUT AMPLIFIER

DIFFERENTIAL GAIN = 10

COMMON MODE GAIN = 1

OUTPUT AMPLIFIER

DIFFERENTIAL GAIN = 1

COMMON MODE GAIN = 1/1000

4.44k

Figure 7. Typical Three Op Amp Instrumentation

Amplifier, Differential Gain = 10

The AD621’s input amplifiers can provide output voltage within

2.5 V of the supplies. To avoid saturation of the input amplifier

the input voltage must therefore obey the equations:

× V

≤ (Upper Supply – 2.5 V)

× V

≥ (Lower Supply + 2.5 V)

Figure 8 shows the trade-off between common-mode and

differential-mode input for

±15 V supplies and G = 10.

By cascading with use of the optional AD621, the circuit of

Figure 6 will provide

±1 V of zero suppression at gains of 10

of

±4 ppm/°C and ±8 ppm/°C, respectively. Therefore, depend-

ing on the magnitude of the differential input signal, either

1.2

0.2

010

2

48

1.0

0.8

0.6

0.4

6

0

12

±15 V, G = 10), for Reference Pin at Ground