AD620

REV. E

–14–

COMMON-MODE REJECTION

Instrumentation amplifiers like the AD620 offer high CMR,

which is a measure of the change in output voltage when both

inputs are changed by equal amounts. These specifications are

usually given for a full-range input voltage change and a speci-

fied source imbalance.

For optimal CMR the reference terminal should be tied to a low

impedance point, and differences in capacitance and resistance

should be kept to a minimum between the two inputs. In many

applications shielded cables are used to minimize noise, and for

best CMR over frequency the shield should be properly driven.

Figures 39 and 40 show active data guards that are configured

to improve ac common-mode rejections by “bootstrapping” the

capacitances of input cable shields, thus minimizing the capaci-

tance mismatch between the inputs.

REFERENCE

AD620

100

100

– INPUT

+ INPUT

AD648

Figure 39. Differential Shield Driver

100

– INPUT

+ INPUT

REFERENCE

AD620

2

2

AD548

Figure 40. Common-Mode Shield Driver

GROUNDING

Since the AD620 output voltage is developed with respect to the

potential on the reference terminal, it can solve many grounding

problems by simply tying the REF pin to the appropriate “local

ground.”

In order to isolate low level analog signals from a noisy digital

environment, many data-acquisition components have separate

analog and digital ground pins (Figure 41). It would be conve-

nient to use a single ground line; however, current through

ground wires and PC runs of the circuit card can cause hun-

dreds of millivolts of error. Therefore, separate ground returns

should be provided to minimize the current flow from the sensi-

tive points to the system ground. These ground returns must be

tied together at some point, usually best at the ADC package as

shown.

DIGITAL P.S.

+5V

C

ANALOG P.S.

+15V C –15V

AD574A

DIGITAL

DATA

OUTPUT

+

1 F

AD620

0.1 F

AD585

S/H

ADC

0.1 F

1 F 1 F

Figure 41. Basic Grounding Practice