AD622

REV. C

–10–

GROUNDING

Since the AD622 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.” The REF pin should however be tied to a low imped-

ance point for optimal CMR.

The use of ground planes is recommended to minimize the

impedance of ground returns (and hence the size of dc errors).

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

environment, many data-acquisition components have separate

analog and digital ground returns (Figure 16). All ground pins

from mixed signal components such as analog to digital converters

should be returned through the “high quality” analog ground

plane. Maximum isolation between analog and digital is

achieved by connecting the ground planes back at the supplies.

The digital return currents from the ADC which flow in the

analog ground plane will in general have a negligible effect on

noise performance.

AD622

12

AGND

DGND

AD7892-2

+5V

–5V

C

GND

C

+5V

0.1 F

0.1 F

0.1 F

DIGITAL P.S.

PROCESSOR

ANALOG P.S.

Figure 16. Basic Grounding Practice

GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are those currents necessary to bias the input

transistors of an amplifier. There must be a direct return path

for these currents; therefore when amplifying “floating” input

sources such as transformers, or ac-coupled sources, there must

be a dc path from each input to ground as shown in Figure 17.

Refer to the

Instrumentation Amplifier Application Guide (free

from Analog Devices) for more information regarding in amp

applications.

1

2

3

8

6

4

5

REFERENCE

7

AD622

–INPUT

+INPUT

LOAD

TO POWER

SUPPLY

GROUND

Figure 17a. Ground Returns for Bias Currents with

Transformer Coupled Inputs

1

2

3

8

6

4

5

REFERENCE

7

AD622

–INPUT

+INPUT

LOAD

TO POWER

SUPPLY

GROUND

Figure 17b. Ground Returns for Bias Currents with

Thermocouple Inputs

REFERENCE

AD622

–INPUT

+INPUT

LOAD

TO POWER

SUPPLY

GROUND

100k

100k

Figure 17c. Ground Returns for Bias Currents with

AC Coupled Inputs