AD621 datasheet(15/16 Pages) AD

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AD621

REV. B

–15–

GROUNDING

Since the AD621 output voltage is developed with respect to the

potential on the reference terminal, it can solve many ground-

ing 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 15). 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 hundreds

of millivolts of error. Therefore, separate ground returns should

be provided to minimize the current flow from the sensitive

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

ANALOG P.S.

+15V C –15V

AD574A

AD621

AD585

S/H

ADC

0.1 F

1 F

DIGITAL

DATA

OUTPUT

0.1 F

1 F1 F

Figure 15. 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 Figures 16a

through 16c. Refer to the Instrumentation Amplifier Application

Guide (free from Analog Devices) for more information regard-

ing in amp applications.

+VS

AD621

LOAD

–VS

REFERENCE

TO POWER SUPPLY GROUND

+INPUT

–INPUT

VOUT

Figure 16a. Ground Returns for Bias Currents when Using

Transformer Input Coupling

+VS

AD621

LOAD

–VS

REFERENCE

TO POWER SUPPLY GROUND

+INPUT

–INPUT

VOUT

Figure 16b. Ground Returns for Bias Currents when Using

a Thermocouple Input

100k

–INPUT

AD621

+INPUT

+VS

–VS

VOUT

LOAD

REFERENCE

TO POWER SUPPLY GROUND

Figure 16c. Ground Returns for Bias Currents when Using

AC Input Coupling

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