AD620

REV. E

–15–

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

AD620

– INPUT

TO POWER

SUPPLY

GROUND

REFERENCE

+ INPUT

LOAD

Figure 42a. Ground Returns for Bias Currents with

Transformer Coupled Inputs

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

be a dc path from each input to ground as shown in Figure 42.

Refer to the Instrumentation Amplifier Application Guide (free

from Analog Devices) for more information regarding in amp

applications.

– INPUT

+ INPUT

LOAD

TO POWER

SUPPLY

GROUND

REFERENCE

AD620

Figure 42b. Ground Returns for Bias Currents with

Thermocouple Inputs

100k

AD620

– INPUT

+ INPUT

LOAD

TO POWER

SUPPLY

GROUND

REFERENCE

100k

Figure 42c. Ground Returns for Bias Currents with AC Coupled Inputs