AD620

Rev. G | Page 18 of 20

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 47, Figure 48, and Figure 49. Refer to A Designer’s Guide

to Instrumentation Amplifiers (free from Analog Devices) for

more information regarding in-amp applications.

AD620

– INPUT

TO POWER

SUPPLY

GROUND

REFERENCE

+ INPUT

LOAD

Figure 47. Ground Returns for Bias Currents with Transformer-Coupled Inputs

– INPUT

+ INPUT

LOAD

TO POWER

SUPPLY

GROUND

REFERENCE

AD620

Figure 48. Ground Returns for Bias Currents with Thermocouple Inputs

100k

Ω

AD620

– INPUT

+ INPUT

LOAD

TO POWER

SUPPLY

GROUND

REFERENCE

100k

Ω

Figure 49. Ground Returns for Bias Currents with AC-Coupled Inputs