Preliminary Technical Data

AD8223

Rev.

PrA | Page 15 of 20

GROUND RETURNS FOR INPUT BIAS CURRENTS

Input bias currents are those dc currents that must flow to bias

the input transistors of an amplifier. These are usually transistor

base currents. When amplifying floating input sources such as

transformers or ac-coupled sources, there must be a direct dc

path into each input so that the bias current can flow. Figure 33

shows how a bias current path can be provided for the cases of

transformer coupling, capacitive ac-coupling and for a

thermocouple application.

In dc-coupled resistive bridge applications, providing this path

is generally not necessary as the bias current simply flows from

the bridge supply through the bridge and into the amplifier.

However, if the impedances that the two inputs see are large and

differ by a large amount (>10 kΩ), the offset current of the

input stage causes dc errors proportional with the input offset

voltage of the amplifier.

THERMOCOUPLE

REF

AD8223

CAPACITIVELY COUPLED

REF

C

C

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TRANSFORMER

REF

AD8223

INCORRECT

CAPACITIVELY COUPLED

REF

C

R

R

C

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1

2

πRC

THERMOCOUPLE

REF

10M

Ω

AD8223

TRANSFORMER

REF

AD8223

CORRECT