Datasheet

AD8223
Rev. 0 | Page 16 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 35
shows how a bias current path can be provided for the cases of
transformer coupling, capacitive ac-coupling, and a thermo-
couple application.
In dc-coupled resistive bridge applications, providing this path
is generally not necessary because 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 k), the offset current of the input
stage causes dc errors proportional to the input offset voltage of
the amplifier.
THERMOCOUPLE
+V
S
REF
–V
S
AD8223
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8223
TRANSFORMER
+V
S
REF
–V
S
AD8223
INCORREC
T
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8223
1
f
HIGH-PASS
=
2πRC
THERMOCOUPLE
+V
S
REF
–V
S
10MΩ
AD8223
TRANSFORMER
+V
S
REF
–V
S
AD8223
CORRECT
0
6925-042
Figure 35. Creating an I
BIAS
Path