AD8223 Analog Devices, AD8223 Datasheet - Page 15

• Manufacturer Part Number: AD8223
• Description: Single Supply, Rail-to-Rail, Low Cost Instrumentation Amplifier
• Manufacturer: Analog Devices

Preliminary Technical Data
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.
Rev. PrA | Page 15 of 20
CAPACITIVELY COUPLED
C
C
THERMOCOUPLE
TRANSFORMER
INCORRECT
AD8223
+V
AD8223
–V
AD8223
+V
–V
+V
–V
S
S
S
S
S
S
REF
Figure 33. Creating an I
REF
REF
f
HIGH-PASS
=
2πRC
1
10MΩ
CAPACITIVELY COUPLED
BIAS
C
C
Path
THERMOCOUPLE
TRANSFORMER
R
R
CORRECT
AD8223
AD8223
AD8223
+V
–V
+V
–V
+V
–V
S
S
S
S
S
S
AD8223
REF
REF
REF