AD620
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
A Designer’s Guide
to Instrumentation Amplifiers
(free from Analog Devices) for
more information regarding in-amp applications.
– INPUT
+V
S
R
G
AD620
LOAD
V
OUT
+ INPUT
–V
S
REFERENCE
+V
S
– INPUT
TO POWER
SUPPLY
GROUND
Figure 48. Ground Returns for Bias Currents with Thermocouple Inputs
R
G
AD620
LOAD
V
OUT
+V
S
– INPUT
+ INPUT
–V
S
REFERENCE
R
G
AD620
LOAD
V
OUT
TO POWER
SUPPLY
GROUND
00775-0-049
Figure 47. Ground Returns for Bias Currents with Transformer-Coupled Inputs
100k
Ω
+ INPUT
100k
Ω
–V
S
REFERENCE
00775-0-050
00775-0-051
TO POWER
SUPPLY
GROUND
Figure 49. Ground Returns for Bias Currents with AC-Coupled Inputs
Rev. G | Page 18 of 20
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