Datasheet

AD621
REV. B
–15–
GROUNDING
Since the AD621 output voltage is developed with respect to the
potential on the reference terminal, it can solve many ground-
ing problems by simply tying the REF pin to the appropriate
“local ground.”
In order to isolate low level analog signals from a noisy digital
environment, many data-acquisition components have separate
analog and digital ground pins (Figure 15). It would be conve-
nient to use a single ground line; however, current through
ground wires and PC runs of the circuit card can cause hundreds
of millivolts of error. Therefore, separate ground returns should
be provided to minimize the current flow from the sensitive
points to the system ground. These ground returns must be tied
together at some point, usually best at the ADC package as shown.
DIGITAL P.S.
+5V
C
ANALOG P.S.
+15V
C
–15V
AD574A
+
AD621
AD585
S/H
ADC
5
9
11
15
6
2
4
7
1
11
7
6
4
0.1
␮
F
1
␮
F
3
DIGITAL
DATA
OUTPUT
0.1
␮
F
1
␮
F1
␮
F
Figure 15. Basic Grounding Practice
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 Figures 16a
through 16c. Refer to the Instrumentation Amplifier Application
Guide (free from Analog Devices) for more information regard-
ing in amp applications.
+V
S
AD621
LOAD
–V
S
REFERENCE
TO POWER SUPPLY GROUND
+INPUT
–INPUT
V
OUT
Figure 16a. Ground Returns for Bias Currents when Using
Transformer Input Coupling
+V
S
AD621
LOAD
–V
S
REFERENCE
TO POWER SUPPLY GROUND
+INPUT
–INPUT
V
OUT
Figure 16b. Ground Returns for Bias Currents when Using
a Thermocouple Input
100k⍀ 100k⍀
–INPUT
AD621
+INPUT
+V
S
–V
S
V
OUT
LOAD
REFERENCE
TO POWER SUPPLY GROUND
Figure 16c. Ground Returns for Bias Currents when Using
AC Input Coupling