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4-14
Cabling and Wiring
Avoiding Ground Loops with Differential Inputs
Frequently, the signal-source ground and the DAS-1800HC Series board
ground are not at the same voltage level because of the distances between
equipment wiring and the building wiring. This difference is referred to as
a
common-mode voltage
(V
cm
) because it is normally common to both
sides of a differential input (it appears between each side and ground).
Since a differential input responds only to the difference in the signals at
its high and low inputs, its common-mode voltages cancel out and leave
only the signal. However, if your input connections contain a ground
loop, your input could see the sum of the signal-source and
common-mode voltages. Figure 4-12 shows the proper way to connect a
differential input while Figure 4-13 illustrates the effect of a ground loop.
Figure 4-12. A Differential Input Configuration that Avoids a Ground Loop
Figure 4-13. Differential Input Configuration with a Ground Loop
Channel
n High
Signal
Source
+
-
AGND
DAS-1800HC Series Board
V
c m
= V
g 1
- V
g 2
E
s
Channel
n Low
Signal Source
Ground V
g 1
E
s
V
c m
V
g 2
Do not join Low
to AGND at the
computer
R
wire
Channel
n High
Signal
Source
+
-
AGND
DAS-1800HC Series Board
V
c m
= V
g 1
- V
g 2
E
s
Channel
n Low
Signal Source
Ground V
g 1
E
s
+ V
c m
V
c m
V
g 2
NOTE
This diagram is included
only to illustrate an
incorrectly wired input; do
not use this configuration.
R
wire
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