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31 Chapter 3
If the signal source has one side connected to a local ground, the signal
source ground and the PCI-1718HDU/HGU ground will not be at exactly
the same voltage, as they are connected through the ground return of the
equipment and building wiring. The difference between the ground volt-
ages forms a common-mode voltage.
To avoid the ground loop noise effect caused by common-mode voltages,
connect the signal ground to the LOW input. Do not connect the LOW
input to the PCI-1718HDU/HGU ground directly. In some cases you may
also need a wire connection between the PCI-1718HDU/HGU ground
and the signal source ground for better grounding. The following two dia-
grams show correct and incorrect connections for a differential input with
local ground:
Correct Connection
Incorrect Connection
+
-
HIGH
LOW
+
Vin
-
+
Vs
-
Vcm GND
Vin=Vs
+-
+-
+
Vs
-
Vcm
HIGH
LOW
GND
+
Vin
-Vin=Vs+Vcm
+
-