Advantech PCM-3718 Series User Manual page 35

If the signal source has one side connected to a local ground, the signal
source ground and PCM-3718's 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 voltages form 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 PCM-3718's ground directly. In some cases you may also
need a wire connection between the PCM-3718's ground and the signal
source ground for better grounding. The following two diagrams show
correct and incorrect connections for a differential input with local
ground.
Correct Connection
Incorrect Connection
PCM-3718 Series User Manual
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
+
Vs
-
_
+
Vcm
+
Vs
-
_
+
Vcm
HIGH
+
+
V in
-
LO W
-
GND
HIGH
+
+
V in
-
LO W
-
GND
26
V in=Vs
V in=Vs+Vcm