Advantech PCI-1710HGU, User Manual

Page: 31 / 52

25 Chapter 3
Differential Channel Connections
Differential input connections use two signal wires per channel. The card
measures only the voltage difference between these two wires, the HI
wire and the LOW wire. If the signal source has no connection to ground,
it is called a "floating" source. A connection must exist between LOW
and ground to define a common reference point for floating signal
sources. To measure a floating sources connect the input channels as
shown below:
If the signal source has one side connected to a local ground, the signal
source ground and the PCI-1710U/HGU/UL 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-1710U/HGU/UL ground directly. In some cases you
may also need a wire connection between the PCI-1710U/HGU/UL
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:
+
-
HIGH
LOW
A.GND
+
Vin
-
+
Vs
-