User manual
PCL-818 Series User Manual 32
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 PCL-818HD/HG/L 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 PCL-818HD/HG/L ground directly. In some cases you may
also need a wire connection between the PCL-818HD/HG/L 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:
+
-
HIGH
LOW
A.GND
+
Vin
-
+
Vs
-