User`s manual

Differential Inputs
Differential inputs measure the voltage between two distinct input signals. Within a certain range (referred
to as the common mode range), the measurement is almost independent of signal source to
PCI-DAS1602/12 ground variations. A differential input is also much more immune to EMI than a single-
ended one. Most EMI noise induced in one lead is also induced in the other, the input measures only the
difference between the two leads, and the EMI common to both is ignored. This effect is a major reason for
twisted pair wire because the twisting ensures that both wires are subject to virtually identical external
influence. The diagram below shows a typical differential input configuration.
8
+
-
Input
Amp
To A /D
LL GND
CH IN
~
1
2
Vs
Vs + Vg2 - Vg1
Any voltage differential between grounds
g1 and g2 shows up as an error signal
at the input amplifier
Single-ended input with Common Mode Voltage
g
g
+
-
Input
Amp
To A /D
Differential Input
I/O
Connector
LL GND
CH High
CH Low
+
-
Input
Amp
To A /D
Differential
Input
LL GND
CH High
CH Low
~
Vs
Vs
Vcm
Common Mode Voltage (Vcm) is ignored
by differential input configuration. However,
note that Vcm + Vs must rem ain within
the am plifier’s common mode range of ±10V
Vcm = Vg2 - Vg1
gg1 2