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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-DAS1200 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.
9
+
-
Input
Amp
To A/D
LL GND
CH IN
~
1
2
Vs
Vs + V
g
2 - V
g
1
Any volta
g
e differential between
g
rounds
g
1 and
g
2 shows up as an error si
g
nal
at the input amplifier
Sin
g
le-ended input with Common Mode Volta
g
e
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 Volta
g
e (Vcm) is i
g
nored
b
y
differential input confi
g
uration. However,
note that Vcm + Vs must remain within
the amplifier’s common mode ran
g
e of ±10V
Vcm = V
g
2 - V
g
1
g
g
12