Measurement Computing CIO-DAS16/M1 - Manual (Page 31)

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6. ELECTRONICS AND INTERFACING
This short, simple introduction to the electronics most often needed by I/O board users covers a few key
concepts.
6.1 COMMON MODE
Differential inputs have a common mode range (CMR or Vcm). Common mode range is the voltage range
over which differences in the low side of the signal and A/D input ground have no impact on the A/D's
measurement of the signal voltage. A differential input can reject differences between signal ground and
PC ground (Figure 6-1).
Figure 6-1. Differential Input Theory
The maximum difference which can be rejected is the CMR (Figure 6-2).
Figure 6-2. CMR Defined
HI
LO
Vs Vs+Vc
Vc
Vcm = Vg1 -
G
S
I
G
N
A
L
g1 g2
+
-
DIFFERENTIAL INPUT
LL GND
CH 0 HI
CH 0 LO
+
-
DIFFERENTIAL
AMP
TO A/D
GND
V+
V-
19
18
17
16
15
37
36
35
34
PC GROUND
MAX CMR
MAX CMR + SIGNAL
CUMULATIVE SIGNAL RANGE
10V
11.5V
EXAMPLE 1
EXAMPLE 2
PC GROUND
MAX CMR + SIGNAL
CSR 11.5V
COMMON MODE 4V
MAX SIGNAL 5V
UNUSED CMR 3.5V