Measurement Computing CIO-DAS1601/12 Manual page 14

Before moving on to the discussion of grounding and isolation, it is important to explain the concepts of
common mode, and common mode range (CM Range). Common mode voltage is depicted in the diagram
above as Vcm. Though differential inputs measure the voltage between two signals, without (almost)
respect to the either signal's voltages relative to ground, there is a limit to how far away from ground
either signal can go. Though the CIO-DAS1600 has differential inputs, it will not measure the difference
between 100V and 101V as 1 Volt (in fact the 100V would destroy the board!). This limitation or
common mode range is depicted graphically in Figure 5-3. The CIO-DAS1600 common mode range is
+/- 10 Volts. Even in differential mode, no input signal can be measured if it is more than 10V from the
board's low level ground (LLGND).
+13V
+12V
+11V
+10V
+9V
+8V
+7V
+6V
+5V
+4V
+3V
+2V
+1V
-1V
-2V
-3V
-4V
-5V
-6V
-7V
-8V
-9V
-10V
~
Vs
Vcm = Vg2 - Vg1
g1
Com m on M ode Voltage (Vcm ) is ignored
by differential input configuration. However,
note that Vcm + Vs must rem ain w ithin
the am plifier's comm on m ode range of ±10V
Figure 5-2b. Differential Input Theory
W ith Vcm = +5VD C,
+Vs m ust be less than +5V, or the com m on m ode range will be exceeded (>+10V)
Figure 5-3. Common Mode Range
CH High
+
Inp ut
Vs
A m p
CH Low -
Vcm
LL G ND
D ifferential
g 2
Input
Gray area represents com m on m ode ran
Both V+ and V- m ust alw ays rem ain w ithi
the com m on m ode range relative to LL G
Vcm
10
To A/D