User`s manual

Figure 5-2b. Differential Input Theory
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 PCIM-DAS1602/16 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 PCIM-DAS1602/16 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).
Figure 5-3. Common Mode Range
11
+
-
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
by 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
+1V
+2V
+3V
11V
+4V
-10V
+5V
-9V
+6V
-8V
+7V
-7V
+8V
-6V
+9V
-5V
+10V
-4V
+11V
-3V
+12V
-2V
+13V
-1V
Gray area represents common mode ran
Both V+ and V- must always remain with
i
the common mode range relative to LL G
Vcm
With Vcm= +5VDC,
+Vs must be less than +5V, or the common mode range will be exceeded (>+10V)