Measurement Computing PCM-DAS16x/16 - Manual (Page 22)

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). Com-
mon mode voltage is depicted in the diagram below 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 PCM-DAS16x/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
the following diagram. The PCM-DAS16x/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).
6.1.4 System Grounds and Isolation
There are three scenarios possible when connecting your signal source to your PCM-
DAS16x/16 board.
1. The PCM-DAS16x/16 and the signal source may have the same (or common)
ground. This signal source may be connected directly to the PCM-DAS16x/16.
2. The PCM-DAS16x/16 and the signal source may have an offset voltage between
their grounds (AC and/or DC). This offset it commonly referred to a common
18
+ 1V
-13V
+ 2V
-12V
+ 3V
-11V
+ 4V
-10V
+ 5V
-9V
+ 6V
-8V
+ 7V
-7V
+ 8V
-6V
+ 9V
-5V
+1 0V
-4V
+11V
-3V
+1 2V
-2V
+1 3V
-1V
G ra y are a re p re se nts com m o n m o d e ra ng e
B o th V + an d V - m u st a lw a y s re m a in w ith in
th e c om m o n m o d e ra n ge re la tive to L L G n d
V cm (C o m m o n M o d e Vo ltag e ) = + 5 Volts
V cm
W ith V cm = + 5 V D C ,
+ V s m u st b e le ss th a n + 5 V, o r th e co m m o n m o d e ran g e w ill b e e xce e d e d (> + 1 0V )