Measurement Computing CIO-DAS16 - Manual (Page 15)

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 Figure 3-2 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-DAS16/F 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 3-3. The CIO-DAS16/F 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 3-3. Common Mode Range Diagram
System Grounds and Isolation
There are three scenarios possible when connecting your signal source to your CIO-DAS16/F board.
1. The CIO-DAS16/F and the signal source may have the same (or common) ground. This signal source may be
connected directly to the CIO-DAS16/F.
2. The CIO-DAS16/F and the signal source may have an offset voltage between their grounds (AC and/or DC). This
offset it commonly referred to a common mode voltage. Depending on the magnitude of this voltage, it may or may not
be possible to connect the CIO-DAS16/F directly to your signal source. We will discuss this topic further in a later
section.
3. The CIO-DAS16/F and the signal source may already have isolated grounds. This signal source may be connected
directly to the CIO-DAS16/F.
Which system do you have?
Perform the following test: Using a battery powered voltmeter 1 , measure the voltage (AC and DC) between the ground
signal at your signal source and at your PC. Place one voltmeter probe on the PC ground and the other on the signal
source ground. Measure both the AC and DC Voltages.
11
1
If you do not have access to a voltmeter, skip the experiment and take a look at the following three sections. You may be able to identify your system type from the
descriptions provided.
+ 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 )