User`s manual

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 PCI-DAS1602/12 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 PCI-DAS1602/12 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).
4.1.2 System Grounds and Isolation
There are three scenarios possible when connecting your signal source to your PCI-DAS1602/12 board.
1. The PCI-DAS1602/12 and the signal source may have the same (or
common
)
ground. This signal source may be connected directly to the PCI-DAS1602/12.
2. The PCI-DAS1602/12 and the signal source may have an offset voltage
between their grounds (ac and/or dc). This offset is commonly
referred to as
common mode voltage
. Depending on the magnitude of
this voltage, it may or may not be possible to connect the PCI-DAS1602/12
directly to your signal source. We will discuss this topic further in a later
section.
3. The PCI-DAS1602/12 and the signal source may already have
isolated
grounds
. This signal source may be connected directly to the
PCI-DAS1602/12.
9
+1V
-13V
+2V
-12V
+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 range
Both V+ and V- must always remain within
the common mode range relative to LL Gnd
Vcm (Common Mode Voltage) = +5 Volts
Vcm
With Vcm= +5VDC,
+Vs must be less than +5V, or the common mode range will be exceeded (>+10V)