Measurement Computing PCM-DAS16D/12 - Manual (Page 17)

As a safe rule of thumb, if your signal source or sensor is not connected
directly to an LLGND pin on your PCM-DAS16x/12, it’s best to assume that
you do not have a common ground even if your voltmeter measured 0.0 Volts.
Configure your system as if there is ground offset voltage between the source
and the PCM-DAS16x/12.
4.1.6 Systems with Common Mode (ground offset) Voltages
The most frequently encountered grounding scenario involves grounds that
are somehow connected, but have AC and/or DC offset voltages between the
PCM-DAS16x/12 and signal source grounds. This offset voltage my be AC,
DC or both and may be caused by a wide array of phenomena including EMI
pickup, resistive voltage drops in ground wiring and connections, etc. Ground
offset voltage is a more appropriate term to describe this type of system, but
since our goal is to keep things simple, and help you make appropriate con-
nections, we’ll stick with our somewhat loose usage of the phrase Common
Mode.
4.1.7 Small Common Mode Voltages
If the voltage between the signal source ground and PCM-DAS16x/12 ground
is small, the combination of the ground voltage and input signal will not
exceed the CIO-DAS800’s +/-10V common mode range, (i.e. the voltage
between grounds, added to the maximum input voltage, stays within +/-10V),
This input is compatible with the PCM-DAS16x/12 and the system may be
connected without additional signal conditioning. Fortunately, most systems
will fall in this category and have a small voltage differential between
grounds.
4.1.8 Large Common Mode Voltages
If the ground differential is large enough, boards +/- 10V common mode
range will be exceeded (i.e. the voltage between PCM-DAS16x/12 and signal
source grounds, added to the maximum input voltage you’re trying to meas-
ure exceeds +/-10V). In this case the PCM-DAS16x/12 cannot be directly
connected to the signal source. You will need to change your system ground-
ing configuration or add isolation signal conditioning. (Please look at our
ISO-RACK and ISO-5B-series products to add electrical isolation, or give our
technical support group a call to discuss other options).
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