Measurement Computing PCM-DAS16x/16 - Manual (Page 24)

If you cannot obtain a reasonably stable DC voltage measurement between the
grounds, or the voltage drifts around considerably, the two grounds are most likely
isolated. The easiest way to check for isolation is to change your voltmeter to it’s ohm
scale and measure the resistance between the two grounds. It is recommended that you
turn both systems off prior to taking this resistance measurement. If the measured
resistance is more than 100 Kohm, it’s a fairly safe bet that your system has electri-
cally isolated grounds.
6.1.7 Systems with Common Grounds
In the simplest (but perhaps least likely) case, your signal source will have the same
ground as the PCM-DAS16x/16. This would typically occur when providing power
or excitation to your signal source directly from the PCM-DAS16x/16. There may be
other common ground configurations, but it is important to note that any voltage
between the PCM-DAS16x/16 ground and your signal ground is a potential error volt-
age if you set up your system based on a common ground assumption.
As a safe rule of thumb, if your signal source or sensor is not connected directly to an
LLGND pin on your PCM-DAS16x/16, 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/16.
6.1.8 Systems with Common Mode (ground offset) Voltages
The most frequently encountered grounding scenario involves grounds that are some-
how connected, but have AC and/or DC offset voltages between the PCM-DAS16x/16
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 connections, we’ll stick with our somewhat loose usage of the
phrase Common Mode.
6.1.9 Small Common Mode Voltages
If the voltage between the signal source ground and PCM-DAS16x/16 ground is
small, the combination of the ground voltage and input signal will not exceed the
PCM-DAS16D/16 maximum input voltage, stays within +/-10V), This input is com-
patible with the PCM-DAS16x/16 and the system may be connected without addi-
tional signal conditioning. Fortunately, most systems will fall in this category and have
a small voltage differential between grounds.
6.1.10 Large Common Mode Voltages
If the ground differential is large enough, the PCM-DAS16D/16 +/- 10V common
mode range will be exceeded (i.e. the voltage between PCM-DAS16x/16 and signal
source grounds, added to the maximum input voltage you’re trying to measure
exceeds +/-10V). In this case the PCM-DAS16x/16 cannot be directly connected to
the signal source. You will need to change your system grounding configuration or
add isolation signal conditioning. (Please look at our ISO-RACK and ISO-5B-series
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