User`s manual

The most frequently encountered grounding scenario involves grounds that are somehow connected, but
have ac and/or dc offset voltages between the PCI-DAS1602/12 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 appro-
priate connections, we’ll stick with our somewhat loose usage of the phrase Common Mode.
Small Common Mode Voltages
If the voltage between the signal source ground and PCI-DAS1602/12 ground is small, the combination of
the ground voltage and input signal will not exceed the PCI-DAS1602/12’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 PCI-DAS1602/12 and the system may be connected without additional signal condi-
tioning. Fortunately, most systems will fall in this category and have a small voltage differential between
grounds.
Large Common Mode Voltages
If the ground differential is large enough, the PCI-DAS1200’s +/- 10V common mode range will be
exceeded (i.e. the voltage between PCI-DAS1602/12 and signal source grounds, added to the maximum
input voltage you’re trying to measure exceeds +/-10V). In this case the PCI-DAS1602/12 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 products to add
electrical isolation, or give our technical support group a call to discuss other options.)
NOTE
Relying on the earth prong of a 120 VAC for signal ground connections is not advised.
Different ground plugs may have large and potentially even dangerous voltage
differentials. Remember that the ground pins on 120 VAC outlets on different sides of the
room may only be connected in the basement. This leaves the possibility that the “ground”
pins may have a significant voltage differential (especially if the two 120 VAC outlets
happen to be on different phases!)
PCI-DAS1602/12 and signal source already have isolated grounds
Some signal sources will already be electrically isolated from the PCI-DAS1602/12. The diagram below
shows a typical isolated ground system. These signal sources are often battery powered, or are fairly
expensive pieces of equipment (since isolation is not an inexpensive proposition), isolated ground systems
provide excellent performance, but require some extra effort during connections to ensure optimum
performance is obtained. Please refer to the following sections for further details.
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