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If both ac and dc readings are 0.00 volts, you may have a system with common grounds. However, since voltmeters
will average out high frequency signals, there is no guarantee. Please refer to the section below titled Common
Grounds.
If you measure reasonably stable ac and dc voltages, your system has an offset voltage between the grounds
category. This offset is referred to as a Common Mode Voltage. Please to read the following warning carefully, then
proceed to the section describing Common Mode systems.
WARNING
If either the ac or dc voltage is greater than 10 volts, do not connect the PCI-DAS1200 to this
signal source. You are beyond the board’s usable common mode range and will need to either
adjust your grounding system or add special isolation signal conditioning to take useful
measurements. A ground offset voltage of more than 30 volts will likely damage the
PCI-DAS1200 board and possibly your computer. Note that an offset voltage much greater than
30 volts will not only damage your electronics, but it may also be hazardous to you.
This is such an important point, that we will state it again. If the voltage between the ground of
your signal source and your PC is greater than 10 volts, your board will not take useful
measurements. If this voltage is greater than 30 volts, it will likely cause damage, and may
represent a serious shock hazard! In this case you will need to either reconfigure your system to
reduce the ground differentials, or purchase and install special electrical isolation signal
conditioning.
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 electrically isolated grounds.
Systems with Common Grounds
In the simplest (but perhaps least likely) case, your signal source will have the same ground as the PCI-DAS1200.
This would typically occur when providing power or excitation to your signal source directly from the
PCI-DAS1200. There may be other common ground configurations, but it is important to note that any voltage
between the PCI-DAS1200 ground and your signal ground is a potential error voltage 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
PCI-DAS1200, 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 PCI-DAS1200. This is
especially true if you are using high gains, since ground potentials in the sub millivolt range will be large enough to
cause A/D errors, yet will not likely be measured by your handheld voltmeter.
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 PCI-DAS1200 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.
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