Measurement Computing CIO-DAS16 - Manual (Page 16)

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 be careful to read the following warning and 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 CIO-DAS16/F to this signal
source. You are beyond the boards 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 CIO-DAS16/F board and possibly your
computer. Note that an offset voltage much greater than 24 volts will not only damage your electronics,
but it may also be hazardous to your health. In this case you will need to either reconfigure your system
to reduce the ground differentials, or 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 CIO-DAS16/F. This
would typically occur when providing power or excitation to your signal source directly from the CIO-DAS16/F. There
may be other common ground configurations, but it is important to note that any voltage between the CIO-DAS16/F
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
CIO-DAS16/F, 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 CIO-DAS16/F. This is especially
true if you are using either the CIO-DAS1402/16 or the CIO-DAS1402/12 at 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 CIO-DAS16/F 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 use the phrase Common Mode.
Small Common Mode Voltages
If the voltage between the signal source ground and CIO-DAS16/F 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 CIO-DAS16/F 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.
Large Common Mode Voltages
If the ground differential is large enough, the CIO-DAS800’s +/- 10V common mode range will be exceeded (i.e. the
voltage between CIO-DAS16/F and signal source grounds, added to the maximum input voltage you’re trying to measure
exceeds +/-10V). In this case the CIO-DAS16/F cannot be directly connected to the signal source. You will need to
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