Measurement Computing CIO-DAS16Jr/16 - Manual (Page 10)

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 entitled Common
Grounds.
If you measure a reasonably stable AC and/or DC voltage, your system has an offset voltage between the two grounds.
This offset is referred to as a common mode voltage. Please 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-DAS16JR/16 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-DAS16JR/16 board and possibly your
computer. You must either reconfigure your system to reduce the ground differentials, or purchase and
install special electrical isolation signal conditioning.
Note: An offset voltage greater than 30 volts will not only damage your electronics, but may be
hazardous to your health.
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. Turn both systems off prior to taking this
resistance measurement. If the measured resistance is more than 100 KOhm, assume your system has electrically isolated
grounds.
2.2.5 Systems with Common Grounds
In the simplest (but perhaps least likely) case, your signal source will have the same ground as the CIO-DAS16JR/16.
This would typically occur when providing power or excitation to your signal source directly from the CIO-DAS16JR/16.
There may be other common ground configurations, but it is important to note that any voltage between the
CIO-DAS16JR/16 ground and your signal ground is a potential error voltage if you set up your system based on a
common ground assumption.
In general, if your signal source or sensor is not connected directly to an LLGND pin on your CIO-DAS16JR/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 CIO-DAS16JR/16. 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.
2.2.6 Systems with Common Mode (ground offset) Voltages
The most frequently encountered grounding problem involves grounds that are somehow connected, but have AC and/or
DC offset voltages between the CIO-DAS16JR/16 and signal source grounds. This offset voltage may be AC, DC, or
both, and may be caused by a variety of things 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 we’ll use the
phrase Common Mode.
2.2.7 Small Common Mode Voltages
Even if the voltage between the signal source ground and CIO-DAS16JR/16 ground is small, the combination of the
ground voltage and input signal still must not exceed the CIO-DAS800’s +/-10V common mode range. (The voltage
between grounds, added to the maximum input voltage, must stay within +/-10V.) If this is the case, the system can safely
be connected without additional signal conditioning. Fortunately, most systems fall in this category and have a small
voltage between grounds.
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