Measurement Computing CIO-DAS160x/1x - Manual (Page 15)

5.1.2 System Grounds and Isolation
There are three scenarios possible when connecting your signal source to your CIO-DAS1600 board.
1. The CIO-DAS1600 and the signal source have the same (or common) ground. This signal
source can be connected directly to the CIO-DAS1600.
2. The CIO-DAS1600 and the signal source have an offset voltage between their grounds (AC
and/or DC). This offset it commonly referred to a common mode voltage. Depending on the magnitude
of this voltage, it may or may not be possible to connect the CIO-DAS1600 directly to your signal
source. We will discuss this topic further in a later section.
3. The CIO-DAS1600 and the signal source already have isolated grounds. This signal source
can be connected directly to the CIO-DAS1600.
Which system do you have?
Try the following experiment. Using a battery powered voltmeter*, measure the voltage (difference)
between the ground signal at your signal source and at your PC. Place one voltmeter probe on the PC
ground and the other on the signal source ground. Measure both the AC and DC Voltages.
*If you do not have access to a voltmeter, skip the experiment and take a look a the following three
sections. You may be able to identify your system type from the descriptions provided.
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-DAS1600 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-DAS1600 board and possibly your computer.
Note that an offset voltage much greater than 30 volts will not only damage your
electronics, but it can also be hazardous to your health.
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 can 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
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