Measurement-computing CIO-DAS08-PGH User Manual download pdf (Page 12)

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-DAS08-PGA 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 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.

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.

3.1.5 Systems with Common Grounds

In the simplest (but perhaps least likely) case, your signal source will have the same ground as the CIO-DAS08-PGx. This would

typically occur when providing power or excitation to your signal source directly from the board. There may be other common

ground configurations, but it is important to note that any voltage between the board ground and your signal source 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 board, 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 board. This is especially true if you are using the board 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.

3.1.6 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 board 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.

3.1.7 Small Common Mode Voltages

If the voltage between the signal source ground and board 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-DAS08-PGx and the system can be connected without

additional signal conditioning. Fortunately, most systems will fall in this category and have a small voltage differential between

grounds.

3.1.8 Large Common Mode Voltages

If the ground differential is large enough, the +/- 10V common mode range will be exceeded (i.e. the voltage between the board

and signal source grounds, added to the maximum input voltage you’re trying to measure exceeds +/-10V). In this case the board

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).

WARNING

Relying on the earth prong of a 120VAC 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

120VAC 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!)

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Measurement-computing CIO-DAS08-PGH User Manual Page 12

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