5.2
SYSTEM GROUNDS and ISOLATION
There are three possibilities when connecting a signal source to the CIO-DAS1400 board:
The CIO-DAS1400 and the signal source may have the same (or common) ground. This signal source can be connected directly to the
board.
The CIO-DAS1400 and the signal source may have an offset voltage between their grounds (AC and/or DC). This offset is commonly
referred to as a common mode voltage. Depending on the magnitude of this voltage, it may or may not be possible to connect the
board directly to your signal source. We will address this topic further in a later section.
The CIO-DAS1400 and the signal source may have isolated grounds. This signal source can be connected directly to the board.
5.2.1
Ground Testing
Perform the following test: Using a battery powered voltmeter*, measure the voltage 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 test and read 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. This offset is referred to
as a Common Mode Voltage. Read and observe the following warning, then proceed to the section describing Common Mode systems.
WARNING
An offset voltage greater than 30 volts can cause injury or death. It may also only damage board and computer circuitry. Use extreme
care. If either the AC or DC voltage is greater than 10 volts, do not connect a CIO-DAS1400 series board 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 signal isolation
conditioning in order to take useful measurements.
If you cannot obtain a reasonably stable DC voltage measurement between the grounds, or the voltage drifts considerably, the two
grounds are most likely isolated. To check for isolation, change your voltmeter to a resistance 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, it’s a fairly safe bet that your system has electrically isolated grounds.
5.2.2
Systems with Common Grounds
In the simplest (but perhaps least likely) case, your signal source will have the same ground as the CIO-DAS1400. 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 CIO-DAS1400 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 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 CIO-DAS1400. 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.
5.2.3
Systems with Common Mode (Ground-Offset) Voltages
The most frequently encountered grounding scenario involves grounds that are loosely connected, but have AC and/or DC offset
voltages between the board and the signal source ground. This offset voltage may be AC, DC, or both and may be caused by a number
of things including EMI pickup or resistive voltage drops in ground wiring and connections.
5.2.4
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-DAS1400’s +/-10V common mode range, (i.e. the voltage between grounds, added to the maximum input voltage,
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