Field Wiring Considerations; Minimizing Drift In Differential Mode - National Instruments DAQ X Series User Manual

Field Wiring Considerations

Environmental noise can seriously affect the measurement accuracy of the
Simultaneous MIO X Series device if you do not take proper care when
running signal wires between signal sources and the device. The following
recommendations apply mainly to AI signal routing, although they also
apply to signal routing in general.
Minimize noise pickup and maximize measurement accuracy by taking the
following precautions:
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•
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Refer to the NI Developer Zone document, Field Wiring and Noise
Considerations for Analog Signals, for more information.

Minimizing Drift in Differential Mode

If the readings from the DAQ device are random and drift rapidly, you
should check the ground-reference connections. The signal can be
referenced to a level that is considered floating with reference to the device
ground reference. Even though you are in DIFF mode, you must still
reference the signal to the same ground level as the device reference. There
are various methods of achieving this reference while maintaining a high
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Use individually shielded, twisted-pair wires to connect AI signals to
the device. With this type of wire, the signals attached to the AI+ and
AI– inputs are twisted together and then covered with a shield. You
then connect this shield only at one point to the signal source ground.
This kind of connection is required for signals traveling through areas
with large magnetic fields or high electromagnetic interference.
Route signals to the device carefully. Keep cabling away from noise
sources. The most common noise source in a PCI DAQ system is the
video monitor. Separate the monitor from the analog signals as far as
possible.
Separate the signal lines of the Simultaneous MIO X Series device from
high-current or high-voltage lines. These lines can induce currents in or
voltages on the signal lines of the Simultaneous MIO X Series device if
they run in close parallel paths. To reduce the magnetic coupling
between lines, separate them by a reasonable distance if they run in
parallel, or run the lines at right angles to each other.
Do not run signal lines through conduits that also contain power lines.
Protect signal lines from magnetic fields caused by electric motors,
welding equipment, breakers, or transformers by running them
through special metal conduits.
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Chapter 4 Analog Input
X Series User Manual