Chapter 4
Analog Input
4-12
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Minimize noise pickup and maximize measurement accuracy by taking the
following precautions.
•
Use differential AI connections to reject common-mode noise.
•
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 S Series device from high-current or
high-voltage lines. These lines can induce currents in or voltages on
the signal lines of the S 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.
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
common-mode rejection ratio (CMRR). These methods are outlined in the
Connecting Analog Input Signals
section.
AI GND is an AI common signal that routes directly to the ground
connection point on the devices. You can use this signal if you need a
general analog ground connection point to the device.
