Computer Hardware User Manual

Chapter 4 Signal Connections
AT-MIO/AI E Series User Manual 4-58  National Instruments Corporation
Field Wiring Considerations
Environmental noise can seriously affect the accuracy of measurements
made with your AT E Series board if you do not take proper care when
running signal wires between signal sources and the board. The
following recommendations apply mainly to analog input signal routing
to the board, although they also apply to signal routing in general.
You can minimize noise pickup and maximize measurement accuracy
by taking the following precautions:
• Use differential analog input connections to reject common-mode
noise.
• Use individually shielded, twisted-pair wires to connect analog
input signals to the board. With this type of wire, the signals
attached to the CH+ and CH- 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 board carefully. Keep cabling away from noise
sources. The most common noise source in a PC data acquisition
system is the video monitor. Separate the monitor from the analog
signals as much as possible.
The following recommendations apply for all signal connections to
your AT E Series board:
• Separate AT E Series board signal lines from high-current or
high-voltage lines. These lines are capable of inducing currents in
or voltages on the AT E Series board signal lines if they run in
parallel paths at a close distance. 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.
For more information, refer to the application note, Field Wiring and
Noise Consideration for Analog Signals, available from National
Instruments.