National Instruments E Series - Manual (Page 80)

Chapter 4 Signal Connections
DAQCard E Series User Manual 4-44  National Instruments Corporation
Field Wiring Considerations
Environmental noise can seriously affect the accuracy of
measurements made with your DAQCard if you do not take proper care
when running signal wires between signal sources and the DAQCard.
The following recommendations apply mainly to analog input signal
routing to the DAQCard, 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 DAQCard. 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 DAQCard carefully. Keep cabling away from
noise sources. The most common noise source in a computer 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 DAQCard:
• Separate DAQCard signal lines from high-current or high-voltage
lines. These lines are capable of inducing currents in or voltages
on the DAQCard 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.