Shielding techniques
Shielding against noise must address both capacitive (electrical) and inductive (magnetic) coupling. The
addition of a grounded shield around the conductor is highly effective against capacitive coupling. In
switching networks, this shielding often takes the form of coaxial cables and connectors. For frequencies
above 100 MHz, double-shielded coaxial cable is recommended to maximum shielding effectiveness.
Reducing loop area is the most effective method to shield against magnetic coupling. Below a few hun-
dred kilohertz, twisted pairs may be used against magnetic coupling. Use shielded twisted pair for
immunity from magnetic and capacitive pickup. For maximum protection below 1 MHz, make sure that the
shield is not one of the signal conductors.
Separation of high-level and low-level signals
Signals whose levels exceed a 20-to-1 ratio should be physically separated as much as possible. The entire
signal path should be examined including cabling and adjacent connections. All unused lines should be
grounded (or tied to LO) and places between sensitive signal paths. When making your wiring connections
to the screw terminals on the module, be sure to wire like functions on adjacent channels.
Source of system cabling errors
Radio frequency interference
Most voltage-measuring instruments can generate false readings in the presence of large, high-frequency
signals. Possible sources of high-frequency signals include nearby radio and television transmitters, com-
puter monitors, and cellular telephones. High-frequency energy can also be coupled to the internal DMM
on the system cabling. To reduce the interference, try to minimize the exposure of the system cabling to
high-frequency RF sources.
If your application is extremely sensitive to RF1 radiated from the instrument, use a common mode choke
Keysight DAQ970A User's Guide 179
4Measurement Tutorials
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