XITRON 2000I, User Manual

Page: 51 / 96

Applications 51
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In most circumstances, the use of a tightly twisted pair of 28AWG wires
is recommended, as this both reduces the effects of thermal E.M.F.
voltages and achieves the close coupling required to reject interference.
•
Route all line wiring and any digital wiring as distant as possible from the
analog instrument connections (particularly computer interface and
networking cables). For routing distances greater than three feet, use a
shielded cable or twisted pair to connect to the terminals
.
When the instrument is driving a load, consider the impedance of the wiring.
With a 10mA load, the cabling will drop lì V for every 0.1mÙ of cable
resistance. Check that sufficient wire gauge is used and that the connections
to the terminals are clean and tight.
Ensuring Stable Current Connections
The considerations for low-level DC currents are similar to those for
voltages, but thermal E.M.F. voltages and lead resistances have no effect on
performance. The primary effects in this mode of operation are those of
interference, cable leakage and dielectric storage.
•
The effect of interference on low level currents is far more pronounced
than for DC voltages.
•
For high performance use tightly coupled twisted pairs and/or shielded
cables. When using shielded cables, note that while electro-magnetic
interference is essentially eliminated, the capacitive injection is
eliminated in the center conductor of the cable (only). The twisted pair
balances the injection thus has less effect.
•
Some instrumentation has far different noise rejection characteristics in
one terminal than the other—commonly, one terminal is the virtual earth
of an amplifier. Reverse the connections to find the best orientation,
particularly when shielded cables are being used.