XITRON 2000I, User Manual

Page: 52 / 96

52
2000I & 2000M User Guide, Rev B
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When dealing with low-level DC currents at high compliances, the effects
of leakage currents become dominant. To limit leakage use the correct
insulation throughout the cabling, and keep all connectors and front
panels free of grease and dirt. Note that most low-level leakage currents
are non-ohmic in nature and tend to be a constant current leakage for
compliances above a few hundred millivolts. Insulators with high-
insulation resistance ratings measured at several hundred volts may not
exhibit this performance when operated at low levels.
ü
For general use P.T.F.E. (known as Teflon®) is effective for low
leakage performance. Keep free of grease.
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For high compliance voltages use Polyethylene or Polypropylene to
lessen the effects of dielectric storage, which results in long settling
times. Polyethylene is the insulation used in most high-quality
shielded cables.
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To eliminate leakage problems without long settling times use
non-insulated wiring and maintain an air gap between wires.
However, this method may cause excessive interference problems if
the cables are long or the environment is particularly adverse.
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Route connecting cables with regard to line wiring. Low-level DC
currents are sensitive to interference where high impedance levels are
involved. In extreme cases the connection of a high quality
Polypropylene capacitor (usually 10nF) across the terminals of the
instrument may be used, but the effect on settling time (particularly at
high compliances) must be considered.
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A primary source of interference is the movement of personnel near the
cables. This effect is caused both by the perturbation of the line
frequency field near the person and the direct change in capacitance
between the person and the cabling. These effects are reduced by using
shielded or twisted pair cables, and may be further reduced by employing
anti-static procedures in the surrounding environment to reduce the DC
potential between the moving person and the cabling. In general, the only
method of eliminating this problem is to avoid excessive movement near
the cables, and to keep these cables as short as possible.