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2.13.2 EIectrostatic interference
Electrostatic interference occurs when an electrically
charged object is brought near an uncharged object, thus in-
ducing a charge on the previously uncharged object. Usually,
effects of such electrostatic action are not noticeable because
low impedance levels allow the induced charge to dissipate
quickly. However, the high impedance levels of many Model
6512 Electrometer measurements do not allow these charges
to decay rapidly, and erroneous or unstable readings may re-
sult. These erroneous or unstable readings may be caused in
the following ways:
1. DC electrostatic field can cause undetected errors or
noise in the reading.
2. AC electrostatic fields can cause errors by driving the in-
put preamplifier into saturation, or through rectification
that produces DC errors.
Electrostatic interference is first recognizable when hand or
body movements near the experiment cause fluctuations in
the reading. Pick-up from AC fields can also be detected by
observing the electrometer preamp output on an oscillo-
scope. Line frequency signals on the output are an indication
that electrostatic interference is present.
Means of minimizing electrostatic interference include:
1. Shielding. Possibilities include: a shielded room, a
shielded booth, shielding the sensitive circuit, and using
shielded cable. The shield should always be connected
to a solid connector that is connected to signal low. If
circuit low is floated above ground, observe safety pre-
cautions, and avoid touching the shield. Meshed screen
or loosely braided cable could be inadequate for high
impedances, or in strong fields. Note, however, that
shielding can increase capacitance in the measuring cir-
cuit, possibly slowing down response time.
2. Reduction of electrostatic fields. Moving power lines or oth-
er sources away from the experiment reduces the amount of
electrostatic interference seen in the measurement.
2.13.3 Thermal EMFs
Thermal EMFs are small electric potentials generated by dif-
ferences in temperature at the junction of two dissimilar met-
als. Although thermal EMFs are most troublesome with low-
voltage signals, they can also affect measurements made at
higher levels in extreme cases.
Low-thermal connections should be used whenever thermal
EMFs are known to be a problem. Crimped copper-to-copper
connections should be used to minimize these effects. Make
certain that all connecting surfaces are kept clean and free of
oxides, since copper-to-copper oxide junctions generate
much higher thermal EMFs than do pure copper-to-copper
connections.
2.13.4 Electromagnetic interference (EMI)
The electromagnetic interference characteristics of the Mod-
el 6512 Electrometer comply with the electromagnetic com-
patibility (EMC) requirements of the European Union as
denoted by the CE mark. However, it is still possible for sen-
sitive measurements to be affected by external sources. In
these instances, special precautions may be required in the
measurement setup.
Sources of EMI include:
• radio and television broadcast transmitters
• communications transmitters, including cellular phones
and handheld radios
• devices incorporating microprocessors and high speed
digital circuits
• impulse sources as in the case of arcing in high-voltage
environments
The effect on instrument performance can be considerable if
enough of the unwanted signal is present. The effects of EMI
can be seen as an unusually large offset, or, in the case of im-
pulse sources, erratic variations in the displayed reading.
The instrument and experiment should be kept as far away as
possible from any EMI sources. Additional shielding of the
instrument, experiment, and test leads will often reduce EMI
to an acceptable level. In extreme cases, a specially con-
structed screen room may be required to sufficiently attenu-
ate the troublesome signal.
External filtering of the input signal path may be required. In
some cases, a simple one-pole filter may be sufficient. In
more difficult situations, multiple notch or band-stop filters,
tuned to the offending frequency range, may be required.
Connecting multiple capacitors of widely different values in
parallel will maintain a low impedance across a wide fre-
quency range. Such filtering, however, may have detrimental
effects (such as increased response time) on the measure-
ment.
2.13.5 Leakage resistance effects
At normal resistance levels, the effects of leakage resistance
are seldom seen because any leakage resistance present is
generally much higher than the resistance levels encountered
Operation
2-31
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