F-22
Measurement Considerations
Model 2520 User’s Manual
Electrostatic interference
Electrostatic interference occurs when an electrically charged object is brought near an
uncharged object, thus inducing 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 mea-
surements do not allow these charges to decay rapidly, and erroneous or unstable readings
may result. These erroneous or unstable readings may be caused in the following ways:
•
DC electrostatic field can cause undetected errors or noise in the reading.
•
AC electrostatic fields can cause errors by driving the input preamplifier into satu-
ration, 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. Means of minimizing electrostatic interfer-
ence 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 precautions, 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 other sources away from
the experiment reduces the amount of electrostatic interference seen in the
measurement.
Magnetic fields
A magnetic field passing through a loop in a test circuit will generate a magnetic EMF
(voltage) that is proportional to the strength of the field, the loop area, and the rate at
which these factors are changing. Magnetic fields can be minimized by following these
guidelines:
•
Locate the test circuit as far away as possible from such magnetic field sources as
motors, transformers and magnets.
•
Avoid moving any part of the test circuit within the magnetic field.
•
Minimize the loop area by keeping leads as short as possible and twisting them
together.
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