Measurement Considerations F-15
Light
Some components, such as semiconductor junctions and MOS capacitors on semiconductor
wafers, are excellent light detectors. Consequently, these components must be tested in a light-
free environment. While many test fixtures provide adequate light protection, others may allow
sufficient light penetration to affect the test results. Areas to check for light leaks include doors
and door hinges, tubing entry points, and connectors or connector panels.
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 measurements 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 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. Means of minimizing electrostatic interference
include:
1. Shielding. Possibilities include: a shielded room, a shielded booth, shielding the sensi-
tive 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 circuit, 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 (volt-
age) 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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