Keithley 6482, Справочное руководство

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6482-901-01 Rev. A / August 2012 Return to
Section Topics
D-7
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual Appendix D: Measurement Considerations
Figure D-3
Eliminating ground loops
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 sensitive cir-
cuit, 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 exper-
iment 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.
Instrument 1 Instrument 2 Instrument 3
Power Line Ground