Electrostatic Interference - Keithley 2500 User Manual

Models 2500 and 2502 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 satura-
tion, 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 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, how-
ever, 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.
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Measurement Considerations F-9