Magnetic Fields; Electromagnetic Interference (Emi) - Keithley 2500 User Manual

F-10 Measurement Considerations
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.

Electromagnetic Interference (EMI)

The electromagnetic interference characteristics of the Model 2500 comply with the elec-
tromagnetic compatibility (EMC) requirements of the European Union as denoted by the
CE mark. However, it is still possible for sensitive measurements to be affected by exter-
nal sources. In these instances, special precautions may be required in the measurement
setup.
Sources of EMI include:
• Radio and TV 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 sig-
nal is present. The effects of EMI can be seen as an unusually large offset, or, in the case of
impulse 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 constructed screen room
may be required to sufficiently attenuate 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 frequency range. Keep in mind, however, that such filtering may have detrimental
effects (such as increased response time) on the measurement.
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