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Electrostatic interference is first recognizable when hand or body movements near the exper-
iment cause fluctuations in the reading. Pick-up from AC fields can also be detected by observ-
ing the electrometer preamp output on an oscilloscope. Line frequency signals on the output are
an indication that electrostatic interference is present. Means of minimizing electrostatic inter-
ference 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 string 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.
Electromagnetic Interference (EMI)
The electromagnetic interference characteristics of the Model 6514 comply with the electro-
magnetic 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 external 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.
•
Impulses sources as in the case of arcing in high-voltage environments.
The effect on instrument performance can be considerable if enough of the unwanted signal
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.
General Measurement Considerations
C-5
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