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Section 4: Basic measurements
To prevent ground loops, instruments should be connected to ground at only a single point, as shown
in the following figure. Note that only a single instrument is connected directly to power line ground.
Experimentation is the best way to determine an acceptable arrangement. For this purpose,
measuring instruments should be placed on their lowest ranges. The configuration that results in the
lowest noise signal is the one that should be used.
Triboelectric effects
Triboelectric currents are generated by charges created between a conductor and an insulator due to
friction. Here, free electrons rub off the conductor and create a charge imbalance that causes the
current flow. For example, bending a triaxial cable causes friction between the center conductor (HI)
and its surrounding insulator, resulting in triboelectric currents. Triboelectric currents can be
minimized as follows:
Use low noise cables. These cables are designed to minimize charge generation and use
graphite to reduce friction. The Keithley Instruments Model 7078-TRX triaxial cables are low
noise.
Use the shortest cables possible, and secure them (for example, taping or tying) to a non-
vibrating surface to keep them from moving.
Piezoelectric and stored charge effects
Piezoelectric currents are generated when mechanical stress is applied to certain insulating materials
(for example, crystalline). In some plastics, pockets of stored charge cause the material to behave in
a similar manner.
When building test fixtures, choose good insulating materials and make connecting structures as rigid
as possible. Make sure there are no mechanical stresses on the insulators.
4-36
Figure 52: Eliminating ground loops
Model 6517B Electrometer Reference Manual
6517B-901-01 Rev. C / August 2015
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