General Measurement Considerations C-3
To prevent ground loops, instruments should be connected to ground at only a single point,
as shown in Figure C-2. 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 pur-
pose, 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 cur-
rents can be minimized as follows:
• Use “low noise” cables. These cables are specially designed to minimize charge gener-
ation and use graphite to reduce friction. The Keithley Model 7078-TRX triax cables are
low noise.
• Use the shortest cables possible, and secure them (i.e., 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 (i.e., 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.
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
gure
-
liminating ground loops
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