Model 6485 Picoammeter Instruction Manual General Measurement Considerations C-3
Figure C-2
Eliminating ground loops
Triboelectric effects
Triboelectric currents are generated by charges created between a conductor and an insula-
tor 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 specially designed to minimize charge
generation and use graphite to reduce friction. The Keithley Model 4801–4803
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 struc-
tures as rigid as possible. Make sure there are no mechanical stresses on the insulators.
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
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