Model 6485 and 6487 User’s Manual General Measurement Considerations B-3
this purpose, measuring instruments should be placed on their lowest ranges. The configu-
ration that results in the lowest noise signal is the
one that should be used. A convenient
way to make this connection uses the ground link at the rear of the Model 6485/6487.
Figure B-2
Eliminating g
Instrument 1 Instrument 2 Instrument 3
Power Line Ground
round loops
Triboelectric effects
Triboelectric currents are generated by charges created between a conductor and an insu-
lator due to friction. Here, free electrons rub
off the conductor and create a charge imbal-
ance that causes the current flow. For example, bendi
ng 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. Use Keithley Models 4801, 4802,
and 4803 coax cables for the Model 6485; and Models 237 and 7078 triax cables
for the Model 6487 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 stres
ses on the insulators.
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