Electrochemical Effects; Triboelectric Effects; Piezoelectric And Stored Charge Effects - Keithley 6430 Instruction Manual

F-8 Measurement Considerations

Electrochemical effects

Error currents also arise from electrochemical effects when ionic chemicals create weak bat-
teries on a circuit board. These batteries could generate a few nanoamps of current between
conductors. Ionic contamination may be the result of body oils, salts or solder flux. The prob-
lem is further enhanced by high humidity (moisture) that decreases insulation resistance.
When building test fixtures, select insulators that resist water absorption, and use the fixture
in a moderate humidity environment. Also, be sure that all insulators are kept clean and free of
contamination.
Humidity
Excess humidity can reduce insulation resistance on PC boards and in test connection insu-
lators. Reduction in insulation resistance can, of course, seriously affect high-impedance mea-
surements. Also, humidity (moisture) can combine with contaminants to produce offset
currents caused by electrochemical effects (see Electrochemical effects). To minimize the
effects or moisture, keep humidity to a minimum (ideally <50%), and keep components and
connectors in the test system clean.

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:
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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.
Dielectric absorption
Dielectric absorption in an insulator can occur when a voltage across that insulator causes
positive and negative charges within the insulator to polarize because various polar molecules
relax at different rates. When the voltage is removed, the separated charges generate a decaying
current through circuits connected to the insulator as they recombine.
Use "low noise" cables. These cables are specially designed to minimize charge gener-
ation and use graphite to reduce friction. The triax cable supplied with the Model 6430
is 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.