Models 2500 and 2502 User’s Manual Measurement Considerations F-5
Electrochemical effects
Error currents also arise from electrochemical effects when ionic chemicals create weak
batteries 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 problem is further enhanced by high humidity (moisture) that decreases insula-
tion resistance.
When building test fixtures, select insulators that resist water absorption, and use the fix-
ture 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
insulators. Reduction in insulation resistance can, of course, seriously affect high-
impedance measurements. Also, humidity (moisture) can combine with contaminants to
produce offset currents caused by electrochemical effects (see “Electrochemical effects”).
To minimize the effects of 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 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 genera-
tion and use graphite to reduce friction.
• 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.
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176
TestEquipmentDepot.com
To Next Page
Loading...
Need help?
General
