Model 6517B Electrometer Reference Manual Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015 4-37
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 deceases 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 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. To minimize the effects of moisture, keep humidity to a minimum (ideally < 50
%), and keep components and connectors in the test system clean.
Light
Some components, such as semiconductor junctions and MOS capacitors on semiconductor wafers,
are excellent light detectors. Consequently, these components must be tested in a light-free
environment. While many test fixtures (Keithley Instruments test fixtures, for instance) provide
adequate light protection, others may allow enough light penetration to affect the test results. Areas to
check for light leaks include doors and door hinges, tubing entry points, and connectors or connector
panels.
Electrostatic interference
Electrostatic interference occurs when an electrically charged object is brought near an uncharged
object, thus inducing a charge on the previously uncharged object. Usually, effects of such
electrostatic action are not noticeable because low impedance levels allow the induced charge to
dissipate quickly. However, the high impedance levels of many Model 6517B measurements do not
allow these charges to decay rapidly, and erroneous or unstable readings may result. These
erroneous or unstable readings may be caused in the following ways:
ï‚· DC electrostatic field can cause undetected errors or noise in the reading.
ï‚· AC electrostatic fields can cause errors by driving the input preamplifier into saturation, or through
rectification that produces DC errors.
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