Keithley 6485, User Manual

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B-4 General Measurement Considerations Model 6485 and 6487 User’s Manual
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 insu-
lators. 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 provide adequate light protection, others
may allow sufficient 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. With
this in mind, the Model 6485/6487 display may be turned off by sending the :DISP:ENAB
OFF command.
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
measurements do not allow these charges to decay rapidly and erroneous or unstable read-
ings may result. These erroneous or unstable readings may be caused in the following
ways:
1. DC electrostatic field can cause undetected errors or noise in the reading.
2. AC electrostatic fields can cause errors by driving the input preamplifier into
saturation or through rectification that produces DC errors.