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C-4
General 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 con-
ductors. Ionic contamination may be the result of body oils, salts or solder flux. The problem 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. See “Handling and Cleaning Test Fixtures” in Section 2 for cleaning tips.
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 of
moisture, keep humidity to a minimum (ideally <50%), and keep components and connectors in
the test system clean. See “Handling and Cleaning Test Fixtures” in Section 2 for cleaning tips.
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.
Electrostatic interference
Electrostatic interference occurs when a 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 readings may result. These erro-
neous 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.
Summary of Contents for 6514
Page 203: ...14 CommonCommands...
Page 207: ...15 SCPISignalOriented MeasurementCommands...
Page 235: ...18 PerformanceVerification...
Page 254: ...19 Calibration...
Page 274: ...20 RoutineMaintenance...
Page 278: ...A Specifications...
Page 282: ...B StatusandErrorMessages...
Page 288: ...C GeneralMeasurement Considerations...
Page 294: ...D DDCEmulationCommands...
Page 299: ...E ExamplePrograms...
Page 306: ...F IEEE 488BusOverview...
Page 320: ...G IEEE 488andSCPI ConformanceInformation...
Page 324: ...H CalibrationOptions...