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Section 5: Source-Measure Concepts
Triboelectric effects
Triboelectric currents are generated by charges created by friction between a conductor and an
insulator. Here, free electrons rub off the conductor and create a charge imbalance that causes the
current flow.
The triax cables supplied with the SMU and preamp greatly reduce this effect by using
graphite-impregnated insulation underneath the outer shield. The graphite provides lubrication and
a conducting cylinder to equalize and minimize charges generated by frictional effects of cable
movement. However, even this type of triax cable creates some noise when subjected to vibration
and expansion or contraction. Therefore, all connections should be kept short, away from
temperature changes (which would create thermal expansion forces), and supported by taping or
wiring the cable to a non-vibrating surface such as a wall, bench, or rigid structure.
Other solutions to movement and vibration problems include:
•
Remove or mechanically decouple vibration sources such as motors, pumps, and other
electromechanical devices.
•
Securely mount or tie down electronic components, wires, and cables.
•
Mount the preamps as close as possible to the DUT.
NOTE
Piezoelectric and stored charge effects
Piezoelectric currents are generated when mechanical stress is applied to certain crystalline
materials used for insulated terminals and interconnecting hardware. In some plastics, pockets of
stored charge cause the material to behave in a manner similar to piezoelectric materials.
To minimize the current due to this effect, remove mechanical stresses from the insulator, and use
insulating materials such as polyethylene that have minimal piezoelectric and stored charge
effects.
Contamination and humidity
Error currents also arise from electrochemical effects when ionic chemicals create weak batteries
between two conductors on a circuit board. For example, commonly-used epoxy-printed circuit
boards, when not thoroughly cleaned of etching solution, flux, or other contamination, can
generate currents of a few nanoamps between conductors.
Insulation resistance can be dramatically reduced by high humidity or ionic contamination.
High-humidity conditions occur with condensation or water absorption, while ionic contamination
may be the result of body oils, salts, or solder flux.
To avoid the effects of contamination and humidity, select insulators that resist water absorption
(such as Teflon), and keep humidity to <50 % relative humidity. Also be sure that all insulators are
kept clean and free of contamination. If insulators become contaminated, clean them thoroughly
with a pure solvent such as methanol.
5-22
A temporary triboelectric current is generated when a triax cable is first connected.
This current is typically tens or hundreds of femtoamperes and can last as long as 5
to 10 minutes.
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Model 4200-SCS Reference Manual
4200-901-01 Rev. S / May 2017
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