Sources that contribute errors
A variety of sources can contribute errors due to generated currents (see figure 6). Triboelectric currents, for example, are generated by charges created between a conductor and an insulator due to friction when free electrons rub off the conductor and create a charge imbalance that causes the current to flow. This noise current can be in the range of tens of nanoamps. Low noise cables greatly reduce this effect by using graphite-impregnated insulation beneath the outer shield. Even low noise cables create some noise when subjected to vibration or temperature-related expansion or contraction, however. Support cables by taping or wiring them to a non-vibrating surface such as a wall, bench, or rigid structure and always isolate them from vibration sources such as motors, pumps, and other electromechanical devices.
Click image to enlarge.
Figure 6. Current-generating phenomena.
Piezoelectric currents are generated when mechanical stress is applied to certain crystalline materials when used for insulated terminals and interconnecting hardware. To minimize these effects, remove mechanical stresses from the insulator and choose insulating materials with minimal piezoelectric and stored charge.
A test fixture’s insulation resistance can be dramatically reduced by high humidity or ionic contamination introduced by body oils, salts, solder flux, etc. Humidity or moisture can combine with any contaminants present to create electrochemical effects that can produce offset currents. To prevent problems, select insulators that resist water absorption, and keep humidity to moderate levels (ideally less than 50%). Keep all components and test fixturing free of contamination.
Both the source resistance and source capacitance of the DUT can affect the noise performance of a feedback ammeter. As the source resistance is reduced, the noise gain of the ammeter will increase. Table 1 summarizes typical recommended source resistances by range for a feedback ammeter, but it’s advisable to check the reference manual of a particular ammeter to find the recommended values.
Table 1. Recommended source resistance values based on current measurement range.
The source capacitance of the DUT will affect the noise performance of a feedback ammeter: as the source capacitance increases, so does the noise gain. Although there is a limit as to the maximum source capacitance value, it’s usually possible to measure the leakage current of a capacitor by connecting a resistor or a forward-biased diode in series with the device.
Optimizing the accuracy of your low current and high resistance measurement applications depends in large part on choosing instrument that offers the appropriate level of sensitivity, ensuring that all cables, connectors, and test fixtures used are specified for high impedance applications, minimizing the source of measurement errors, and verifying test system performance at every stage of the integration process. For further information on optimizing these measurements, consult Keithley’s Low Level Measurements Handbook
or view the informative webinar on this topic: Techniques for Making Optimal Low Current and High Resistance Measurements
About the author
Mary Anne Tupta is a Senior Staff Applications Engineer at Keithley Instruments, Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. She earned a B.S. in physics/electronic engineering and an M.S. in physics from John Carroll University. She has assisted Keithley customers with instrument applications since 1988. She can be reached at 440-498-2715 or firstname.lastname@example.org