Operation4-9detrimental effect on measurement accuracy unless steps aretaken to avoid them. Thermal EMF effects can be virtuallyeliminated by taking two voltage measurements, E1 and E2,the first with the current, I, flowing in one direction, and thesecond with a current, I, of the same magnitude flowing inthe opposite direction. The resistance can then be calculatedas follows:Note that simply reversing the current source polarity will re-sult in a 2× accuracy specification change. To avoid thisproblem, matrix switching could be added to the test systemto reverse the current. See paragraph 4.4.4.R E2 E– 12I----------------=Figure 4-6Low-resistance testingKEITHLEY 182 SENSITIVE DIGITAL VOLTMETERTRGSRQREMTALKLSTNModel 1821Dual 1x20 MUXDUTs(20)20120120HILOHILO220 Current Source182 Input220 OutputR220 CurrrentSource7015 DUTV182Nanovoltmeter7015Low-level resistance measurementsMany times, it is necessary to make resistance measurementswith either lower voltage sensitivity or higher currents thanare available with ordinary DMMs. Examples of cases wherelow-level resistance measurements may be necessary includethe testing of PC board traces, contacts, bus bars, and low-re-sistance shunts.Figure 4-6 shows a typical test configuration for a switchingsystem capable of testing a number of low resistance devices.The Model 220 Current Source forces current through thedevice under test, while the Model 182 Sensitive DigitalVoltmeter measures the resulting voltage across the device.Since low voltage levels are being measured, thermal EMFoffsets generated by relay and connector contacts will have aArtisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
