Accurate self-checking digital teraohmmeter

We describe a digital teraohmmeter achieving state-of-the-art linearity, stability, and accuracy. Proper shielding, guarding, and measurement reproducibility having been addressed by hardware design, the instrument employs automatic self-checking circuits and routines to help maintain a linearity of 50 ppm when working over 10/sup 6/-10/sup 12/ /spl Omega/ in resistance, 1-1000 V in test voltage, and 1 pA-1 /spl mu/A in test current. It can measure high-resistance standards up to 10/sup 15/ /spl Omega/ with a standard uncertainty not exceeding 100 ppm after a traceable calibration of its internal reference at the 1-M/spl Omega/ level. In a laboratory, an annual calibration should suffice.<
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