Determination of High-Resolution Digital Voltmeter Input Parameters

High-resolution digital voltmeters (DVMs) can be widely used when precise measurements are needed, but input circuitry can contribute to the results of measurement. Therefore, to make uncertainty of measurement as small as possible for a particular measurement, it is necessary to characterize its input parameters. In this paper, the methods for the determination of input resistance, input capacitance, and input offset current of widely used precise DVMs HP 3458A with 8 1/2-digit resolution are presented. The method with voltage source and high-ohmic divider resistor has been developed for the determination of both input offset current $I_{S}$ and input resistance $R_{V}$. The second method with the source of the linear voltage ramp and the picoampermeter is primarily used for the determination of input capacitance $C_{V}$. It was shown that the input offset current of voltmeters is in the range of picoamperes with relative instability of approximately $\pm$0.1, the input resistance is in the range of teraohms with the same relative instability of $\pm$0.1, and the input capacitance was measured to be in the range of a few hundred picofarads with uncertainty of a few picofarads.