Determination of digital voltmeter input parameters

The digital voltmeter input circuitry can contribute to measurement uncertainty, and is therefore important to characterize it. In this paper the methods for the determination of input resistance, capacitance and input bias current of the precise digital voltmeters HP 3458A are presented. The method with voltage source and high-ohmic divider resistor has been developed for the determination both the input offset currents I_S, as well as the input resistance R_V. The second method with source of linear voltage ramp and picoampermeter is primary used for the determination of input capacitance C_V. It was shown that: internal current of voltmeters is in the range ofpicoamperes, with relative instability of approximately plusmn0,1; input resistance is in the range of TOmega with the relative instability of plusmn0.15; and the input capacitance was measured with an uncertainty of a few picofarads.