IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008 1685 Determination of High-Resolution Digital Voltmeter Input Parameters Ivan Leniˇ cek, Damir Ili´ c, and Roman Malari´ c Abstract—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 di- vider resistor has been developed for the determination of both in- put 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 ±0.1, the input resistance is in the range of teraohms with the same relative instability of ±0.1, and the input capacitance was measured to be in the range of a few hundred picofarads with uncertainty of a few picofarads. Index Terms—Digital voltmeter (DVM), input capacitance, input offset current, input resistance. I. I NTRODUCTION P RECISION digital voltmeters (DVMs) are currently used for a number of highly accurate methods of measurement, including those with Quantum Hall Resistance Standards [1]– [4]. In such measurements, there are many sources that con- tribute to measurement errors, such as environmental condi- tions (changes in temperature, RF signals, and electromagnetic fields), but it is also important to precisely know the values of the voltmeter’s input parameters [5]. It is also evident in the pre- cise determination of resistance by voltage ratio measurements, where the voltmeter input impedance is shunting the resistances being measured [6], [7]. Here, it should also be emphasized that DVMs can be used for low-frequency ac voltage (and voltage ratio) measurement when it is set on the DCV range, where its input capacitance influenced the measurement results [8]. The DVM input circuitry model on the DCV range, as shown in Fig. 1, contains the parallel combination of input resistance R V and input capacitance C V . The DVM also injects small current I S to the device that is connected to its input terminals. If R V , C V , and I S can be precisely determined, their influences on the measurement can be calculated [9]. In Section II, the aforementioned methods and experimen- tal results for the determination of input offset current I S , Manuscript received July 6, 2007; revised March 21, 2008. The authors are with the Faculty of Electrical Engineering and Computing, University of Zagreb, 10000 Zagreb, Croatia (e-mail: ivan.lenicek@fer.hr; damir.ilic@fer.hr; roman.malaric@fer.hr). Digital Object Identifier 10.1109/TIM.2008.923786 Fig. 1. Voltmeter input circuit parameters. Fig. 2. Measurement circuit for the determination of the DVM’s input parameters. input resistance R V , and input capacitance C V are shown in Sections II-A–C, respectively. II. DETERMINATION OF THE DVM’ S I NPUT PARAMETERS The measurement method for input resistance presented in this paper is done according to the brief explanation in [1] but with some modifications to achieve better accuracy and re- peatability of measurements. The DVM selected for this paper is the HP 3458A, which is used in numerous high-precision measurements, but the presented methods for the determination of the input parameters of a voltmeter can be applied to any DVM. The scheme of the measurement circuit is shown in Fig. 2. DC-voltage calibrator FLUKE 5440B has been used as a very stable voltage source (with a long-term drift of less than 10 μV/V), which is connected to the DVM through a resistance R P of 10 GΩ. The resistor R P is of carbon-film type and placed in a thermally insulated metal enclosure that ensures low thermal drift during measurement, which is typically less than 10 μΩ/Ω in 1 h. The enclosure of R P is connected to the shield of the coaxial cable attached to the “high” input terminal of the DVM. The voltage drop across input resistance R V is measured with the DVM itself, but it is also controlled with the electrometer transconductance amplifier TA having an input resistance of 10 15 Ω and extremely low input offset current (I TA ∼ =7 fA). The active guard terminal G of the 0018-9456/$25.00 © 2008 IEEE