Measurement of Leakage Current to Ground in Programmable Josephson Voltage Standards

The voltage error associated with the leakage current of programmable Josephson voltage standards (PJVS) is one of the largest contributions to the uncertainty in direct comparison of voltage standards. Due to the parallel biasing scheme of the PJVS and the resulting multiple leakage paths, the quantities “leakage resistance” and “leakage current” are not necessarily equivalent. A method to measure and model the leakage current to ground for a given PJVS output voltage is presented. By upgrading simple components of the NIST system, the leakage current to ground can be reduced from 250 pA to 50 pAat 10 V.

I. Introduction

A voltage standard that floats relative to Earth potential offers the flexibility to choose the grounding node of a measurement circuit. This feature is important both for a programmable voltage standard (PJVS) [1] used with Kibble balances [2] and more generally for the measurement of a voltage source that is, by construction, referenced to Earth ground (for example a Zener standard on line-power). The leakage resistance to ground of a PJVS system is not infinite. Because a PJVS array requires a current bias, it is always connected to its bias electronics. The bias electronics have an unavoidable leakage current to ground (LCG) error which must be evaluated and reported as a measurement uncertainty. This effect may constitute the largest contributor to the uncertainty budget of direct PJVS comparisons.