A Stable, Programmable Current Source for the NIM-2 Joule Balance

The current source is used to produce a stable electromagnetic force in the joule balance method to realize the kilogram. The stability of the current determines the statistical uncertainty of the force measurement. In this paper, an approach of two-stage current parallel-summation is proposed to realize the coarse and fine tuning separately, which is controlled by two digital-to-analog converters to set-up different voltage references. In addition, the distributed array of resistors and a 1‰ voltage divider are built for higher precision of the output. A programmable current source with the stability better than 0.1 ppm/5min (5 mA) and high-resolution (0.07 nA) is developed.