Development of a High-Stable DC Current Source With Improved Resolution for the Joule Balance

In the joule balance method to realize the kilogram, a programmable current source (CS) is necessary to produce a stable electromagnetic force to balance the Earth’s gravitational force on the mass. The stability and noise level of the current strongly determine the statistical uncertainty introduced by the force measurement phase in the joule balance. An approach to implement a CS with high stability and low noise by a method of two-stage current parallel summation is presented here, with which the CS can be coarsely and finely tuned separately by two subsystems. Based on this idea, a programmable CS with high stability and low noise has been developed. The output current range is from 0 to 14 mA with a resolution of 3.5 nA. The experimental results show that the stability of the current is better than 0.15 ( $\mu $ A/A)/h at 5 mA, which meets the requirement of the joule balance.