Low noise programmable current source for the NIST-3 and NIST-4 watt balance

We give a brief description of a low noise, high resolution programmable current source that was designed for the NIST-3 watt balance to determine the Planck constant h and will be used for the NIST-4 watt balance. The current source is used to control the induction coil position in the magnetic field. It features bipolar operation to ±20 mA, high resistance isolation to ground, and low power consumption.