Constant-current supply of 3 ppm stability and resettability; application for a SQUID

We describe a new design for a constant-current supply which is stable and resettable to 3 ppm over a period of one week, even in the presence of ambient temperature changes of several degrees. These characteristics are achieved by utilizing a digital-to-analog converter for providing a changeable, yet extremely stable, reference voltage. Seven full-scale current ranges are available between 0.1 and 100 mA. It is also shown that this constant-current supply is suitable even for use with a SQUID, meeting all of the exacting requirements for such use.