Design considerations for a CCC bridge with complete digital control

The considerations for designing a completely digitally controlled cryogenic current comparator resistance ratio bridge are presented. Crucial elements in the design include low noise current sources of sufficient stability, and gain and bandwidth in the digital feedback loops, so that the final behaviour of the CCC bridge is limited by the fundamentally inevitable (1/f) noise sources of the SQUID and nanovolt amplifier. The digital control and data acquisition of the bridge strongly facilitates setting up different measurement configurations and finding optimal operational settings.