Subranging digital SQUID ammeter

A digital SQUID (Superconducting Quantum Interference Device) has an extremely large dynamic range that makes it attractive for measurement of widely varying magnetic field or current. However, if we use a single digital SQUID device to simultaneously measure large and small signals that differ by 4-5 orders of magnitude, either resolution or input bandwidth must be compromised. We are implementing a subranging architecture, where multiple SQUID devices, coupled to a common input coil, are employed for different ranges of signal amplitude, to provide high-resolution digitization of signals with much higher bandwidth. The simplest subranging ammeter will consist of two digital SQUIDs, one with a fine resolution and the other with coarse resolution. The coarse resolution SQUID measures the current in larger steps leaving a smaller current, less than its step size (resolution) to be measured by the fine resolution SQUID with smaller steps. Each digital SQUID consists of an analog SQUID followed by a comparator gate that feeds a quantum of magnetic flux (+Φ₀ or -Φ₀ depending on the polarity of the net magnetic flux sensed by the SQUID) back to the SQUID upon clocking. The two SQUIDs are alternately clocked. The digital outputs from the two SQUIDs can be combined off chip