High sensitivity digital SQUID magnetometers
A single-chip digital SQUID magnetometer integrates a sensitive analog SQUID sensor with a comparator gate and feedback circuitry on the same chip. The comparator gate is an asymmetric SQUID gate driving two DC-to-SFQ converters in series with the feedback coil. In an optimized digital magnetometer chip, sensitivity and noise level are determined by the input analog SQUID. The dynamic range of such digital chips is extremely wide enabling them to be operated in a relatively high magnetic field environment without extensive magnetic shielding. Their slew rate is determined by the frequency of an external two-phase clock. This chip simplifies room temperature electronics and, due to its digital output, can be easily multiplexed on-chip. In this paper, we describe this digital SQUID magnetometer chip and summarize our experimental results demonstrating a digital SQUID chip with sensitivity of 20 /spl mu//spl Phi//sub 0///spl radic/(Hz) and a system slew rate of 5/spl times/10/sup 6/ /spl Phi//sub 0//s at the pick up coil.