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A dc superconducting quantum interference device (SQUID) magnetometer has been integrated on a 9×9 mm2 chip with eight pick-up loops in parallel to directly form a SQUID inductance of about 0.5 nH. Very simple feedback electronics have been developed which do not require liquid-helium temperature impedance matching circuits or flux modulation techniques. The magnetometer has a typical white noise of 8 fT/(Hz)1/2 and a 1/f corner frequency below 3.5 Hz. With an additional positive feedback circuit at 4.2 K the white noise level has been further reduced to 4.5 fT/(Hz)1/2. Using a two-pole integrator, a 3 dB bandwidth around 0.5 MHz and a maximum slew rate of 3 mT/s at 1.3 kHz have been attained with a ±0.4 μT feedback range.
Drung et al. (Mon,) studied this question.