Investigation of a Double-Loop DC-SQUID Magnetometer with Additional Positive Feedback

The essential performance parameters (noise, dynamic range, stability) of the PTB direct-coupled magnetometer W5 are presented. The complete magnetometer is integrated on a 7.2 × 7.2 mm2 chip. Using a simplified feedback loop without flux modulation, a very low white noise level of typically \(2.3\;fT/\sqrt {Hz} \) is achieved with sufficiently wide margings for the SQUID parameters. Even large preamplifier offset voltage changes of ±4 µV increase the white noise level by only about 20%. Output signal fluctuations of the order of ±100 fT during periods of 24 hours have been observed with the magnetometer enclosed by a Pb shield.