An integrated DC SQUID magnetometer with variable additional positive feedback

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Abstract
An easily adjustable direct-coupled magnetometer is presented. A typical usable voltage swing of 30 mu V and noise levels of down to 1.13 fT Hz-1/2 at 1 kHz have been achieved with additional positive feedback (APF). A compensation circuit integrated into the magnetometer chip allows the reduction of APF gain in three steps within a factor of 1.8, by simply adding one or two bond connections on the magnetometer chip. In this way the tolerance of the critical current I0 is enlarged from +or-10% to +or-27% at the traditional SQUID working point. If additionally the shunt resistors are trimmed by plasma etching the tolerance of I0 can be increased further to about +or-55%.