AN INTEGRATED DC SQUID MAGNETOMETER WITH VARIABLE ADDITIONAL POSITIVE FEEDBACK

An easily adjustable direct-coupled magnetometer is presented. A typical usable voltage swing of 30 muV 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 +/- 10% to +/- 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 +/- 55%.