INTEGRATED DC SQUID MAGNETOMETER WITH HIGH DV/DB

A directly coupled dc SQUID magnetometer with very simple feedback electronics is presented. The magnetometer has been integrated on a 7.2 x 7.2 mm2 chip and fabricated using a four-level Nb/Si(x)N(y)/Nb process. Eight pick-up loops are connected in parallel to directly form the SQUID inductance of about 0.4 nH which leads to a high sensitivity B/PHI = 0.47 nT/PHI-0. An Additional Positive Feedback (APF) circuit on the magnetometer chip has been used to increase the gradient of the V-PHI characteristic to dV/d-PHI congruent-to 300-mu-V/PHI-0 at the SQUID operating point. The resulting gradient of the transfer function dV/dB congruent-to 640-mu-V/nT makes it possible to directly read out the SQUID without helium temperature impedance matching circuits or flux modulation techniques. Using a highly simplified feedback electronics consisting of an ultra-low noise bipolar preamplifier and an integrator on a 10 cm2 PC board, a white noise level below 3 fT/square-root Hz with a 1/f corner frequency around 3 Hz, a 3 dB bandwidth of about 40 kHz, and a maximum feedback range of +/- 32 nT for frequencies up to 300 Hz have been measured.