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 7.2×7.2 mm² chip and fabricated using a four-level Nb/Si_xN_y/Nb process. Eight pickup loops are connected in parallel to directly form the SQUID inductance of about 0.4 nH which leads to a high sensitivity B/Φ=0.47 nT/Φ₀. An additional positive feedback (APF) circuit on the magnetometer chip has been used to increase the gradient of the V-Φ characteristic to dV/dΦ≃300 μV/Φ₀ at the SQUID operating point. The resulting gradient of the transfer function dV/dB≃640 μV/nT makes it possible to directly readout 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/√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