Integrated magnetometer with a digital output

A magnetometer with a 50-mm/sup 2/ pickup coil, a sensor SQUID, a flux transformer, and a 1-b A/D converter were integrated on a single 6*12 mm/sup 2/ silicon chip in the Nb-Nb/sub x/O/sub y/-PbInAu technology. The pickup coil has a single turn with a 25- mu m-wide Nb strip. Operated in a flux-locked loop and at a clock frequency of 18.4 MHz, the achieved flux noise and magnetic field sensitivity are 3.5 mu Phi /sub 0// square root Hz and 8.5 fT/ square root Hz at 1.5 Hz. With a digital feedback of constant step size a maximum slew rate of 11500 Phi /sub 0//s was measured. Below the maximum slew rate the linearity between input and output was investigated.