Measured performance parameters of gradiometers with digital output

A planar first-order gradiometer and a DC SQUID (superconducting quantum interference device) comparator have been integrated on the same 6*6 mm/sup 2/ chip in Nb-Nb/sub 2/O/sub 5/-PbInAu technology and operated in a flux-locked loop. Owing to the digital gradiometer output, a very low flux noise of 7*10/sup -7/ phi /sub 0// square root Hz, a coupled noise energy of 34 times Planck's constant h, and a gradient noise of 3.8 pT/m square root Hz have been achieved in the white noise region above about 60 Hz. In spite of the very low flux noise, a maximum slew rate of 1.5*10/sup 5/ phi /sub 0//s at a signal frequency of 4 kHz has been measured. A simple modulation scheme has been developed to cancel Josephson junction critical-current fluctuations, yielding 4.5*10/sup -6/ phi /sub 0// square root Hz at 0.1 Hz.<
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