PURPOSE:To reduce the number of the bits of an incremental and decremental counter by allowing positive and negative to almost alternately appear in the output pulse of a digital SQUID, by applying a bias current having an amplitude modulated waveform to the SQUID. CONSTITUTION:A current to be measured is allowed to flow to a SQUID composed of a superconductive loop containing a superconductive inductance L and two Josephson junctions J1, J2 for coupling a magnetic field are provided to said SQUID. The output of an amplitude modulated waveform generator 1A is connected to the asymmetric injection terminal B of the SQUID to apply a bias current to said AQUID and output is taken out from the injection terminal B to be converted to a feedback current If by an incremental and decremental counter 3, an integrator 4 and a D/A converter 5 and fed back to a superconductive inductance Lf to be coupled with the SQUID. By this method, positive and negative output pulses are almost alternately obtained to suppress the number of the bits of the incremental and decremental counter.
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