Abstract: A high-sensitivity superconductive digitizer (10) which utilizes a dc superconducting quantum interference device (SQUID) (38) to convert an input current or magnetic flux into a frequency signal which is converted by a counter (60) into a digital representation of the input. This measurement requires that the SQUID (38) be operated in a linear region of its periodically varying output. To insure locking the system into a fixed point in the periodic signal, a digital flux-locked loop is employed which utilizes a digital integrator (70) and digital to analog converter (72) in a feedback loop. A waveform generator (46) produces a dithering modulation waveform in the SQUID (38) and an up-down counter (60) finds difference signals between alternating waveforms. This difference signal is then fed to a digital integrator (70) and converted to analog by a digital to analog converter (72) and fed back to the SQUID in a feed-back loop to lock the SQUID (38) to a fixed operating point.