Second-order lowpass sigma–delta modulator with parallel multiple-flux-quantum feedback driver

Abstract We have made a prototype superconducting double-loop lowpass sigma–delta modulator with a parallel multiple-flux-quantum feedback driver. The feedback driver consists of multi-stage SFQ splitters and SQUID-gates connected in series. The multi-stage splitters forming a binary-tree structure produce multiple numbers of flux quanta for the feedback gain and switching of the SQUID-gates gives feedback to the first integrator. A simulation showed that the feedback duration of the SQUID-gate type was only 20 ps, whereas our earlier type with a ladder circuit of gain 20 requires 526 ps. This reduction of the feedback duration enables much higher speed operation. The fabricated modulator with SQUID-gates was operated at the sampling frequency of 1.2 GHz and showed noise shaping roughly of the second-order.