Experimental implementation of analog-to-digital converter based on the reversible ripple counter

A new A/D converter which includes a comparator, and a reversible binary counter with DC outputs has been designed, fabricated, and tested. The comparator generates two trains of the single-flux-quantum (SFQ) pulses in response to increasing or decreasing input signal. The pulses are transferred through SFQ transmission lines to the adding and diminishing inputs of a reversible counter. The reversible counter has been realized by supplementing the usual counter with the SFQ transmission lines, splitters; and confluence elements for sending diminished pulses directly to each bit. Nondestructive read-out of the counter contents is carried out by SFQ/DC converters connected to each counter bit. The integrated circuit is fabricated using 5-μm Nb-AlO _x-Nb Josephson junction technology with a critical current density about 500 A/cm². External Mo shunts of the junctions provide the value of β_c⩽1 and I_c R≈300 μV. The A/D conversion is studied for low-frequency signals with an external clock. High-frequency performance of the reversible counter has been tested