A study for an improved design of front-end circuit of superconducting analog-to-digital converter

We have proposed an improved design of the front-end circuit for superconducting analog-to-digital (A/D) converters. The assumed structure of the A/D converter consists of a front-end circuit based on single flux quantum circuitry and a back-end circuit based on semiconductor circuits. To complete the A/D converter, it is necessary to enable synchronous operation between the front-end circuit and assumed back-end circuit. In the present framework, the front-end circuit consists of a ladder-type pulse generator, a modulator, a hybrid DEMUX, shift-registers and stack-type amplifiers. The voltage level of the output data signal is enlarged to about 10 mV by using stack-type amplifiers. Furthermore, the timing margin for synchronization is improved by using a hybrid demultiplexing method. We also have designed a front-end circuit based on the present framework and verified its functionalities at low speed. It was experimentally confirmed that the timing margin in a 1-to-4 hybrid DEMUX was enlarged to three times as large as in the conventional binary-tree DEMUX. A high-voltage output signal of 11 mV was also obtained. From these results, we conclude that functionalities of the designed front-end circuit are correct.