High-speed operation of front-end circuit of analog-to-digital converter

We designed a front-end circuit of an analog-to-digital converter based on single flux quantum circuitry and tested its high-speed operation. The front-end circuit consists of a clock oscillator, a sigma-delta modulator, a 1-to-8 demultiplexer (DEMUX) and SQUID amplifiers (SQUID AMP's). The total number of Josephson junctions is 2518. In the test, a dc current was supplied to the modulator and the output digital data were distributed by the DEMUX and amplified by the SQUID AMP's. The distributed output data was successfully acquired at high-speed by using an on-chip testing method. To confirm the proper function of the circuit, we studied the dependence of the ratio of the logical value ''1'' on the current level. As a result, we found that the ratio was proportional to the magnitude of the dc current. At the same time, we also confirmed the amplifications of the SQUID AMP's. These results indicate that the front-end circuit was operated correctly. The maximum operating frequency of the circuit was 43 GHz