Investigation of a 5-bit Flash-Type SFQ A/D Converter Using 10 $\hbox{kA/cm}^{2}$ Niobium Process and Locally Isolated Grounds

We have investigated technologies for high-frequency and stable operation in a 5-bit flash-type single-flux quantum analog-to-digital (A/D) converter. We designed and evaluated several comparators with different designs of the transformer and found that the length of the primary line was the principal factor to determine the bandwidth, which was improved from 7 to 13 GHz by reducing the length from 860 to 290 μm. We have also investigated methods to avoid incorrect operation caused by ground return current of the bias current: one is locally extracting the return current around the component circuits, and the other is to use locally isolated ground for each component circuit. An integrated 5-bit A/D converter with an error correction circuit was designed using the locally isolated ground structure, which can avoid ground return current inflow from other component circuits. The A/D converter was operated without extracting ground return current and 10 GHz operation was carried out by means of the beat frequency method in our cryocooling system. Operation for an optical input signal was also observed. In addition, 100 GHz sampling clock operation with 1 kHz input signal was confirmed.