Analysis of high-performance counter-type A/D converters using RSFQ logic/memory elements

A new family of counter-type Josephson-junction A/D converters using ultrafast rapid single-flux quantum (RSFQ) logic/memory elements is proposed. The converters are built in two main blocks: a low-bit differential-code A/D converter running with subterahertz clock speed and a RSFQ-logic-based digital processing unit providing digital low-pass filtering and sample rate reduction. It is shown that such converters can be treated as digital SQUIDs possessing an extremely high slew rate (up to 10¹¹ φ₀/s) combined with flux resolution close to that of analog DC SQUIDs (up to 10^-6 φ₀Hz^-1/2). Several ways of implementing both main blocks to make it possible to tradeoff these parameters against converter complexity are presented and discussed