A pipelined gray code-to-natural binary decoder for use in a Josephson A/D converter

A 4-bit A/D conversion system with a sampling rate of up to 5 GHz utilizing Josephson digital technology is being constructed. The system consists of a comparator stage, which utilizes the multiple lobes of Josephson interferometers to achieve 4-bit Gray code encoding of an analog signal with only 4 comparators; a decoding stage, to process the Gray code into natural binary; and an interface stage, to amplify the millivolt level Josephson signals in order to drive room temperature instruments. We report here on the design, construction, and testing of a 2-bit decoder stage. The decoder utilizes alternating latch and logic blocks operating in a pipelined fashion to perform the decoding while maintaining the speed of the input stage. Timing is controlled by a multi-phase overlapping clock. The fabrication technology used is an eight-level Pb alloy process, with 5 μm linewidths. Extension to 4 bits is discussed.