A novel Josephson adder without carry propagation delay

The authors describe a novel Josephson adder based on the radix-2 signed-digit number representation. In the proposed adder, the addition operation can be carried out in a constant time independent of word length owing to the elimination of the carry propagation chain. A critical path for generating the sum signal can be completed through only three series gates. A carry signal and an intermediate sum signal for all digits in the adder are simultaneously generated in the first step, and the final sum signal is obtained in the second step. Features of the proposed adder are a simple construction, ultra-high speed, and low power consumption. The principle of constructing the adder with an 8 b word length using superconducting quantum interference devices (SQUIDs) is described in detail, and simulation results are presented. Results show that reliable operation can be achieved for the longest delay time of 120 ps for the 8 b adder.<>