16-Bit Wave-Pipelined Sparse-Tree RSFQ Adder

In this paper, we discuss the architecture, design, and testing of the first 16-bit asynchronous wave-pipelined sparse-tree superconductor rapid single flux quantum adder implemented using the ISTEC 10 kA/cm² ADP2.1 fabrication process. Compared to the Kogge-Stone adder, our parallel-prefix sparse-tree adder has better energy efficiency with significantly reduced complexity (at the expense of latency) and almost no decrease in operation frequency. The 16-bit adder core (without SFQ-to-dc and dc-to-SFQ converters) has 9941 Josephson junctions occupying an area of 8.5 mm². It is designed for the target operation frequency of 30 GHz with the expected latency of 352 ps at the bias voltage of 2.5 mV. The adder chip was fabricated and successfully tested at low frequency for all test patterns with measured bias margins of +9.8%/-10.7%.