A Super-Pipelined Energy Efficient Subthreshold 240 MS/s FFT Core in 65 nm CMOS

Abstract—This paper proposes a design approach targeting circuits operating at extremely low supply voltages, with the goal of reducing the voltage at which energy is minimized, thereby improving the achievable energy efficiency of the circuit. The proposed methods accomplish this by minimizing the cir-cuit’s ratio of leakage to active current. The first method, super pipelining, increases the number of pipeline stages compared to conventional ultra low voltage (ULV) pipelining strategies, reducing the leakage/dynamic energy ratio and simultaneously improving performance and energy efficiency. Measurements of super-pipelined multipliers demonstrate 30 % energy savings and 1.6 performance improvement. Since super pipelining reduces the logic depth between registers, two-phase latch based design is employed to compensate for reduced averaging effects and pro-vide better variation tolerance. The second technique introduces a parallel-pipelined architecture that suppresses leakage energy by ensuring full utilization of functional units and reduces memory size. We apply these techniques to a 16-b 1024-pt complex-valued Fast Fourier Transform (FFT) core along with low-power first-in first-out (FIFO) design and robust clock distribution network. The FFT core is fabricated in 65 nm CMOS and consumes 15.8 nJ/FFT with a clock frequency of 30 MHz and throughput of 240 Msamples/s at , providing 2.4 better energy efficiency than current state-of-art and higher throughput than typical ULV designs. Measurements of 60 dies show modest frequency (energy) spreads of 7 % (2%). Index Terms—Fast Fourier Transform (FFT), subthreshold CMOS circuits, super-pipelining, ultra low voltage (ULV) design. I