A Novel Energy-Efficient Continuous-Time Hysteretic VCO-Based Comparator

Voltage-controlled oscillator (VCO)-based comparators offer higher energy efficiency as the difference in input magnitudes increase, such as in level-crossing ADCs. Nevertheless, to date, they require a clock signal to perform comparison operations. This is incongruous with continuous-time applications, where inputs are compared continuously. Further, they lack hysteresis, a crucial feature for mitigating spurious switching that compromises energy efficiency. In this paper, we present a novel VCO-based comparator that, for the first time, simultaneously achieves continuous-time operation and high energy efficiency. The former feature is enabled by a novel continuous-time decision circuit, while the latter is achieved through a novel switched-current hysteresis circuit that mitigates spurious switching. The proposed comparator is designed in 65 nm CMOS. Simulation results show that it achieves low energy per comparison, ranging from 0.07 to 4 pJ, with an average propagation delay of ~15 ns. The average energy consumption is 0.19 pJ — ~1.8× lower than the state-of-the-art VCO-based comparator.