Design and Analysis of an Always-ON Input-Biased pA-Current Sub-nW mV-Threshold Hysteretic Comparator for Near-Zero Energy Sensing

This paper presents the design and analysis of an always-ON comparator capable of detecting mV-range input voltage signals reliably with sub-nW power consumption. The comparator compares the two pA currents generated by current mirrors biased by the mV-range input signal. With the input signal near zero at the standby mode, the comparator consumes near-zero energy. A positive feedback is introduced to accelerate the output signal transition to the supply or the ground and to generate the hysteresis to tolerate the noise in the input signal. The comparator threshold and the amount of hysteresis are programmed using control signals to adjust the current mirror transistor sizes. Analytic expressions of comparator characteristics are derived using circuit theory and subthreshold transistor models, and have been validated using simulation and measurement. A prototype comparator implementing these ideas was designed and fabricated in a standard 65-nm CMOS process. Chip measurements have shown that the comparator achieved programmable thresholds from 27 to 46.5 mV with energy per switching from 1.9 to 2.4 nJ using four control bits. The measured power consumption is 270 pW at the input signal of 0.1 mV with the frequency of 10 Hz.