A Compact Sub-nW/kHz Relaxation Oscillator Using a Negative-Offset Comparator With Chopping and Piecewise Charge-Acceleration in 28-nm CMOS

This work presents a compact and power-efficient kHz-range relaxation (RC) oscillator with robust performance against temperature and voltage variations. By deliberately introducing a negative-offset voltage into the comparator, an offset cancellation scheme leveraging chopping and piecewise charge-acceleration facilitates a low temperature coefficient. A low-power comparator with a tail resistor and a low oscillation amplitude improves the energy efficiency. The die area is compact by introducing leakage-based temperature compensation that eliminates bulky resistors and complex calibration. Prototyped in a 28-nm CMOS process and measured at 28.5 kHz, our oscillator occupies 0.0046 mm $<^>{2}$ and dissipates 27.6 nW at a 0.8-V supply. The energy efficiency is 0.97 nW/kHz, and the temperature coefficient is 33.3 ppm/ $<^>{\circ}$ C over $-$ 40 to 85 $<^>{\circ}$ C with 1-point calibration. The corresponding FoM of 164.9 dB compares favorably with the recent arts. The start-up time is rapid, and the period settling time is within one cycle of $\sim$ 5.7 $\mu$ s. The Allan deviation is $\le $ 40 ppm for measurement intervals of $>$ 0.5 s.