A 120nW 18.5kHz RC oscillator with comparator offset cancellation for ±0.25% temperature stability

Integrated low-frequency oscillators can replace crystal oscillators as sleep-mode timers to reduce the size and cost of wireless sensors [1]. Since the timer is one of the few continuously functioning circuits, minimizing its power consumption can greatly reduce sleep-mode power of highly duty-cycled systems. Temperature stability of the oscillator is important in order to minimize timing uncertainly and guard time for the radios, and thus maximizing sleep time. The voltage-averaging feedback method described in [2] achieves high stability in the MHz frequencies, but when scaled to the kHz range, requires very large filters. On the other extreme, gate leakage-based timers have been designed for sub-nW power consumption, but operate in the sub-Hz frequencies [3]. In the past, high accuracy RC oscillators in the kHz range have been designed with feed-forward correction [1] and self-chopped operation [4]. In this work, an offset cancellation architecture achieves long-term frequency stability and temperature stability while operating at lower power.