A 510-pW 32-kHz Crystal Oscillator With High Energy-to-Noise-Ratio

Abstract

This article introduces a 32-kHz crystal oscillator (XO) with high energy-to-noise-ratio pulse injection at sub- harmonic frequency. A T/4-delay clock slicer is proposed to convert the sinusoidal crystal waveform into an output clock of 32 kHz and to introduce a delay of T/4, providing proper timing for energy injections. The output clock feeds frequency dividers and generates pulses to activate the proposed all-NMOS differential driver at 4 kHz. It enables two injections in eight periods at the peak and valley of the crystal oscillation, with the crystal running freely between injections. This configuration achieves a 2-ppb Allan deviation floor. The less frequent injections reduce the injection overhead, enabling the lowest reported power consumption of published nW XOs (0.51 nW). At 0.45 V , the proposed XO operates across a temperature range of −25 ◦C to 125 ◦C, the widest reported range for nW XOs. This design is fabricated in the 40-nm CMOS and occupies 0.02 mm 2.