A 43 nW, 32 kHz, ±4.2 ppm Piecewise Linear Temperature-Compensated Crystal Oscillator With ΔΣ-Modulated Load Capacitance

This article describes an ultralow-power (ULP) temperature-compensated crystal oscillator (TCXO) with a pulsed-injection XO driver for IoT applications. Temperature compensation is achieved by changing the load capacitance (
${C}_{L}$
) between two values using a delta-sigma modulator (
$\Delta \Sigma \text{M}$
). The complex modulation profile across temperature is approximated as piecewise linear elements that is selected by a coarse temperature sensor. As a result, the power and area of fine-grain look-up tables (LUTs) or a polynomial engine used in prior works can be avoided. The proposed pulsed-injection XO driver that directly replenishes the energy of the
${C}_{L}$
sustains the XO oscillation for the two different
${C}_{L}$
states. Implemented in 40-nm CMOS, the proposed 32.768-kHz TCXO achieves an accuracy of ±4.2 ppm from −20 °C to 85 °C with just three-point trimming and an Allan deviation floor of 34 ppb while consuming 43 nW, which is an approximate 8
$\times $
improvement over the recent state-of-the-art TCXOs.