Sensing Voltage at Electrically Floating Nodes: A Path Toward Enhancing Performance and Robustness in Capacitive MEMS Resonators

Abstract

Capacitively transduced micromechanical resonators for timing reference applications are overwhelmingly measured from the current output at their sensing electrodes, using a transimpedance amplifier (TIA). Continuous time floating-voltage measurement in capacitive resonators has not found its reach due to various reasons, the primary drawback being picking up of stray charges through stray/unknown capacitances linked to the electrically floating electrode. In this paper, we introduce a novel concept of bias tuning electrodes which alleviates this issue. Through theoretical modelling and experimental evidence, we show that voltage measurement performed at electrically-floating sensing-electrode using a voltage-amplifier (VA) is superior to TIA topology in terms of robustness, noise performance, and bandwidth. Furthermore, we introduce a new electrical circuit equivalent model for resonator devices with a bias tuning electrode in lieu of the standard Mason and Butterworth-Van Dyke (BVD) models which are unsuitable for our new topology. This new model also offers better insights for the combined system of resonator and sensing-unit. The theoretical and experimental work was carried out using a Epi-seal encapsulated DETF device wherein the superior performance of VA topology in key parameters and equivalent performance in other measures is demonstrated. This work is readily extendable to any general capacitively transduced device.[2024-0156]