From Open-Loop to Closed-Loop Single-VCO-Based Sensor-to-Digital Converter Architectures: theoretical analysis and comparison

Abstract

Sensor interface circuits are traditionally based on analog approaches. Recently, time-based architectures have become more popular thanks to their highly-digital scalable implementation. However, the poor VCO linearity limits the achievable overall linearity. This paper presents a novel single- VCO-based sensor interface architecture in closed-loop configuration, thus ensuring higher linearity. Models are proposed and analytic expressions are derived to estimate the sensor interface resolution based on the characteristics and noise performance of the single building blocks. Based on frequency-domain models, the STF and the NTF are derived for both the open-loop and the closed-loop configuration, and the SNR is calculated. The closed- loop architecture can achieve the same resolution as the open- loop one, provided that the feedback circuit noise is negligible compared to the other sources of noise, while ensuring a better linearity at the expense of lower speed and a small area and power overhead.