System Analysis of Capacitance-to-Digital Converters Based on a Switched-Capacitor Feedback

This work describes the transient and steady-state responses of a voltage-controlled oscillator (VCO)-based analog front-end for capacitive sensors aimed at measuring the distance between the plates. A full VCO-based capacitance-to-digital converter can be obtained by processing the VCO output with a frequency-to-digital converter. The front-end primarily consists of a ring oscillator, whose input is regulated in a closed loop by means of a switched-capacitor (SC)-based feedback. The SC network is also used to couple an unbiased capacitive sensor to the readout without the need for biasing circuitry. This manuscript investigates the ripple generation due to the switching nature of the analyzed architecture, the average behaviour under small-signal variations, and the main noise contributions. The extracted model is validated through transient simulations and can be valuable for design purposes and stability studies.