A Time-Encoded Capacitance-to-Digital Converter Based on a Switched-Capacitor Feedback

Abstract

An innovative method to perform capacitance-to-digital conversion without requiring sensor biasing circuitry is proposed. This work is intended for the readout of MEMS capacitive microphones used in human-to-machine interface applications, where the main constraints are low power consumption and small chip area. The time-encoded sigma-delta ADC described here employs a switched-capacitor-based feedback to linearize the voltage-controlled oscillator and to couple the capacitive sensor to the readout. A prototype was fabricated to test the concept with a CMOS 130nm process. The achieved relative capacitance resolution is 14.7 bits with a rest capacitance value of 2.575 pF and a total power consumption of 343 µW. Linearity measurements (SNDR peak = 51.5dB in the bandwidth from 300 Hz to 6.8 kHz) are limited by the test fixture due to the nonlinearity of the varactor introduced in lieu of a testing input sensor.