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

An innovative method to perform capacitance-to-digital conversion without requiring sensor biasing circuitry is proposed. This letter 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 130-nm process. The achieved relative capacitance resolution is 14.7 b with a rest capacitance value of $2.575 \,\mathrm{p}\mathrm{F}$ and a total power consumption of $343 \,{\mu }\mathrm{W}$. Linearity measurements ($\text{SNDR}_\mathrm{peak}=51.5$ dB in the bandwidth from $300 \,\mathrm{Hz}$ to $6.8 \,\mathrm{k}\mathrm{Hz}$) are limited by the test fixture due to the nonlinearity of the varactor introduced in lieu of a testing input sensor.