A VCO-Based ADC With Direct Connection to a Microphone MEMS, 80-dB Peak SNDR and 438-μW Power Consumption

This article presents a microphone readout chip incorporating a voltage controlled oscillator (VCO)-based ADC that can be directly connected to a capacitive micro electro mechanical systems (MEMSs) sensor without requiring a voltage buffer. The ADC uses an open-loop pseudo-differential architecture with two ring oscillators followed by a coarse–fine frequency-to-digital converter. The proposed coarse–fine architecture optimizes power consumption, thanks to a new algorithm. The MEMS is connected to the ring oscillators with a source follower circuit that can be programed to interchange signal to noise ratio (SNR) by the power consumption in small steps. This feature enables always-on operation in voice recognition applications. The microphone is compatible with standard PDM audio interfaces. Total ADC power consumption ranges between 245 and
$438 \mu \text{W}$
for peak signal-to-noise and distortion ratios (SNDRs) between 74 and 80.3 dB-A, including the ADC and MEMS coupling circuitry. The dynamic range (DR) achieves 108 dB at full performance with a total harmonic distortion (THD) of 1.5% at the acoustic overload point (AOP) of 128 dBSPL. The ADC occupies an area of 0.14 mm2 implemented in 0.13-
$\mu \text{m}$
CMOS.