A time-encoding CMOS capacitive sensor readout circuit with flicker noise reduction

This paper shows a novel capacity to digital measurement circuit that is suited for differential MEMS capacitive sensors such as pressure sensors or condenser microphones. The basic operating principle relies on two differential reactance-controlled multivibrator circuits whose frequency differences are sensed by a Time to Digital Converter. The multivibrator oscillators may be biased in the relaxation oscillation mode, where demodulated flicker noise is greatly attenuated by large scale excitation of CMOS transistors. The paper shows a system level description of the readout circuit, a full transistor design in 0.13u CMOS and an oscillator characterization form measurements on a discrete demonstration circuit.