A sensor-merged oscillator-based readout circuit for pizeo-resistive sensing applications

This paper presents a readout circuit for piezo-resistive sensing applications. It is based on a resistor-based oscillator which converts the resistance information into frequency domain. In the readout circuit, a mixer is employed to down-convert the oscillator frequency to lower frequency for subsequent digitization. The frequency down-conversion operation is equivalent to amplification in time which increases the system sensitivity to resistance variation. A frequency calibration loop is incorporated to set the mixer output frequency to a well-defined initial value. The readout circuit is fabricated in a TSMC 0.35-μm Bio-MEMS CMOS process platform. The oscillation frequency is 24 MHz, and the down-converted output frequency at mixer output is set to 100 kHz. The total readout circuit consumes 1 mW with 3-V supply. The system sensitivity is 7 kHz/Ω.