A 0.7 V Capacitance-To-Digital Converter for Interdigitated Electrode Capacitive Vapor Sensors

This paper presents a CMOS capacitance-to-digital converter (CDC) for capacitive sensors applications. Off-chip capacitive sensors are fabricated by coating sensing materials on interdigitated electrode (IDE) structure. A CDC consists of a capacitance-to-voltage front-end and 12-bit successive approximation register analog-to-digital converter (SAR-ADC) in a single chip is used for converting sensor data into digital codes. The proposed capacitance-to-voltage front-end circuit extends the dynamic range by a select logic for combinations of reference capacitances and voltages. The SAR-ADC uses a monotonic capacitor switching procedure and the input common-mode voltage gradually converges to supply voltage. This chip is fabricated with the TSMC 0.18-um 1P6M CMOS process and the core occupies area of 719 X 276 um2. The CDC achieves measurement interval of 1.5-78 pF and 8.35-bit ENOB with 220 nW consumption at a 0.7 V supply voltage and 0.25 ms measurement time.