A 0.8 µW 8-bit 1.5∼20-pF-input-range capacitance-to-digital converter for lab-on-chip digital microfluidics systems

This paper presents an ultra-low-power capacitance-to-digital converter (CDC) for digital microfluidics systems that demand a wide capacitive sensing range. It is structured with a switched-capacitor capacitance-to-current front-end interface, followed by a differential current-mode SAR analog-to-digital converter (ADC). By using subthreshold-biased transistors, the differential current-mode SAR ADC significantly saves power and reduces sensitivity to supply voltage and clock variation. The 8-bit 1-kS/s CDC designed in 0.35-µm CMOS exhibits a capacitance sensing range from 1.5 to 20 pF while consuming a total power of 0.8 µW. The ENOB of the current SAR ADC is 7.46.