An Ultra Low-Voltage Low-Power Capacitance-to-Digital Converter for Wirelessly Powered Intraocular Pressure Sensor

This paper presents the design and implementation of a low-power and low-voltage capacitance-to-digital converter (CDC) for a wirelessly powered intraocular pressure (IOP) sensor. To minimize power consumption and enable operation with ultra low voltages, a CDC is designed based on a ΔΣ modulation using the inverter amplifier operating in the deep-subthreshold region as integrator amplifiers. The CDC, implemented in 0.13 μm CMOS process and tested in a pressure chamber with MEMS capacitive sensors, obtains an integral nonlinearity of 2.1 mmHg (corresponding to 1.25 fF) over a range of 0-70 mmHg above ambient only consuming 30 nW from a 325-mV supply. This paper presents the highest reported capacitive resolution, the lowest reported power and operating voltage for CDC used with IOP measurement, obtaining a 4.5× reduction in operating voltage and a 3.7× reduction in power consumption over existing state-of-the-art.