Capacitance-to-Digital Converter for Operation Under Uncertain Harvested Voltage down to 0.3V with No Trimming, Reference and Voltage Regulation

Abstract

In low-cost battery-less systems, capacitive sensing via capacitance-to-digital conversion (CDC) needs to operate with minimal or no support from additional circuitry such as voltage regulation, voltage/current references or digital post-processing as shown in Fig. 5.2.1 (e.g., for linearization). At the same time, direct harvesting demands operation down to very low voltages and power, to consistently fit the power available from the environment even when scarce (e.g., down to $\sim \mathrm{nW} / \mathrm{mm}^{2}$ in light harvesters under realistic conditions). To enable continuous monitoring at power lower than the μW-range of state-of-the-art $\sim 12$ -bit CDCs $[1-3], 7$ -to-8-bit architectures with power down to sub-nW have been demonstrated for sensor nodes [4], although their supply voltage requirement $(\geq 0.6 \mathrm{~V})$ is not suitable for direct harvesting, similar to [5]. CDCs for continuous monitoring at lower resolution $(\sim 7$ bit) with sub-nW operation at $0.6 \mathrm{~V}$ have been also demonstrated [6], although their power is burdened by the additional contribution of digital post-processing $(\sim n W s)$ and others. A fully digital CDC has been introduced in [7] in the form of capacitance-to-voltage conversion via capacitor linear discharge due to a ring oscillator and final voltage-to-digital conversion, which requires two supply voltages of $0.45 \mathrm{~V}$ and $1 \mathrm{~V}$. Operation at minimal power also comes with measurement times in the sub-second or second scale [6,8] in addition to the reduced resolution, which are still in the range required by continuous monitoring in several applications [6,8] (e.g., temperature, humidity, proximity, fluid level monitoring).