Capacitance-Based Voltage Regulation- and Reference-Free Temperature-to-Digital Converter down to 0.3 V and 2.5 nW for Direct Harvesting

A temperature-to-digital converter for direct harvesting is proposed, where no DC-DC conversion is required between the DC harvester and the system. Temperature-induced capacitance differences are read out through ring oscillator frequency. PVT variations are suppressed by the differential nature of the temperature sensor architecture, whereas mismatch is compensated via a self-referenced calibration procedure. No reference, regulator, digital post-processing and digital direct temperature readout is needed to retain true-nW and low-$V_{min}$ operation. A 180-nm testchip tested across corner wafers shows 7bit ENOB, 2.5-4.5nW from solar and thermal direct harvesting at 0.3-0.5 V, as representative of a very wide range of environmental conditions.