A 763 pW 230 pJ/Conversion Fully Integrated CMOS Temperature-to-Digital Converter With +0.81 °C/−0.75 °C Inaccuracy

Abstract

A sub-nW fully integrated temperature sensor is presented that digitizes temperature via a capacitive charging time feedback loop controlled by a least significant bit (LSB)-first algorithm. Specifically, an ultra-low-power current reference generator charges two metal–insulator–metal (MIM) capacitors $C_{\mathrm{ top}}$ and $C_{\mathrm{ bot}}$ , generating $V_{\mathrm{ ramp,top}}$ and $V_{\mathrm{ ramp,bot}}$ which are then compared to a constant with temperature (CWT) voltage and a proportional to absolute temperature (PTAT) voltage, respectively. Temperature is then digitized by matching the charging time between the $V_{\mathrm{ ramp,top}}$ and $V_{\mathrm{ ramp,bot}}$ via feedback tuning of $C_{\mathrm{ top}}$ driven by an energy-efficient digital processing unit (DPU) for direct ultra-low-power digital readout. The design is fabricated in 65 nm complementary metal–oxide–semiconductor, and measurement from 12 samples reveals a maximum temperature error of +1.61 °C/−1.53 °C (+0.86 °C/−0.83 °C) and +0.81 °C/−0.75 °C when operating from 0 °C to 100 °C after two-point (three-point) calibration without and with trimming, respectively. Operating from a 0.5 V supply, the 12 samples consumed an average power of 763 pW at 20 °C, which after a 0.3 s conversion time results in 230 pJ/conversion.