A 19 pJ-K2 Temperature Sensor using Sub-VTH Ring Oscillator with 1.28°C/V Line Sensitivity

This paper presents a sub $-V_{\text{TH}}$ ring oscillator (RO) based digital temperature sensor where the ROs are designed using the sub $-V_{\text{TH}}$ transmission gates (TG). Two ROs are designed with different TGs to generate $f_{\mathrm{H}}$ and $f_{\mathrm{L}}$ frequencies, whose frequency ratio $(f_{\mathrm{H}}/f_{\mathrm{L}})$ increases linearly with temperature. The temperature-dependent $f_{\mathrm{H}}/f_{\mathrm{L}}$ ratio is converted to digital code (TCODE) using two asynchronous counters. Further, a cascoded 3T regulator is proposed to power the sensor and reduce its line sensitivity. The sensor prototype is designed in 180 nm while operated at 0.7 V supply it consumes 7.7 nW of power (3.6 nW by core and 4.1 nW by digital block) with $-1.5^{\circ}\mathrm{C}/+1.94^{\circ}\mathrm{C}$ inaccuracy across 0°C to 100°C range after 2-point calibration at 10°C and 80°C. It achieves an 0.15°C resolution and FoM of 19 pJ-K²with 1.28 ° C/V line sensitivity when operated from 0.7 V to 2.2 V supply voltage.