An Ultra-Low-Power 0.8 V, 60 nW Temperature Sensor for Battery-Less Wireless Sensor Networks

This work presents a nano-watt digital output temperature sensor featuring a supply-insensitive, self-biased current source. Second-order temperature dependencies of the MOS diode are canceled to produce a stable reference and a linear temperature-sensitive voltage. The sensor integrates a sensing unit, voltage-controlled differential ring oscillators, and a low-power frequency-to-digital converter, utilizing a resistor-less design to minimize power and area. The delay element in the ring oscillator reduces stage count, improving noise performance and compactness. Fabricated in 65 nm CMOS, the sensor occupies 0.02 mm2 and consumes 60 nW at 25 °C and 0.8 V. Measurements show an inaccuracy of +1.5/−1.6 °C from −20 °C to 120 °C after two-point calibration, with a resolution of 0.2 °C (rms) and a resolution FoM of 0.022 nJ·K−2. Consuming 0.55 nJ per conversion with a 9.2 ms conversion time, the sensor was tested in a battery-less wireless sensor node, demonstrating its suitability for wireless sensing systems.