Full Passive UHF Tag With a Temperature Sensor Suitable for Human Body Temperature Monitoring

TLDR

The tag’s temperature sensor uses a ring‑oscillator whose frequency varies with temperature for sensing. The power‑optimized RFID tag, with an ultralow‑power ring‑oscillator sensor, achieves a 2 m read range from a 2‑W reader, 0.035 °C resolution, <0.1 °C inaccuracy between 35–45 °C, 110 nW average power at 10 Hz, enabling batteryless human body temperature monitoring.

Abstract

A long-range UHF RF identification (RFID) sensor has been designed using a 0.35- ¿m CMOS standard process. The power-optimized tag, combined with the ultralow-power temperature sensor, allows an ID and a temperature reading range of 2 m from a 2-W effective radiated power output power reader. The temperature sensor is based on a ring oscillator, where the temperature dependence of the oscillation frequency is used for thermal sensing. The temperature sensor exhibits a resolution of 0.035°C and an inaccuracy value lower than 0.1°C in the range from 35°C to 45°C after two-point calibration. The average power consumption of the temperature sensor is only 110 nW at ten conversions per second while keeping a high resolution and accuracy. These properties allow the use of the RFID as a batteryless sensor in a wireless human body temperature monitoring system.