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

A long-range UHF RF identification (RFID) sensor has been designed using a 0.35-mu 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 degrees C and an inaccuracy value lower than 0.1 degrees C in the range from 35 degrees C to 45 degrees 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.