A long-range UHF RF identification (RFID) sensor has been designed using a 0.35-µm 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. Index Terms-CMOS analog front end, digital core, high accuracy, low power, RF identification (RFID), temperature sensor, ultrahigh frequency (UHF).

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