The present invention provides an ultra-low power embedded CMOS temperature sensor based on serially connected subthreshold MOS operation particularly well suited for passive RFID food monitoring applications. Employing serially connected subthreshold MOS as sensing element enables reduced minimum supply voltage for further power reduction, which is very important in passive RFID applications. The temperature sensor may be part of a passive RFID tag and incorporates a temperature sensor core, proportional-to-absolute-temperature (PTAT) and complimentary-to-absolute-temperature (CTAT) delay generators, and a time-to-digital differential readout circuit. In one embodiment, the sensor is embedded inside a passive UHF RFID tag fabricated with a conventional 0.18 μm 1P6M CMOS process. With the sensor core working under 0.5 V and digital interfacing under 1 V, the sensor dissipates a measured total power of 119 nW at 33 samples/s and achieves an inaccuracy of +1/−0.8° C. from −10° C. to 30° C. after calibration.