An ultra-low-power CMOS temperature sensor with analog-to-digital readout circuitry for RFID appli- cations was implemented in a 0.18-μm CMOS process. To achieve ultra-low power consumption, an error model is proposed and the corresponding novel temperature sensor front-end with a new double-measure method is presented. Analog-to-digital conversion is accomplished by a sigma-delta converter. The complete system consumes only 26 μA @ 1.8 V for continuous operation and achieves an accuracy of ±0.65 °C from –20 to 120 °C after calibration at one temperature. An ultra-low-power CMOS temperature sensor with analog-to-digital readout circuitry for RFID applications was implemented in a 0.18-μm CMOS process. To achieve ultra-low power consumption, an error model is proposed and the corresponding novel temperature Analog-to-digital conversion is accomplished by a sigma-delta converter. The complete system consumes only 26 μA @ 1.8 V for continuous operation and achieves an accuracy of ± 0.65 ° C from -20 to 120 ° C after calibration at one temperature.