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A time-to-digital-converter-based CMOS smart temperature sensor is proposed for high-accuracy portable applications. Conventional smart temperature sensors rely on an analog-to-digital converter, which consumes much chip area and operating power, for digital output code conversion. For the purpose of cost reduction and power saving, the proposed smart temperature sensor first generates a pulse with a width proportional to the measured temperature. Then, a cyclic time-to-digital converter (TDC) is utilized to convert the pulse into the corresponding digital code. The test chips, with extremely small area of 0.175 mm2, were fabricated by the TSMC CMOS 0.35 μm 2P4M process. Due to the excellent linearity of the digital output, the achieved measurement error is merely -0.6°C to +0.8°C without any curvature correction or dynamic offset-cancellation. The effective resolution is better than 0.15°C, and the power consumption is 10 μW.