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A time-to-digital-converter-based CMOS smart temperature sensor without a voltage/current analog-to-digital converter (ADC) or bandgap reference is proposed for high-accuracy portable applications. Conventional smart temperature sensors rely on voltage/current ADCs for digital output code conversion. For the purpose of cost reduction and power savings, the proposed smart temperature sensor first generates a pulse with a width proportional to the measured temperature. Then, a cyclic time-to-digital converter is utilized to convert the pulse into a corresponding digital code. The test chips have an extremely small area of 0.175 mm2 and were fabricated in the TSMC CMOS 0.35-μm 2P4M process. Due to the excellent linearity of the digital output, the achieved measurement error is merely -0.7∼+0.9°C after two point calibration, but without any curvature correction or dynamic offset cancellation. The effective resolution is better than 0.16°C, and the power consumption is under 10 μW at a sample rate of 2 samples/s.