This paper proposes a BJT-based temperature sensor with a 16-bit SAR ADC fabricated in a standard 55-nm CMOS process. It obtains a resolution of 0.04 °C and a resolution figure of merit (FoM) of 5.12 pJ·K² with the utilization of a 16-bit SAR ADC. The sensor takes 0.1 ms to complete a round of conversion. The switched-capacitor amplifier is applied to scale up the input range of the SAR ADC and improve the effective dynamic range. The sensor achieves an accuracy of ± 0.6 °C from - 40 °C to 125 °C. It occupies an area of 0.234 mm2 and draws a current of 6.4 uA (analog front end) from a 1.2-V supply voltage.