This paper presents the design aspect of a BJT-based temperature sensor implemented in standard CMOS technology that is optimized for its noise-power performance. The interface electronics of the sensor consists of a continuous-time duty-cycle modulator, where a capacitor is periodically charged and discharged, with two temperature-dependent current sources, between two thresholds determined by a Schmitt trigger. In order to optimize the noise properties of the sensor, the major noise sources have been analyzed and optimized using target specifications of the manufacturer. Experimental results are in agreement with those of simulations and analytical calculations. The sensor has been implemented in 0.7μm CMOS technology. At 3.3V supply, the measured temperature resolution amounts to 3mK for a measurement time of 1.8ms. The test results show that a Resolution Figure of Merit (RFoM) of 3.2pJK2 has been achieved in this design, which is the best reported result for BJT-based temperature sensors in the market.