This paper presents a CMOS temperature sensor intended for the temperature compensation of MEMS frequency references. It is based on a Wien bridge RC filter, whose phase-shift, due to the temperature dependency of the resistors used, is temperature dependent. This phase shift is then digitized by a phase domain sigma-delta modulator. The sensor was implemented in 0.18μm CMOS, consumes 36 μW and achieves 6mK resolution in a conversion time of 100msec. After batch calibration and a 3-point trim, it achieves ±0.15°C (3σ) inaccuracy over the industrial range (-40°C to 85°C).