This paper presents a high-precision, low-power CMOS temperature-to-digital converter (TDC) for detecting MEMS reference frequency sources on-chip temperature. This TDC uses a bipolar transistor as the core device for temperature measurement and a second-order Sigma-Delta ADC to read the temperature information. In order to improve the accuracy, the PTAT bias circuit with finite current gain compensation resistor and Dynamic Element Matching (DEM) circuit with Bank-Swap structure is employed in the temperature front-end circuit, respectively. In the ADC design, Analog T-switches (AT-Switch) with complementary structures are employed to reduce the leakage current of MOS switches in a fully differential switched-capacitor integrator to improve the accuracy further. Implemented in TSMC 180nm CMOS, the TDC occupies 0.135mm². The measurement results show that the average power consumption of the circuit is 95µW (@27°C) at a supply voltage of 1.8V. After temperature curve fitting, an inaccuracy of ±0.85°C (3σ) is achieved from -40°C to 85°C, which meets the requirements of the FBAR (Flim Bulk Acoustic-wave Resonator) oscillator for temperature compensation.