A CMOS temperature-to-frequency converter with. an inaccuracy of less than ±0.5 °C (3σ) from -40 °C to 105 °C

This paper describes a temperature-to-frequency converter (TFC) implemented in a standard CMOS process. Its output frequency is determined by the phase-shift of an electrothermal filter, which consists of a heater and a temperature sensor realized in the substrate of a standard CMOS chip. The filter's phase-shift is determined by the geometry of the thermal path between the heater and the sensor, and by the temperature-dependent rate at which heat diffuses through the substrate. The resulting temperature-dependent phase-shift is quite well-defined, since filter geometry is defined by lithography, while the thermal diffusivity of the high-purity lightly-doped silicon substrate is essentially constant. The filter was used as the frequency-determining component of a frequency-locked loop (FLL), whose output frequency is then a well-defined function of temperature. Using this approach, a TFC with an inaccuracy of +/- 0.5 degrees C (3 sigma) over the industrial temperature range (-40 degrees C to 105 degrees C) has been realized in a standard 0.7-mu m CMOS process.