A CMOS Temperature-to-Frequency Converter With an Inaccuracy of Less Than $\pm \hbox{0.5}\,^{\circ}{\hbox{C}}$ (3$\sigma$) From $-\hbox{40}\,^{\circ}\hbox{C}$ to 105$\,^{\circ}\hbox{C}$

Abstract

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 plusmn0.5degC (3sigma) over the industrial temperature range (-40degC to 105 degC) has been realized in a standard 0.7-mum CMOS process