A CMOS Temperature-to-Frequency Converter With an Inaccuracy of Less Than 0.5 C( 3/27) From

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 tempera- ture 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 C( 3 ) over the industrial temperature range ( 40 Ct o 105 C) has been realized in a standard 0.7- m CMOS process.