A Thermal-Diffusivity-Based Frequency Reference in Standard CMOS With an Absolute Inaccuracy of ±0.1% From -55 °C to 125 °C

An on-chip frequency reference exploiting the well-defined thermal-diffusivity (TD) of IC-grade silicon has been realized in a standard 0.7 mu m CMOS process. A frequency-locked loop (FLL) locks the frequency of a digitally controlled oscillator (DCO) to the process-insensitive phase shift of an electrothermal filter (ETF). The ETF's phase shift is determined by its geometry and by the thermal diffusivity of bulk silicon (D). The temperature dependence of D is compensated for with the help of die-temperature information obtained by an on-chip band-gap temperature sensor. The resulting TD frequency reference has a nominal output frequency of 1.6 MHz and dissipates 7.8 mW from a 5 V supply. Measurements on 16 devices show that it has an absolute inaccuracy of +/-0.1% (sigma = +/-0.05%) over the military temperature range (-55 degrees C to 125 degrees C), with a worst case temperature coefficient of +/-11.2 ppm/degrees C.