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An on-chip frequency reference exploiting the well-defined thermal-diffusivity (TD) of IC-grade silicon has been realized in a standard 0.7 μm 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 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% (σ = ±0.05%) over the military temperature range (-55°C to 125°C ), with a worst case temperature coefficient of ± 11.2 ppm/°C.