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This paper describes a new method for determining the effective value of the thermal diffusivity, D eff, of a CMOS chip. D eff is a parameter that describes the rate at which heat diffuses through a chip, and hence its knowledge is essential for the thermal management of systems on chip and the design of thermal sensors. By embedding an electrothermal filter (ETF) in a frequency-locked-loop (FLL), its phase response, which is determined by its (fixed) geometry and D eff, can be measured. D eff can then be accurately determined from the measured phase response. For an ETF implemented in a 0.7 Â¿m CMOS process, the resulting values of D eff were 1.405, 0.755, and 0.495 cm2/s at -55, 27, and 125Â°C respectively.