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An application specific integrated circuit for the pressure and temperature measurement at the high temperature oil well bottom conditions is presented in this paper. The circuit is mainly composed of three high performance automatic level controlled (ALC) oscillator circuits, which drive three external crystals (the sensitive elements), as well as mixing and filtering stages. The integrated circuit was successfully tested up to 220/spl deg/C, showing a frequency resolution of 0.0003 ppm (0.0007 psi, for the pressure measurement) and a drift of 1.5 Hz/month (0.5 psi/month) concerning, respectively, the short- and long-term measurement stability. A drastic reduction of the whole measurement tool size and cost was then allowed by means of this application specific BiCMOS integrated circuit.