A method and a circuit for processing a signal supplied by a piezoelectric sensor (10) indicating the cylinder pressure of a piston engine such as a heat engine. Undesirable variations in the value of the signal supplied by the sensor and corresponding to plateaux in the pressure are compensated by a closed-loop servo control which is suitable for setting an output signal to a predetermined constant reference value, independently of undesirable variations in the signal from the sensor (10). The value of the output signal is sampled, and at each sampling moment, if the absolute value of the variation in the output signal is less than a comparison value, the servo control is kept active. A circuit for processing and a pressure-measuring device including a circuit of this kind is also described.