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Describes a new modulation-demodulation technique for isolation amplifiers, particularly useful for multichannel data acquisition systems. Input and output stages are transformer coupled using an improved version of the amplitude-modulation technique of a symmetric square wave, which requires a simple rectifier bridge for demodulation. Characteristics of the isolation stage obtained in this way are excellent linearity and gain stability, typical of magnetic coupling, and a frequency bandwidth ranging from DC to an upper limit which, at least in principle, is not fixed by the demodulation technique; moreover, common-mode voltages in the kilovolt range are obtainable. Details of an amplifier are given, designed as the analog input stage of a multichannel data acquisition system.