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An innovative balanced photodiode structure made in amorphous silicon/amorphous silicon carbide, suitable for detecting small current variations on a large background signal, is presented and characterized. The structure is a three-terminal device, constituted by two series-connected n-i-p photosensors, where the output signal is the difference between the currents flowing through the two diodes. The layer thickness and optical properties of the thin-film materials and the geometry of the structure have been optimized for the detection of ultraviolet radiation. Common mode rejection ratio (CMRR) values ranging between 30 dB at 254 nm and 42 dB at 365 nm have been measured, independent on the bias voltage. The decrease of the CMRR at lower wavelengths has been ascribed to differences in the surfaces of the two diodes exposed to the light.