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This paper describes the per-pixel readout circuit of an imaging matrix and compares it with other solutions. The per-pixel readout circuit consists in a digital pixel sensor array constituted by a photodiode and a one-bit first-order sigma-delta analog to digital converter for each pixel. The output of each pixel is a digital bit stream, containing information about the intensity of the light that falls into its photodiode. The sigma-delta A/D converters use only ten small size MOSFETs and one capacitor. The comparison between the solution presented here and other solutions show that the circuit complexity is similar but the performance, in terms of signal to noise ratio, is superior.