Pixel readout circuit for X-ray imagers

Abstract

This paper describes the per-pixel readout circuit of an X-ray imaging matrix and compares it to other solutions. The per-pixel readout circuit consists of a digital pixel sensor array constituted by a photodiode and a one-bit first-order sigma-delta A/D 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 19 small size MOSFETs and one capacitor. In order to perform the X-ray detection, a scintillator crystal must be placed above each photodiode for converting the X-ray energy into visible light with wavelengths near to the quantum efficiency peak of the photodiodes. Then, the visible light is converted into electronic charges by each photodiode, which is part of digital pixel sensor. The comparison between the solution presented here and other solutions show that the implemented circuit is simpler and its resolution is similar or even superior.