CMOS x-ray image sensor with pixel level A/D conversion

Abstract

This paper describes a pixel array for x-rays imaging consisting in 400 um x 400 um photodiodes fabricated in CMOS technology, with an A/D converter for each one. Above the photodiodes, an array of scintillating CsI:Tl crystals are placed. So, the x-ray energy is first converted to visible light by the scintillating crystals which is then detected by the photodiodes. The photocurrent produced by each photodiode is finally converted to a digital form by a sigma-delta analog to digital converter. The sigma-delta a/d converter uses 18 minimum-size MOSFETs and one capacitor. 8 to 10 bits of resolution can be achieved.