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.

A Review of Optical Sensors in CMOS

“…Figure 11c depicts the general structure of a digital pixel sensor (DPS), where the data conversion from the analog to the digital domain is carried out at the pixel level by a dedicated ADC controlled by the reset and select signals [65,[99][100][101][102][103]. The output of each pixel is a digital signal with all ADCs working in parallel.…”

Narrow-band pass filter array for integrated opto-electronic spectroscopy detectors to assess esophageal tissue

“…However, since the maximum center to center separation of photodiodes will be around 150 μm it is reasonable to believe that no significant spatial heterogeneity will be introduced across wavelengths. Moreover, the analog photodiodes array readout electronics signals will be converted to digital using a compact Sigma-delta converter, already studied by the research group [ 24 ]. Each photodiode will have one converter.…”

“…Each photodiode will have one converter. It is important to notice that the addition of readout electronics will not increase the size of the optical microsensors since this converter can be designed with only 19 MOSFETs [ 24 ]. This scheme will be able to make a conversion in less than 1 microsecond.…”

Experimental measurement of a high resolution CMOS detector coupled to CsI scintillators under X-ray radiation

“…There are two basic pixel structures that are widely used as CMOS imaging elements: passive pixel sensors (PPSs), which were developed first and the active pixel sensors (APS), which have been developed with the goal of improving the image quality. Usually, an APS has three transistors for each pixel, whereas the PPS has only one (Bohndiek et al, 2008;Elbakri et al, 2009;Rocha et al, 2010). With respect to passive pixel, APS technology offers an increased signal-to-noise ratio of the sensor and dynamic range, at the price of a smaller fill factor (Endrizzi et al, 2013).…”