Towards a single-photon energy-sensitive pixel readout chip: pixel level ADCs and digital readout circuitry

Unlike conventional CMOS imaging, a singlephoton imager detects each individual photon impinging ona detector, accumulating the number of photons during acertain time window and not the charge generated by the allthe photons hitting the detector during said time window.The latest developments in the semiconductor industryare allowing faster and more complex chips to be designedand manufactured. With these developments in mind we areworking towards the next step in single photon X-ray imaging:energy sensitive pixel readout chips. The goal is not only to detect and count individual photons, but also to measure the charge deposited in the detector by each photon, and consequently determine its energy. Basically, we are aiming at a spectrometer-in-a-pixel, or a 'color X-ray camera'. The approach we have followed towards this goal is the design of small analog-to-digital-converters at the pixel level, together with a very fast digital readout from the pixels to the periphery of the chip, where the data will be transmitted off-chip. We will present here the design and measurement on prototype chips of two different 4-bit pixel level ADCs. The ADCs are optimized for very small area and low power, with a resolution of 4-bits and a sample rate of 1 Msample/s. The readout architecture is based around current-mode sense amplifiers and asynchronous token-passing between the pixels. This is done in order to achieve event-by-event readout and, consequently, on-line imaging. We need to read eventby-event (photon-by-photon), because we cannot have memory on the pixels due to obvious size constraints. We use current-mode sense amplifiers because they perform very well in similar applications as very fast static-RAM readout.