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Description
CoRDIA is an X-ray imager being developed, for Photon Science experiments at 4th generation Synchrotron Rings. Its goal is to be capable of continuous operation at 150k frame/s. Its Analog Front-End consists in a battery of adaptive-gain amplifiers and Analog-to-Digital converters, arranged in a pipelined, modular structure compatible with a compact pixel pitch (110um). A test structure has been designed using a 65nm process and was characterized, confirming expected performances in terms of noise, linearity, and adaptive gain operation at the operational speed. Some cross-talk issues have been identified, and an updated shielded layout was designed to solve them.
Summary (500 words)
CoRDIA (Continuous Readout Digitising Imager Array) is a detector being developed as a collaboration between DESY and University of Bonn. The purpose of a collaboration is to provide an X-ray imager for Photon Science experiments at 4th generation Synchrotron Rings and at Free Electron Lasers considering Continuous Wave operation.
The imager goals include continuous operation at a frame rate of ~150 kHz, while keeping a compact pixel pitch (~110um); and single-photon discrimination capability at 12 keV, while also being compatible with high-Z sensors for experiments at higher energies.
The imager uses an adaptive gain circuit to extend its dynamic range to ~2k photons per pixel per image (~300 Mcount/s) by an adaptive gain circuit. Additional dynamic range extension is possible by selective blinding, in case of constant-flux experiments (High Dynamic Range mode).
Its structure includes Analog-to-Digital converters integrated in the pixel array, to provide full digital readout while maintaining 4-side buttablilty. In order to fulfill the continuous frame rate specification, it adopts a pipelined signal-processing architecture, so that one image is digitized while the next one is being acquired. The data is then read out using the the GWT-CC circuit developed by NIKEF for Timepix4.
A first prototype including circuit blocks (Charge integrating Amplifier, Analog-to-Digital Converter) had been manufactured and tested in TSMC65nm technology [1].
A second prototype had been designed ([2]) and has been now tested.
The second prototype includes the CoRDIA pipelined Analog Front-End, arranged as a modular structure that can be arrayed a 2D matrix with the target pixel pitch. It also include calibration circuits (a pulsed capacitor and a pulsed current source circuit) that allow for electrical characterization in absence of a real photon input, and have been used for the characterization.
Characterization of the pipelined Front-End circuit confirm a readout noise (255e) that is compatible with the single-photon discrimination goal. The adaptive-gain circuit was confirmed to behave as expected at the operational frame rate (150 kframe/s). The ratio between gains (~31) and the gain switch point (~50 12keV photons) match expectations. Linearity was also evaluated and found to remain mostly within the +/-2% cone. The noise was confirmed to remain below the Poisson limit.
Some crosstalk shortcomings have been identified in the circuit, due to insufficient shielding between the Sample/Hold cells (that act as an interface between the Charge Amplifiers and the Analog-to-Digital converters) and the top-metal layers connecting to the bump-bond pads for sensor integration. Similarly, the readout line carrying the gain-switch status was found to perturb the analog output of selected pixels before digitization.
The effects have been reproduced in simulation, and a better-shielded layout have been simulated, that suppresses the cross-talk effect while not affecting the performances, and will be used in future arrays.
[1] A Marras et al Journal of Physics: Conference Series 2380 (2022) 012093
[2] A Marras et al 2024 JINST 19 C03006