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Description
The design and measurement results of a SoC readout ASIC, called FLAXE, developed for the ECAL-p, the electromagnetic calorimeter at the LUXE experiment are presented. The FLAXE consists of 32 channels with programmable gain front-end, fully differential shaper, and a 10-bit SAR ADC in each channel, working nominally at 20 MSps. Due to a very low bunch crossing rate of 10Hz foreseen for the LUXE experiment, the ASIC is equipped with an internal DAQ memory, read via the SPI bus. The detailed characterization of the FLAXE, performed with a dedicated test setup is presented and discussed.
Summary (500 words)
The LUXE (Laser Und XFEL Experiment) is an experiment planned as an extension to the European XFEL facility at DESY, Hamburg. Its main target is to explore the phenomena associated with strong-field QED (SFQED) above the Schwinger limit, and to search for new particles, beyond the Standard Model, coupling to photons. The two modes of operation are foreseen, colliding the European XFEL electron-beam directly with a high-power, tightly focused laser beam or converting the electron beam to a high-energy photon beam and then colliding it with the laser.
A granular and compact (small Moliere radius) sampling electromagnetic calorimeter, called ECAL-p is foreseen as a part of the LUXE detector system. It will consist of 16 to 20 layers of 3.5 mm (1X0) thick tungsten absorber plates, and silicon sensors placed in a 1 mm gap between absorbers. Each sensor layer covers 55x5.5 cm2 and is segmented with 5x5 mm2 pads, resulting in approximately 25 to 30 thousands of channels per whole calorimeter. For readout electronics, a 32-channel SoC readout ASIC, called FLAXE, was developed in CMOS 130 nm technology, with the die size of 4020 um x 3700 um. Each readout channel is composed of an analogue front-end with two gain modes, a CR-RC shaper, and a 10-bit SAR ADC. The ASIC is equipped with biasing and trimming DACs, and a calibration injection circuitry.
Under experimental conditions, the charge deposition per channel varies from few fC (for MIP) to few pC at the electromagnetic shower maximum. To cover this wide deposition range, two programmable gain modes are developed in the front-end, one optimized for good SNR for MIP depositions, and the second designated for larger depositions. For the pedestal stabilization and to accommodate for the sensor leakage current, the Krummenacher feedback is used. Its circuitry was modified to allow for coarse pedestal setting, whereas the fine-tuning feature is provided in each channel by trimming DACs. The front-end is followed by a pole-zero cancellation circuit, and a fully differential CR-RC shaper, with 50 ns peaking time, matching the SAR ADC input without need of an additional single-ended to differential signal conversion. The ADC samples the shaper output with a nominal 20 MSps rate, with the possibility of increasing the rate up to 50 MSps. The channel layout occupies 2780 um x 80 um (1350 um x 80 um is used for decoupling of the ADC reference voltage).
Due to the very low repetition rate of 10Hz of the LUXE experiment, the data is read through the SPI-based bus between bunch crossings. The SPI-based circuitry comprises the slow control registers and the internal DAQ memory, collecting up to the 64 consecutive ADC samples from each channel around each bunch crossing. Additionally, to significantly reduce the power consumption, ASIC works in a so-called power-pulsing mode, where majority of the circuitry (analogue front-end, ADC) is switched off between bunch crossings.
The FLAXE has been characterized with a dedicated test setup, and the results will be presented in this contribution.