Comparative evaluation of analogue front-end designs for the CMS Inner Tracker at the High Luminosity LHCThe CMS Inner Tracker, made of silicon pixel modules, will be entirely
replaced for the operation at the High Luminosity LHC. One of the crucial
components of the new Inner Tracker system is the readout chip, being developed
by the RD53 Collaboration, and in particular its analogue front-end that
receives the signal from the sensor and digitizes it. Three different analogue
front-ends called Synchronous, Linear, and Differential were designed and
implemented in the RD53A demonstrator chip. A dedicated evaluation program was
carried out to select the most suitable design to build a radiation tolerant
pixel detector able to sustain high particle rates with high efficiency and a
small fraction of spurious pixel hits. The test results showed that all three
analogue front-ends presented strong points, but also limitations. The
Differential front-end demonstrated very low noise, but the threshold tuning
became problematic after irradiation. Moreover, a saturation in the
preamplifier feedback loop was limiting the signal return to baseline having an
impact on the dead time. The Synchronous front-end showed very good timing
performance, but also higher noise. The Linear front-end had all the studied
performance parameters within requirements, but it had the largest time walk.
This limitation was addressed and mitigated in an improved design. The analysis
of the advantages and disadvantages of the three front-ends in the context of
the CMS Inner Tracker operation requirements led to the selection of the Linear
front-end for integration in the final CMS readout chip.
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