The performance of the CMS electromagnetic calorimeter upgraded readout electronics,
developed for the High-Luminosity phase of the LHC, is discussed. Data collected in two beam
test campaigns conducted in 2018 and 2021 at the H4 and H2 beam lines of the CERN SPS are
analyzed. Time and energy resolutions are measured on a 5 × 5 matrix of lead tungstate crystals
equipped with prototypes of the new front end readout electronics, using electron and pion beams of
energies spanning from 25 to 250GeV. In both campaigns the constant term of the energy resolution is
measured to be better than 0.6% and the time resolution for electrons with energies above 50GeV
is measured to be better than 30ps, fulfilling the design requirements.
