The digital readout system for the CMS electromagnetic calorimeter

The CMS Electromagnetic Calorimeter is a high-precision detector demanding innovative solutions in order to cope with the high dynamic range and the extreme high resolution of the detector as well as with the harsh environment created by the high level of radiation and the 4 T magnetic field. The readout system is partly placed within the detector and partly in the adjacent counting room. As the on-detector electronics must cope with the harsh environment the use of standard components is excluded for this part of the system. This paper describes the solutions adopted for the high-precision analogue stages, the A–D conversion, the optical transfer of the raw data from the on-detector part to the so-called Upper Level Readout, placed in the counting room, and the functionality of the latter. The ECAL is instrumental in providing information to the first-level trigger process and the generation of this information will be described. Also, the problem of reducing the raw data volume (6×1012 bytes/s) to a level that can be handled by the central DAQ system (105 bytes/s) without degrading the physics performance will be discussed.