Prototype of readout electronics for the LHAASO KM2A electromagnetic particle detectors

The KM2A (one kilometer square extensive air shower array) is the largest detector array in the LHAASO (Large High Altitude Air Shower Observatory) project. The KM2A consists of 5242 EDs (Electromagnetic particle Detectors) and 1221 MDs (Muon Detectors). The EDs are distributed and exposed in the wild. Two channels, anode and dynode, are employed for the PMT (photomultiplier tube) signal readout. The readout electronics designed in this paper aims at accurate charge and arrival time measurement of the PMT signals, which cover a large amplitude range from 20 P.E.(photoelectrons) to 2 × 10⁵ P.E. By using a 'trigger-less' architecture, we digitize signals close to the PMTs. All digitized data is transmitted to DAQ (Data Acquisition) via a simplified White Rabbit protocol. Compared with traditional high energy experiments, high precision of time measurement over such a large area and suppression of temperature effects in the wild become the key techniques. Experiments show that the design has fulfilled the requirements in this project. (authors)