Optimization of the ATLAS (s)MDT readout electronics for high counting rates

In the ATLAS muon spectrometer, Monitored Drift Tube (MDT) chambers are used for precise muon track measurement. For the high background rates expected at HL-LHC, which are mainly due to neutrons and photons produced by interactions of the proton collision products in the detector and shielding, new small-diameter Muon Drift Tube (sMDT)-chambers with half the drift tube diameter of the MDT-chambers and ten times higher rate capability have been developed. In order to avoid baseline shifts of the shaped signal the standard MDT readout electronics uses bipolar shaping which, however, leads to a deterioration of signal pulses due to preceding background hits at high counting rates, leading to losses in muon efficiency and drift tube spatial resolution. These so-called signal pile-up effects can be mitigated by active baseline restoration (BLR), which can also eliminate the baseline shift in the case of unipolar shaping. Discrete multi-channel prototype electronics of both types have been tested with generated input pulses and in the Gamma Irradiation Facility at CERN under high $\gamma$-irradiation rates.