A modular electronic readout system for the OPAL electromagnetic presampler

The readout system used to sample and digitize the 21 504 channels of the OPAL electromagnetic barrel presampler is described. The system consists of semi-custom analog electronics mounted directly on the detector to provide integrate-and-hold capability and custom VME boards to implement the digitization functions. The data from one event is digitized completely in under 7 ms. This meets the OPAL requirement of a sustained 10 Hz trigger rate with less than 10% dead time. A minimum ionizing particle deposits 2.7 pC in a single layer of the presampler, and the largest signal commonly seen in 200 pC. The dynamic range of the readout system is 0.032 to 1000 pC. The noise on a single channel is typically 0.2 pC. A linear analog calibration is automatically applied on a channel-by-channel basis. Programmable logic is used extensively.