A pair of small angle silicon-tungsten (Si-W) calorimeters has been bu
ilt to measure the luminosity to a precision better than 0.1% in the O
PAL experiment at the Large Electron Positron (LEP) collider at CERN n
ear Geneva. Each calorimeter contains 19 layers of tungsten (W) plates
and silicon (Si) detectors, corresponding to a total of 22 radiation
lengths, sampled by about 1 m2 of detectors divided into 304 x 64 inde
pendently read out channels. A complete electronics system has been de
veloped, from the preamplifier up to the VME read out and control inte
rface. It includes a fast trigger based on analogue sums. This paper d
escribes how a large number of channels have been implemented in a den
se environment, thanks to the use of ASIC's directly bonded on the det
ector.