Opera_ROC: A front-end chip for opera multi-anode photomultipliers

This paper presents the ASIC (OperaROC) developed to readout the multi-anode phototubes (PM) used in the Target tracker of the Opera neutrino experiment. It is made of 32 channels of variable gain preamplifiers, shapers, track and hold, multiplexed output and an auto-trigger capability. The channel to channel gain adjustment allows to compensate with 6 bits and up to a factor of 4 the non-uniformity of the PM. This optimizes the dynamic range and allows the use of a common threshold on the chip. The auto-trigger capability discriminates at 1/3 of photoelectron (p.e.) with a dispersion of 0.05 p.e. and performs the hold for the multiplexed charge readout on a dynamic range of 0.1 to 100 p.e. Design considerations and measured performance are presented.