The ALTRO Chip: A 16-channel A/D Converter and Digital Processor for Gas Detectors

The ALTRO (ALICE TPC Read Out) chip is a mixed-signal integrated circuit designed to be one of the building blocks of the readout electronics for gas detectors. Originally conceived and optimised for the Time Projection Chamber (TPC) of the ALICE experiment at the CERN LHC, its architecture and programmability makes it suitable for the readout of a wider class of gas detectors. In one single chip, the analogue signals from 16 channels are digitised, processed, compressed and stored in a multi-acquisition memory. The Analogue-to- Digital converters embedded in the chip have a 10-bit dynamic range and a maximum sampling rate in the range of 20 to 40MHz. After digitisation, a pipelined hardwired Processor is able to remove from the input signal a wide range of systematic and non-systematic perturbations, related to the non-ideal behaviour of the detector, temperature variation of the electronics, environmental noise, etc. Moreover, the Processor is able to suppress the signal tail within 1mus after the pulse peak with 0.1% accuracy, thus narrowing the pulses to improve their identification. The signal is then compressed by removing all data below a programmable threshold, except for a specified number of pre- and post-samples around each peak. This produces non-zero data packets. Eventually, each data packet is marked with its time stamp and size - so that the original data can be reconstructed afterwards - and stored in the multi-acquisition memory that has a readout bandwidth of 300Mbyte/sec. This paper addresses the algorithms of the implemented digital functions and the performance of the ALTRO chip on measured data. 5 Refs.