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 optimized for the time projection chamber (TPC) of the ALICE experiment at the CERN LHC, its architecture and programmability make it suitable for the readout of a wider class of detectors. In one single chip, the analog signals from 16 channels are digitised, processed, compressed, and stored in a multi-acquisition memory. The analog-to-digital converters embedded in the chip have a 10-bit dynamic range and a maximum sampling rate in the range of 20-bit. After digitization, a pipelined Data Processor is able to remove from the input signal a wide range of perturbations, related to the nonideal behavior of the detector, temperature variation of the electronics, environmental noise, etc. Moreover, the Data Processor is able to suppress the pulse tail within 1 (mu)s after the peak with 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 nonzero 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 300 Mbyte/s. This paper addresses the algorithms of the implemented digital functions and the performance of the ALTRO chip.