Versatile free-running ADC-based data acquisition system for particle detectors

A high-density and low-power (<300 mW/channel) data acquisition (DAQ) system integrating 2240 readout channels inside a single OpenVPX crate is presented. It is intended to be used in various applications, e.g. gaseous or scintillator-based particle detectors. 14 payload slots, controller and data concentrator communicate via a multi-gigabit backplane. Each payload slot consists of a front module for digital and a rear transition module for analog processing. A pair of modules implements 160 full readout chains including amplification/shaping, sampling, and feature extraction. The sampling rate and ADC resolution are configurable for 100-1000 MS/s and 14-8 bit, respectively. To extract the pulse arrival time we used a proper shaping of signals and a timing algorithm based on a non-linear rise approximation, which strongly reduces the sampling phase error, thus providing a good performance at moderate (100-250 MS/s) sampling rates. This allows raw data acquisition, arrival time extraction, energy calculation, and pile-up reconstruction for all 160 channels to be implemented in a single FPGA. It occupies less than 30% of its processing resources. The system was tested using the proton beam at COSY (Cooler Synchrotron) at Juelich Research Center (Germany). An off-detector DAQ with a 12-m long cabling was investigated. It introduces considerable additional thermal noise and crosstalk affecting the timing performance. For a sampling rate of 150 MS/s, the time resolution was found to be about 270 ps.