VME-based data acquisition system for high-resolution position-sensing silicon drift detectors

We present the design guidelines and the experimental characterization of a complete acquisition system for the measurement of the amplitude and time-of-arrival of the signal pulses delivered by high-resolution silicon drift detectors (SDDs). The readout system has been equally developed for multi-channel silicon drift detectors and for controlled drift detectors (CDDs) intended for spectroscopic imaging of X-rays or charged particles. The system architecture comprises an analog section, constituted by a VLSI front-end preamplifier and bias current generator for the on-detector JFET follower, and a digital back-end. The digital back-end is realized with 12 bit 100 MS/s 8 channel ADC VME boards which digitize the analog waveforms and store 256 ksample/waveform in memory banks. Each sampled waveform is numerically elaborated to extract the amplitude and time measurements also in presence of an unwanted background signal. The system can be upgraded up to 128 channels per VME crate. The overall linearity error is better than 0.05% and the mean noise over all channels, expressed in terms of equivalent noise charge, is about 4 electrons r.m.s.. The measured time resolution is 1 ns r.m.s. at a signal charge of 15,000 electrons, corresponding to a position resolution of 3 mum r.m.s. along the drift direction. The developed system has been used for X-ray imaging tests with CDDs and SDDs at Sincrotrone Trieste.