Novel Fully Integrated OTA Based Front-End Analog Processor for X-rays Silicon Strip Detectors T. Noulis, C. Deradonis, S. Siskos Electronics Laboratory of Physics Department Aristotle University of Thessaloniki Thessaloniki, Greece tnoul@physics.auth.gr, siskos@physics.auth.gr G. Sarrabayrouse Laboratoire d’Analyse et d’Architecture des Systèmes LAAS – CNRS Toulouse, France sarra@laas.fr Abstract—A space application fully integrated preamplifier - shaper system for X-rays silicon strip detectors is developed. An operational transconductance amplifier (OTA) based shaper architecture with continuous variable peaking time and programmable gain is designed using advanced filter design techniques. Considerations about the noise performance, the bandwidth and the output signal processing flexibility are also presented. The system was designed in 0.6 m AMS process. Analysis is supported by simulation results, which confirm the exceptional system performance. I. INTRODUCTION Radiation detection has been increasingly developed in various fields of radioactivity control, medical imaging and space science. The current trend is towards smaller, higher channel density and lower power systems that provide better position resolution and may also be required to be radiation hard. CMOS technologies are chosen for the implementation of integrated front end systems due to their high integration density, relatively low power consumption, and capability to combine analog and digital circuits on the same chip. The preamplifier–shaper structure (Fig. 1) is commonly adopted in the design of the above systems. Integrating both preamplifier and shaper features such as variable pulse shaping and programmable gain into a single application specified integration circuit (ASIC) could provide a compact and low-cost detector system with excellent noise performance [1] - [3]. Although the above architecture was sufficiently studied mainly in terms of the charge sensitive preamplifier (CSA) input transistor for noise reduction, few studies have been performed on pulse shapers. Despite that fact, semi-Gaussian (S-G) shapers are the most common pulse shapers employed in readout systems [4], [5]. In this paper an analysis concerning front-end systems semi-Gaussian shapers is carried out. Design considerations about noise performance, bandwidth and output signal processing flexibility are also examined. An advanced OTA based shaper design technique, which provides full integration, is used and a novel CR-RC 2 implementation with programmable gain and variable peaking time is proposed. II. SEMI-GAUSSIAN SHAPER ANALYSIS A semi-Gaussian shaper principal schema is shown in Fig. 2. A high-pass filter (HPF) sets the duration of the pulse by introducing a decay time constant. The low-pass filter (LPF), which follows, increases the rise time to limit the noise bandwidth. Although pulse shapers are often more sophisticated and complicated, the CR-RC n shaper contains the essential features of all pulse shapers, a lower frequency bound and an upper frequency bound and it is basically a (n+1) order band pass filter (BPF), where n is the integrators number. The transfer function of an S-G pulse shaper consisting of one CR differentiator and n integrators (Fig. 2) is given by [6]: BPF n i d d s H s A s s H(s) ) ( 1 1 (1) where d is the time constant of the differentiator, i of the integrators, and A is the integrators dc gain. The number n of the integrators is called shaper order. Peaking time is the time that shaper output signal reaches the peak amplitude and is defined by s = n i . The order n and peaking time s , depending on the application, can be predefined by the design specifications or not. This work is financially supported by the program Platon 2003-2005 (scientific cooperation between GSRT – Greece and CNRS – France). Figure 1. Block diagram of a detector readout system. IEEE MELECON 2006, May 16-19, Benalmádena (Málaga), Spain 1-4244-0088-0/06/$20.00 ©2006 IEEE 47