Low-Noise High-Speed CMOS Detector Readout Electronics

Abstract

In elementary-particle physics, the energies of elementary particles or radiation events are measured or detected by means of a semiconductor detector readout system. A principle schematic of such a detection system is depicted in Fig. 5.1. An inverse biased photo sensitive diode (Si or Ge) detects charged particles or radiation events by generating electron-hole pairs within the detector material. The generated electrons drift in a high electrical field towards the positive-biased n+ contact of the detector and are finally collected. Since the collection time is very short, being in the order of several ns [5.1], [5.2], the detector output signal can be represented as a Dirac current impulse, the integral of which equals the total generated charge Q. The generated charge Q is integrated onto a small feedback capacitor C f . by means of a low noise charge sensitive amplifier (CSA) giving rise to a voltage step at the CSA output with an amplitude Q/C f . The wide use of the CSA at the front end stems from its low noise configuration and insensitivity of the gain to the detector capacitance variations. The step signal is fed to a main amplifier where pulse shaping is performed, primarily to optimize the S/N ratio of the readout system. Therefore, it is called a pulse shaper. In all cases, the resulting output signal is a rather narrow pulse suitable for further processing. Depending on the application, the pulse processing unit can simply be a Multi-Channel Analyzer (MCA) in radiation spectroscopy applications, a series of discriminators for event detections or sample-hold circuits in a multi-channel readout system to store the informations for serial read out. However, for resolution analyses, the detail of the circuits in the processing unit is of no concern and only the CSA and the pulse shaper will therefore be considered in this chapter.