Fully Differential Readout Electronics for Coplanar-grid Detectors James E. Toney* a , Christopher M. Baer a , Nader Kalkhoran b a Battelle Memorial Institute, 505 King Ave., Columbus, OH 43201 b Spire Corporation, One Patriots Park, Bedford, MA 01730 ABSTRACT We have demonstrated a new amplifier topology for coplanar-grid detectors that provides true differential readout with a single, fully-differential, charge-sensitive preamplifier. A prototype, multi-detector system with adjustable gain for each detector has been demonstrated using this fully differential approach. In its initial implementation using general-purpose amplifier chips, this system produces comparable noise performance to the traditional two-amplifier readout with specialized, charge-sensitive preamplifiers. In lieu of a differential gain to correct for electron trapping, the circuit uses a differential sampling scheme. This method enables a symmetrical photopeak to be obtained, but introduces some undesired filtering that limits energy resolution. Keywords: Coplanar-grid, charge-sensitive preamplifier, fully differential amplifier 1. INTRODUCTION Of the numerous electrode structures and readout schemes that have been demonstrated for large-volume cadmium zinc telluride gamma-ray spectrometers, the coplanar grid 1 (CPG) represents the best trade-off between energy resolution, active volume and simplicity. One drawback of the CPG is the need for two charge-sensitive preamplifiers to integrate the charge on the separate grids. In addition to increasing power consumption and circuit complexity, the second amplifier increases the minimum obtainable noise level by a factor of √2. An alternative approach using a unipolar readout has also been demonstrated 2 , but in that scheme the electrode design is dependent on the crystal properties, which makes it difficult to optimize performance for each detector. We have demonstrated a fully differential charge-sensitive preamplifier configuration that performs a true differential readout of CPG detectors with a single amplifier. The traditional approach to processing signals from coplanar detectors involves two separate charge sensitive preamplifiers to integrate the charge on the individual grids followed by an instrumentation amplifier (IA). The output of the IA is connected to the input of a pulse shaping amplifier, which in turn is connected to the multi-channel analyzer, as illustrated in Figure 1(a). We have adopted a novel approach that maintains a fully differential signal path throughout, thereby replacing the two preamplifiers and the IA with a single, fully-differential, charge-sensitive preamplifier. This approach is illustrated in Figure 1(b). The fully differential configuration is maintained through the shaping amplifier stage. Conversion to a single- ended output is performed in the final stage, which also sums the signals from four detector channels. 2. PREAMPLIFIER DESIGN Our differential, charge sensitive amplifier is based on the THS4131 fully differential op amp from Texas Instruments. The noise performance (1.3 nV/√Hz, input-referred) and gain-bandwidth product (150 MHz) of this chip make it well suited to the application. However, an external FET input stage is needed to provide the high input resistance needed for charge sensing. The dual external FET (Siliconix SST441) used for the preamplifier input stage was chosen based on its superior noise and gain characteristics at the bandwidth required for the preamplifier. A low- noise, well-matched FET pair with both low input capacitance and high input resistance was required to attain superior noise performance. Hard X-Ray and Gamma-Ray Detector Physics V, edited by Larry A. Franks, Arnold Burger, Ralph B. James, Paul L. Hink, Proceedings of SPIE Vol. 5198 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.504882 172