3.3-MHz charge sensitive preamplifier

Abstract

An optical monitor for the European Space Agency satellite project XMM is under development at MSSL. One option being investigated is the use of an intensifier with a ‘‘SPAN’’ anode readout. The SPAN anode gives 2048×2048 resolution elements from 6 analog signals each digitized to 8 bits. A count rate of 200 000 counts/s is anticipated for the optical monitor which places major constraints upon the analog signal processing electronics. To accommodate the optical monitor’s requirements a charge sensitive electronic system is being developed. The capacitive coupling (∼100 pF) between anodes, which has already been minimized, causes a cross-coupled signal to be observed in the impulsive phase of the pulse. It is shown both theoretically and experimentally that this cross coupling can be removed with a perfect integrator. The principle of operation is to employ an analog switch after the charge integrator to reject the impulsive phase. To obtain good noise characteristics, a long integration time (∼6 μs) has been chosen. However, this leads to pulse pileup in the feedback capacitor of the integrator. Since the rate of decay will depend upon the number of pulses piled up, switched compensation in the form of an RC circuit is employed in parallel with the signal line. Detailed circuit design and performance characteristics are given.