Signal Processing for High Granularity Calorimeter: Amplification, Filtering, Memorization, and Digitalization

A dedicated very-front-end (VFE) readout channel for high-granularity calorimeters has been designed, fabricated and evaluated. This electronics is composed of a low-noise charge-sensitive amplifier followed by a bandpass filter based on a gated integrator. The efficiency of the gated integrator, a time-variant filter, in terms of noise rejection has been compared to an equivalent time-invariant filter, a CRRC shaper. Their serial and parallel noise indexes have been evaluated by simulation for different values of integration and peaking times. Results show that the gated integrator is a more efficient filter than the CRRC shaper, thanks to the time of integration which can be expanded to approach the time interval between two successive bunch crossings, whereas the peaking time of the CRRC shaper is limited by pile-up considerations. Moreover, the gated integrator intrinsically includes the capacitor required to perform the analog memorization of the signal before its delayed digital conversion. This analog-to-digital conversion is carried out by a 12-bit cyclic ADC specifically developed for this application. The very-front-end channel has been evaluated using a 0.35 μm CMOS technology. Measurements show a global non-linearity better than 0.1%. The Equivalent Noise Charge at the input of the channel is evaluated to 1.8 fC, to be compared to the maximum input charge of 10 pC. The power consumption of the complete channel is limited to 6.5 mW.