The read-out chip needed to be able to exploit the full potential of a picosecond pixel sensor is very challenging. The basis of our approach is to use the relatively large area of the pixel to perform all signal processing already at the pixel level. An Application Specific Integrated Circuit (ASIC) solution is the natural choice for the read-out electronics of the UFSD detector. Such a solution will benefit from the performances of the most advanced technologies both in terms of circuit density (for space resolution) and of circuit speed (for time resolution).

We will consider several possible architectures; in particular we will carry out a comparison of an analogue approach based on Time to Amplitude Converter (TAC) with a digital approach based on Phase Locked Loops (PLL) and Delay Locked Loops (DLLs). A very important step to be performed during the first year of the project is the selection of the best-suited technology. The industry is now moving from the 130 nanometres to the 65 nanometres production technology, with a consequent increase in speed, which might be necessary to achieve our goal. We will also explore the use of strained lattice bi-CMOS technologies based on hetero-junction bi-polar devices (SiGe), since their superior analog performance allows the use of previous generation CMOS processes keeping the costs under control. This is an important consideration given the sharp increase in cost that newer technologies require.

Due to the unprecedented time resolution requirement of our project, while considering the various options outlined above, one needs to take into account the timing errors due to the jitter induced by noise, the non-linearity introduced in digitization and the time-walk deriving from statistical fluctuation in the total charge released by the particle in the sensor. Likewise, due to the large amount of logic needed for computation, the substrate noise is a critical issue. A very interesting shielding option, consisting in embedding the full digital logic in a dedicated well, will be explored.