1. A. Rivetti Gigatracker meeting, dec 2009 Charge measurement with the TDC per pixel architecture A. Rivetti, G. Dellacasa S. Garbolino, F. Marchetto, G. Mazza I.N.F.N. Sezione di Torino, via P. Giuria 1 Torino, 10125, Italy

2. A. Rivetti Gigatracker meeting, dec 2009 Assuming 200  m thick silicon (1 mip=2.4 fC): The fraction of particle releasing more than 4 mips is 9 x 10 -3 The fraction of particle releasing more than 5 mips is 6.1 x 10 -3 Basic facts (1) Key questions: Where to put the threshold? How many levels?

3. A. Rivetti Gigatracker meeting, dec 2009 In the TDC per pixel architecture the duration of the CFD pulse is not proportional to the input charge, so it can not be used to put a threshold on high charges. Basic facts (2) CFD pulse duration as a function of the input charge The information must be extracted at the front-end amplifier Simplest approach: leading edge comparator after the front-end amplifier

4. A. Rivetti Gigatracker meeting, dec 2009 Reminder: front-end amplifier topology Transimpedance amplifier with fully differential outputs It can handle signals of either polarity. Linear from 5000 to 60000 electrons Leakage compensation works for ± 200 nA of leakage current

5. A. Rivetti Gigatracker meeting, dec 2009 Front-end amplifier schematics Fully differential amplifier with triple well NMOS transistors for maximum insulation from the substrate Class AB output stage to reduce the power consumption

6. A. Rivetti Gigatracker meeting, dec 2009 Front-end linearity The gain has been set to saturate at 10 fC (maximum expected for “good” signals) Hard to set a reliable threshold at 10 fC in this case and practically impossible to have a higher one

7. A. Rivetti Gigatracker meeting, dec 2009 Powering the front-end at 1.4 V Linear dynamic range is significantly extended. Front-end behavior can be tested in the present chip. Bias current is the same, power increases by 15 %.

8. A. Rivetti Gigatracker meeting, dec 2009 Reducing the preamplifier gain Impact on timing performance must be assessed carefully, but preliminary simulations show marginal increase (time walk: from 385 to 420 ps peak to peak, jitter from 66 ps to 75 ps peak-to-peak).

9. A. Rivetti Gigatracker meeting, dec 2009 Simple ToT Preamplifier+leading edge comparator+3 bits counter Not enough to get rid of the CFD for timing information, but it provides more level of charge discrimination. Counting on both edges: 3.125 ns time bin. 1 bit = 5.3 fC

10. A. Rivetti Gigatracker meeting, dec 2009 Summary  Only approaches that require minimal hardware modifications have been considered so far.  My personal conclusions:  For a single threshold increase the power supply at the front-end 1.Marginal increase in power 2.No modification to the preamplifier. Timing properties can be assessed in the present prototype.  If more than one levels are needed a small counter is preferable.  Only one comparator  Same number of bits to be stored