1. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 1 HCAL Upgrade 2016(18?) SiPM to QIE10 Coupling / FY2012 Sergey Los FNAL/CMS/HCAL

2. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 2 IntroductionIntroduction AC coupled scheme for SiPMs was a success at the test beam tests, and this scheme works for HO Upgrade Now that time is running towards SiPM upgrade for HB/HE, it’s time to look again, and select appropriate coupling for that application Damn AC ! Customer is ALWAYS right!

3. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 3 Basic AC/DC Coupling for SiPM/QIE DC coupling is just it, but with some complications: SiPM output is directly fed into QIE Full charge, no additional shaping QIE input has an offset from 0V AC coupling uses a series capacitor, which attenuates, and shapes signal going into QIE External drain resistor is used to drain SiPM leakage current Performance of capacitive attenuation scheme is well understood, but compromises have to be made (say undershoot amplitude vs. BV drop across the drain resistor)

4. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 4 AC/DC Pros and Cons IssueDCAC AttenuationNot practical, the best solution is to have QIE sensitivity/dynamic range tailored to a specific SiPM gain Simple and stable Leakage current measurement High end measurement, not trivial for 100V bias voltage, but doable, some non- linearity should be expected Low end measurement across the drain resistor Voltage drop on drain resistor Virtually does not exist, as QIE input impedance is much smaller than typical drain resistor Luminosity dependent, can be adjusted with slow control. High value drain resistor is needed to lower undershoot value, and improve accuracy of leakage current measurement Radiation induced leakage current effects (10uA for Zecotec, 3E12, G=50,000, 300uA for a 2x2mm Hamamatsu, G=650,000 ) Pedestal goes up (QIE10 80 LSBs for 10uA) Baseline goes up, RMS increases Pedestal stays the same Baseline stays the same RMS increases (same as DC) Luminosity dependent effects (Minbias event spectrum per single diode?) Pedestal goes up Signals sit on top of leakage current, and signal pile-up Signal pile-up depends on luminosity Pedestal stays the same Signals sit on average on the same pedestal value Signal pile-up depends on luminosity Pedestal – what you measure with a random trigger Baseline – moments when there are no signals, tails, or undershoots from minbias events

5. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 5 AC/DC Pros and Cons Individual signal effectsSignal sits on top of leakage current (made mostly by single pe signals), and the rest of minbias events Signal has an undershoot, which can peg pedestal value into zero, duration of undershoot is anti proportional to it amplitude SiPM recovery timeDiode recovery time is equal (almost) to the individual cell recovery time, determined by the device itself After a signal, when a big percentage of cells was fired, BV sags for all cells for a long time (Cd x Rdrain), similar effect happens for SiPMs with fast recovery, and multi-hit capability A solution was found for this in a form of a ballast capacitor in parallel with the diode Att = Catt / (Cd + Cballast + Catt) Cballast Rdrain

6. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 6 Rate, and Pulse Shape Effects Radiation induced leakage current (10uA for Zecotec at 3E12 p/cm2, G=50,000) –That is 30 single pe signals per TS at the end of the life (40fC/5pe noise per TS) –For DC coupled solution that means pedestal increase by 80 LSB counts, and being in bigger size bins. Might be o.k. as is, as the noise at that point is 13 LSB. Leakage current can be compensated for inside the QIE10, or externally, or not at all –For AC coupling this current generates a voltage drop across the drain resistor, and has to be compensated for (say 10uA*10Kohm=100mV, at 25mV resolution of a 12bit, 100V BV DAC). Pedestal value does not change, noise increases the same way as in DC case Luminosity dependence (up to 1uA (25fC, 8LSB, 10mV@10Kohm) per tower for a Zecotec diode at G=50,000, and SLHC luminosity –Most of those signals are MIP signals (around 10 pe, or 30 LSB, and 12.5MHz) –For DC coupling that gives a pile-up of MIP-style signals, effective pedestal increase, and noise increase –For AC coupling there is no pedestal increase, but the same noise increase Large signal effects –For DC coupled system there are no surprises here besides familiar pile-up –For AC coupled system the mostly notable issue is that for a small pedestal value, and a reasonably sized undershoot (1-10% of signal amplitude), QIE readout is going to be pegged to zero for a certain duration after signals above some amplitude. My gut feeling is that this is going to introduce a negligible dead time, but it is better if somebody can run a simulation for this (spectrum of higher amplitude signals times undershoot time, which is proportional to the amplitude)

7. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 7 Technical Issues BV generation/regulation –HO solution for this is applicable to both AC/DC coupling, and has been tested to 4.2E12 p/cm2 (going to BV <100V will help a lot, KETEK rocks!) Leakage current measurement –HO solution will work for AC coupling –DC coupling will require high end current measurement, which is not as trivial, given a 100V BV (think have a reasonable solution, will be ready for the 2012 Summer TB) –Do we need more than 12 bit resolution in sight of higher radiation induced leakage currents? Signal attenuation –Passive component attenuation for AC coupling, not sensitive to the QIE Rin –A whole R&D is required if we want to have attenuation for DC coupled QIE10, we’d better just redesign QIE sensitivity for the selected SiPM gain: –QIE10 input impedance changes with input signal amplitude (10  5 ohm) –QIE10 input is not referenced to zero, but to a 1.2V reference voltage –Current mirror splitter is probably the best option for DC attenuation, but linearity, bandwidth, and real estate are the issues (which Tom Z. has already solved inside the QIE) Signal saturation –Main AC coupling disadvantage of having additional saturation source due to voltage drop across the diode, when many pixels fire was figured out how to deal with during the summer 2011 TB – ballast capacitor in parallel to the diode (extra gain of SiPM only helps here, as it allows to lower the value of the attenuating capacitor, speeding up the QIE response!!!

8. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 8 SummarySummary There is still pretty much a single issue with AC coupling – undershoot after a big pulse Signal saturation due to a voltage drop has a ballast capacitor solution Undershoot effect should be studied with numbers in hand (estimating dead time caused by undershoot for different undershoot, and pedestal values) DC coupling presents a number of technical challenges Signal attenuation scheme Hopefully not an issue anymore /Leakage current measurement/ Pedestal shift compensation (may be not needed for radiation induced leakage current, as it is stable, but can be a plus for luminosity dependent shift) Are there indeed issues with physics information extraction when using AC coupled design

9. S. Los HCAL Upgrade Workshop, Nov. 08, 2011, FNAL 9 SummarySummary