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SiPM to QIE10 Coupling for HCAL Upgrade

Explore the pros and cons of AC/DC coupling for SiPM/QIE10 coupling in the HCAL upgrade. Discuss technical issues such as signal attenuation, leakage current measurement, and signal saturation.

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SiPM to QIE10 Coupling for HCAL Upgrade

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  1. HCAL Upgrade 2016(18?) • SiPM to QIE10 Coupling / FY2012 • Sergey Los • FNAL/CMS/HCAL

  2. Introduction • 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. 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. AC/DC Pros and Cons Pedestal – what you measure with a random trigger Baseline – moments when there are no signals, tails, or undershoots from minbias events

  5. AC/DC Pros and Cons Att = Catt / (Cd + Cballast + Catt) Rdrain Cballast

  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. 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. Summary • 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. Summary

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