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TDC/ADC vs analog memories for barrel PID. D. Breton

TDC/ADC vs analog memories for barrel PID. D. Breton. PID barrel session - Frascati Meeting December 1 st 2009. Introduction. A robust and simple TDC based solution (BABAR- like ) can be considered as a baseline for the PID readout .

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TDC/ADC vs analog memories for barrel PID. D. Breton

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  1. TDC/ADC vs analog memories for barrel PID.D. Breton PID barrel session - Frascati Meeting December 1st 2009

  2. Introduction • A robust and simple TDC based solution (BABAR-like) canbeconsidered as a baseline for the PID readout. • Charge measurementcouldbeadded if necessary, at least for PMT agingsurvey. • There is no obvious use of it for physics up to now. • Couldbenecessary for discriminator time walk correction • Analogmemories are another good solution, which has been proven to bevery effective and useful for the characterizationbenches. • They are the baseline for the TOF electronics • The question is, canwe envisage usinganalogmemories for the barrel with the followingconstraints: • High number of channels => 30,000 • High trigger rate => 150 kHz • No T0 for trigger matching => ~ 200 ns window • Littlephysicalspace and radiation on the detector • => limitation of electronics in terms of quantityand « intelligence » on detector • => limitation of power

  3. Pros and cons … • Here follow some feelings (from Jerry and me) about analog memories: • The positive side of analog memories: • One gets the signal shape, thus all the information! • Capability to apply a digital filter to remove a known noise, if present. • Incredible flexibility to come up with very clever timing schemes (CFD, reference pulse timing, spline interpolations, etc.). • To Jerry, at the moment, a software CFD algorithm is a winner as it can be done very fast. In fact one gets used to spline interpolation very fast... • Capability to deal with overlapping pulses, cross-talk, etc. • Capability to deal with unforeseen events. • The negative side: • One gets a lot of samples if: • One doesn’t know where the useful samples do reside • The sample rate is very high compared to the signal shape • One can spend incredible amount of time with just two channels, as Jerry proved to himself. Not yet clear how to deal with 30,000 pixels. • As one has to digest the waveforms in real time, not clear how fast the algorithm has to be.

  4. Back to barrel PID • No T0 for trigger matching => ~ 200 ns window • If we sample at ~2.5GS/s, we would need 500 samples to cover the time window (each sample has to be converted into 12 bits by an ADC) • => totally unreasonable! • Signal from the MAPMT is much shorter (~10 ns useful) • => we need to tag the position of the signal inside the trigger window with a discriminator/TDC in order to be able to come back for reading the corresponding samples after L1 decision. • If we don’t want this TDC to be very precise (10ns steps), and as we need to cover at least three time steps in order to recover from the digitization uncertainty, we end up with ~75 samples/event. • The whole signal shape is not useful permanently during physics run, mostly for debugging and monitoring purpose • => Time and Charge could be extracted from samples thanks to digital filtering • => but this operation is expensive in terms of processing and power if performed by FPGAs on the detector (without forgetting radiation mitigation) • => it should better be done in the ROM • => the readout links have to digest the whole sample information

  5. Summary • TDC/ADC solution is the simplest working solution • We already have a 16-channel/100-ps TDC in hand • We need to optimize it for SuperB • We get ~32 bits per hit without charge, 48 bits with charge. • The only difficulty here is the discriminator • Could be power consuming to get the necessary precision • If there is a time walk, charge is necessary for its correction • Could be done on-detector, or in the ROM, or offline … • We have ideas about walk-less solutions => to be tested … • Analog memories give a much richer information • but their implementation is trickier, especially with such a high number of channels and no pointing to events’ T0 • they would require more computing and power on the detector • if we want to limit the latter, the corresponding dataflow will be significantly higher, thus the number of readout links (factor >> 10) • There is no definitive leadership of TDC/ADC solution as of today • => we need to farther study the case to make up our minds !

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