6 Mar 2002Readout electronics1 Back to the drawing board Paul Dauncey Imperial College Outline: Real system New VFE chip A simple system Some questions

6 Mar 2002Readout electronics2 Real system I assume the final TESLA calorimeter will have: A total of 32 million channels –Ignor possible reduction in layers for now. Analog preamplifier for every channel –I see no alternative to this; presumably will be in an ASIC. Digital readout via fibre-optic cables –Need 15 bit range, 10 bit resolution; not easy using analog fibre system. Assume digital for now.

6 Mar 2002Readout electronics3 Requirements Looking at both 500 GeV and 800 GeV running: Maximum number of bunches per train ~5k. 500 GeV, 800 GeV Minimum bunch spacing 176 ns – GeV, GeV Total time ~1 ms in both cases – GeV, GeV

6 Mar 2002Readout electronics4 Cosmics HCAL requires cosmic data between trains: Not defined how this will be done Assume data readout within ~10 ms Do multiple “faked” trains before next real train – GeV, GeV allows 10’s of fake trains to be read out per real train ~1 ms live-time every ~10 ms –~10% efficiency; sufficient?

6 Mar 2002Readout electronics5 Data volume Noise dominates rate above threshold Assume ~3  cut so ~0.1% readout, i.e. 32k channels per sample, or 160M per bunch train. Physics rate low; ~10’s physics events per train. Even at 100 showers of 1000 channels each per event, this is small compared with the noise. With 4 bytes/channel, this is ~700 MBytes per bunch train (real or fake) –Physics event data lower by factor ~100. –Cosmics data lower by factor ~10 6

6 Mar 2002Readout electronics6 Data volume (2) Using Gbit links, i.e. ~100 MBytes/s, this needs ~700 links total. –~10 per detector module; sounds feasible Each link serves ~50k channels and moves ~1 MByte.

6 Mar 2002Readout electronics7 On detector tasks On detector, need to do three things: “Digitise” - digitise data. “Cut” - remove data below threshold. “Buffer” - save data during the bunch train. Six permutations of the order of these: Most make sense (but vary in cost) Original idea: digitise, (buffer), cut, buffer –Otherwise need analog threshold and buffer

6 Mar 2002Readout electronics8 Can cut be done on analog data? Unfeasible to have threshold voltage per channel Need uniformity across ASIC (128 channels?) –Pedestal and gain; to what level? 1%? 10%? If not possible, must digitise first –Buffer can be before or after digitise If possible, only one sensible arrangement –Cut, buffer, digitise

6 Mar 2002Readout electronics9 Assume no cut on analog data Digitise before buffer; assume ~10 ns required for analog multiplex settling –16 channels per (F)ADC (~100 MHz) –Total of 2M FADC’s needed Digitise after buffer; assume digitise and cut done as part of readout –Need 5k deep analog pipeline –Can take 10 ms, not 1 ms; 160 channels per FADC –Total of 200k FADC’s needed

6 Mar 2002Readout electronics10 Assume cut on analog data Cut and store analog data –Short analog pipeline; ~5 samples on average, maximum ~100 samples? (NB CMS APV25 has 192-sample analog pipeline) Total data rate down by factor 1000 –Need 160 FADC’s for average, i.e. 200k channels per FADC –Fluctuations; more like 10k channels per FADC so 3000 FADC’s.

6 Mar 2002Readout electronics11 What we now expect Each VFE chip will handle 18 channels All 18 will be multiplexed onto a single line at 1 MHz (may be speeded up to 5 MHz). Each row of diodes takes 6 VFE chips, mounted on the PCB holding the diodes. The VFE chip has single gain, so need ADC’s with at least 12 bits, preferably 14. Pre-amp peaking time will be ~150 ns, so a trigger is needed within this time.

6 Mar 2002Readout electronics12 Simplest system 15 6U cards in a VME crate –Each covers 2 layers. Each card handles 36 VFE chips –Total is 540 chips. Use one ADC per VFE chip –14 or 16 bit? Cost ~ £20/chip, gives ~ £11k total Signals to/from PCB’s using twisted pair cables, around 100 pins. –Analog, multiplex clock, calibration signals

6 Mar 2002Readout electronics13 Simplest system (2) No data suppression in hardware –Total data volume per event is 2 bytes x 9720 channels = 19 kBytes. –This is ~1 kByte/card so can just use FPGA VME readout of 19 kBytes ~ 1ms –Limited to < 1 kHz, realistically < 500 Hz Rough cost; must be < £50k –ADC’s = £15k, FPGA’s = £5k, PCB’s = £10k, crate = £5k, PC/PCI interface/disks = £10k

6 Mar 2002Readout electronics14 Questions How to do trigger distribution and handling? –Should multiplex clock be generated in hardware on the cards? CPU then just polls for data –HCAL may need trigger within 20 ns! Calibration signal distribution and handling? –Externally generated by FCT? –Timing of trigger after pulse? Just use NIM crate for the above? Do we expand system to include HCAL? –I don’t know what this would entail...