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MGPA status Mark Raymond (17/10/02) Significant issue arising last time:

MGPA status Mark Raymond (17/10/02) Significant issue arising last time: ADC common mode input voltage now specified at (VDD-VSS)/2 => no change to MGPA O/P stage required Areas identified last time as still needing attention: noise (lower gain channels)

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MGPA status Mark Raymond (17/10/02) Significant issue arising last time:

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  1. MGPA status Mark Raymond (17/10/02) Significant issue arising last time: ADC common mode input voltage now specified at (VDD-VSS)/2 => no change to MGPA O/P stage required Areas identified last time as still needing attention: noise (lower gain channels) simulations including process spread transmission line effects (APD -> MGPA)? power supply rejection target package Concentrate this time on: noise target package CMS Ecal

  2. Noise Rpf diff. O/P gain stage transconductance gain stage vRpf2 CI Cpf iCFET2 s.f. RG RI vFET2 CIN charge amp. iRG2 VCM ENC due to charge amp. noise sources: Rpf: note: Rpf constrained by Cpf (RpfCpf= t = 2RICI = 40 nsec. ) -> 4900 electrons (barrel) -> 2700 electrons (endcap) I/P FET: (CTOT = CIN + CFET + Cpf) -> 1800 electrons (barrel, CIN=300pF (200 + 60 + 40)) -> 660 electrons (endcap, CIN=112pF (40 + 60 + 12)) =>no strong dependence on CIN 1/2 K1t Rpf K2vFETCTOT t 1/2 CMS Ecal

  3. Noise Rpf diff. O/P gain stage transconductance gain stage vRpf2 CI Cpf iCFET2 s.f. RG RI vFET2 CIN charge amp. iRG2 VCM ENC due to transconductance stage sources: RG -> Cpf dependence because relative magnitude depends on charge amp gain. Keep RG as small as poss. but has to vary for different gain stages Cascode FET -> Cpf and RG dependence V/I stage noise sources become more important for lower gains (bigger RG) other noise sources exist but these are the main ones 1/2 K3Cpf RG t 1/2 K4CpfRG gm t CMS Ecal

  4. Re-distribute gains to keep RG small previous now distribution of gain between value of RG and diff O/P gain is trade–off between noise and power in output stage (8x -> 10 mA, 1x -> 18 mA, but 0.25x -> 54 mA!) => can’t make RG same for all stages and implement different gains in O/P stage gain re-arrangement not completely trivial – pulse shape variations for different gains due to different parasitics in O/P circuit needed careful compensation (not needed when all O/P stages were identical). CMS Ecal

  5. Simulated noise dependence on gain keeping RG as small as poss. numbers in red exceed 10,000 (3500) but signal size means relative contribution to overall energy resolution less significant CMS Ecal

  6. MGPA noise contribution to energy resolution BARREL ENDCAP CMS Ecal

  7. Possible noise improvements for endcap if reduce dynamic range requirement to 1500 GeV Cpf -> Cpf/2, Rpf -> 2.Rpf charge amp. gain doubled and subsequent noise sources less significant 1.5 TeV fullscale 3 TeV fullscale CMS Ecal

  8. MGPA layout issues ‘artists impression’ here, proper layout not yet begun segmented approach minimise crosstalk between different gain stages multiple power pads all bias lines decoupled (bias generator in centre) diff. O/Ps separated layout (chip and hybrid) needs care different stray capacitance -> different pulse shapes/gain range) gain resistors now internal +/- 10 % tolerance but external probably not feasible => gain ranges need decision before manufacture 52 pin (FPPA) package not enough approach here uses 80 pins – can still be TQFP CMS Ecal

  9. Summary of areas still needing attention Simulations to date mainly for nominal process parameters need more detailed study of process variation on performance Transmission line effects between APD and MGPA? more info needed (characteristic impedance, C/length, length, …) rough nos. will do Power supply rejection – not looked at yet Target package – any objections to 80 pin TQFP? Layout – should begin soon  ……… CMS Ecal

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