January, 2003 CMS Ecal 1

MGPA Specification Discussion – 9th Jan. 03

OUTLINE

3 or 4 gain channels discussion

technical background - why 3 gains could be preferred? simulation comparisons of 3 and 4 channel versions – effect of process variations implications for layout summary

possible CAL circuit (very brief): proposed circuit and simulation result

January, 2003 CMS Ecal 2

Noise (http://www.hep.ph.ic.ac.uk/~dmray/pptfiles/Ecalprog2.ppt)

CIN

vFET2

vRpf2

Rpf

1 charge amp. stage

s.f. RG

4 diff. O/P gain stages

Cpf

VCM

CI

RI

4 transconductance(VI) gain stages

iCFET2

iRG2

Original design 4 channel version

all diff. O/P stages identicaldifferent gains implemented by values of RG

Main noise sources: Rpf, VFET, gain resistor (RG) and VI FET

relative importance depends on channel gain -> value of RGfor low gain ranges RG large, noise becomes unacceptably large

proposed solution: keep RG small and vary diff O/P stage gain

January, 2003 CMS Ecal 3

Re-distributing gains between diff O/P and RG to keep RG small

RG diff O/P gain Overall gain

RG diff O/P gain

20 8 32 20 8

80 8 8 20 2

160 8 4 40 2

640 8 1 80 1

4 chan previous 4 chan now

gain re-arrangement not completely trivial

need to compensate pulse shape variations for different gains due to different parasitics in O/P circuit

not needed when all O/P stages identical

January, 2003 CMS Ecal 4

Diff O/P stage gain compensation

O/P termination defines dominant time constant

inherent high frequency bandwidth determined by input resistance and capacitance -> parasitic (short) time constant (few nsec) depends on W/L ratios and drain currents but gain also depends on W/L ratios and currents -> different gain channels have different parasitic time constants

can compensate by adding extra internal capacitance

works OK but process variations affect W/L ratios (effective length varies)

=> external termination capacitance needs tuning to compensate for internal variations

can be done but leads to different termination capacitors for different channels

2.5 pF/ns

January, 2003 CMS Ecal 5

Any way to improve?

-> make use of FPPA spec review (“Memo on FPPA specifications”, C.Seez (August, 2002)) -> conclusions:

1. not possible to relax 60 pC full range signal

2. three gain ranges adequate for barrel 0 – 140 instead of 0 – 50 note: highest gain required

140 – 300 50 – 200 reduced by factor ~3300 – 1250 GeV 200 - 400

400 – 1500 3. three gain ranges also acceptable for endcap

January, 2003 CMS Ecal 6

Possible improvements from going to 3 gains

using FPPA spec review conclusions can re-instate equal diff O/P gains, since highest gain can be reduced

Overall gain

RG diff O/P gain

32 ~20 8

8 ~20 2

4 ~40 2

1 ~80 1

Overall gain

RG diff O/P gain

Gain range (barrel) [GeV]

~10 ~20 ~3 0 - 125

~5 ~40 ~3 125 - 250

~1 ~200 ~3 250 - 1250

4 – chan version 3 – chan version

implications

channel to channel pulse shape variation dependence on process spread goes (matching guaranteed by design) no internal compensation required & no process dependent external component selection

R = 200 -> slightly increased noise for lowest gain range; 28,000 -> 34,000 electrons

noise performance for other 2 ranges remains < 10,000 electrons (7000 – 8000)

January, 2003 CMS Ecal 7

Simulated pulse shape (4 gain channel version)

results here for nominal process parameters: = 0

½ fullscale signals shown for each gain range

use gain matching spec. to compare

(Vpk-25ns)/Vpk should match to 1%

highest: -0.2%higher: +0.2%lower: +0.2%lowest: -0.2%

note: sigma (continuous variable +ve & –ve) selects process variation (L,VT) from distribution specified by manufacturer.

Vpk

Vpk-25ns

Signal sizeshighest gain channel: 1 pChigher: 4 pClower: 8 pClowest gain channel: 32 pC

= 0

January, 2003 CMS Ecal 8

= -1.5

highest: -1.7%higher: +0.5%lower: +0.5%lowest: +0.7%

Pulse shape 4 chan. gain version, = -1.5

highest gain pulse shape (solid line) rise time now too slow

=> need to tune diff O/P stage external termination components to speed up (reduce Cdiff)

can be done (precision 0402 capacitors available)

Vcm

Cdiff

January, 2003 CMS Ecal 9

process parameter variation pulse shape example (4 gain channel version)(without any external compensation)

= -1.5 = 0 = +1.5highest: 1 pChigher: 4 pClower: 8 pClowest: 32 pC

highest: -1.7%higher: +0.5%lower: +0.5%lowest: +0.7%

highest: -0.2%higher: +0.2%lower: +0.2%lowest: -0.2%

highest: +0.7%higher: -0.03%lower: -0.03%lowest: -0.6%

PulseShapeMatching

January, 2003 CMS Ecal 10

3 gain channel version - no external compensation necessary because diff O/P stage parasitics same for all 3 chans

= -1.5 = 0 = +1.5highest: 3 pCmiddle: 6 pClowest: 30 pC

highest: -0.04%middle: -0.04%lowest: +0.08%

PulseShapeMatching

highest: -0.3%middle: -0.2%lowest: +0.5%

highest: 0%middle: 0%lowest: 0%

January, 2003 CMS Ecal 11

4 channel 3 channel

Layout benefits of 4 -> 3 channels

80 pin packages

January, 2003 CMS Ecal 12

Noise justification for gain of 32?

barrel: 1250 GeV -> 60 pC

highest gain range 32 10

fullscale signal 40 Gev (2pC) 125 GeV (6 pC) least significant bit 10 MeV 31 MeV digitisation noise (root 12) 2.9 MeV 8.9 MeV + 40 MeV electronic noise 40.1 MeV 41 MeV

January, 2003 CMS Ecal 13

Summary (1)

original 4 channel design worked well from pulse shape matching viewpointdifferent gains realised by different RG values in VI stage -> all diff O/P gains identical

but low gain channel noise too high

redistributing gains between RG and diff O/P stage solves noise problempulse shapes for different gain channels matched by internal compensation

but process variations give effects which can only be compensated by selecting slightlydifferent output termination capacitors for different gain channels

difficult to quantify how big a problem but likely to complicate production e.g. production testing, VFE module assembly (won’t have standard set of component values)

January, 2003 CMS Ecal 14

Summary (2)

benefit of 4 -> 3 channels

pulse shape matches inherently (by design): no need to “tune” pulse shape to cope with process spread by selecting different O/P termination capacitance (now checked for wide range of parameter variations (np mismatch, supply voltage, temp.)

layout: minimum pin count reduced – can use more power pins(will need some extra pins for CAL circuit and I2C test)

power: ~ 600 mW - > ~500 mW

simplistic conclusion (from electronics perspective only)

3 gain channels -> all diff O/P stages identical -> more robust design -> less risk

(note: all previous talks can be found at: http://www.hep.ph.ic.ac.uk/~dmray)

January, 2003 CMS Ecal 15

Possible simple CAL circuit

can adjust resistor values to get 2 or 3 points per MGPA gain range

requires external trigger (where from?)

On-chipOff-chip

January, 2003 CMS Ecal 16

CAL circuit simulation

10pF

1nF

DAC valuee.g. 100mV

MGPA I/P

10k RtcRtc:0 ->10

Highest gain channel O/P for 1 pC input signal

Can use Rtc to simulate real signal risetime

external components