The mgpa ecal readout chip for cms
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The MGPA ECAL readout chip for CMS. Multi–Gain Pre-Amplifier - 0.25 m m CMOS chip for CMS ECAL. OUTLINE Introduction & background Design Measured Performance Conclusions. Mark Raymond , Geoff Hall, Imperial College London, UK.

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The MGPA ECAL readout chip for CMS

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The mgpa ecal readout chip for cms

The MGPA ECAL readout chipfor CMS

Multi–Gain Pre-Amplifier - 0.25 mm CMOS chip for CMS ECAL

OUTLINE

Introduction & background

Design

Measured Performance

Conclusions

Mark Raymond, Geoff Hall, Imperial College London, UK.

Jamie Crooks, Marcus French, Rutherford Appleton Laboratory, UK.

IEEE Nuclear Science Symposium, Rome 2004

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

CMS Electromagnetic Calorimeter

ECAL X-section PbWO4 crystals

ECAL

barrel

end-cap

Compact Muon Solenoid

~ 75,000 Lead Tungstate scintillating crystals

60,000 barrel, 15,000 end-cap

Hostile radiation environment

PbWO4 crystals

2.2 x 2.2 cm2

23 cm

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Crystal Readout

2 different types

Barrel - Avalanche Photodiode (APD)

good for high transverse magnetic field

not so radiation hard

2/crystal -> ~ 200 pF detector capacitance

60 pC full-scale signal

End-cap - Vacuum Photo-Triode (VPT)

better radiation hardness

OK for lower transverse magnetic field in end-cap

v. low capacitance but cabling adds ~ 50 pF

16 pC full-scale signal

challenge for front end readout chip

2 different signal sizes and input capacitance

prefer to have just one chip for both

APDs

VPT

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Background

CMS ECAL dynamic range requirement

~ 16 bits to cover range from noise

to full-scale signal

General approach

use multiple gain ranges

-> high resolution with only 12 bit ADC

only transmit value for highest gain

channel-in-range

=> have to take decision on front end

every 25 ns (LHC bunch spacing)

Earlier version of CMS ECAL architecture

range decision taken in preamplifier (complex chip), followed by single channel commercial ADC

New architecture proposed following major ECAL electronics review, early 2002

3 parallel gain channels (MGPA), multi-channel ADC, range decision taken by logic in ADC chip

use 0.25 mm CMOS to achieve:

system simplifications: single 2.5V supply for all on-detector chips, power savings

well known radiation hardness

short production turnaround, high yield, cost savings

Short timescale for development

design begun mid 2002, first submission early 2003, fortunately worked well

final version (only minor design revisions) available Spring 2004

12 bit ADCs

12

12 bits

LOGIC

6

2 bits

range

1

MGPA

APD/VPT

Multi-channel ADC

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

MGPA Target Specifications

Barrel/Endcap read out using APD/VPT

different capacitance and photoelectric

conversion factors

3 gain ranges (1:6:12) sufficient

to deliver required physics performance

40 ns pulse shaping trade-off between

pile-up and noise (25 ns LHC bunch spacing)

linearity and pulse shape matching specs

demanding

Vpk-25

Vpk

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

MGPA Architecture

input stage

CF chosen for max. poss. gain

depending on barrel/end-cap

RF chosen for 40 ns decay

avoids pile-up

CFRF external components

=> 1 chip suits barrel & end-cap

differential current O/P stages

external termination

2RICI = 40 nsec.

=> low pass filtering on all

noise sources within chip

3 gain channels 1:6:12

set by resistors (on-chip),

for linearity, feeding common-

gate stages

I2C interface to program:

output pedestal levels

DAC for test pulse (ext. trig.)

i

I2C and

offset

generator

RI

CI

i

VCM

RG1

RI

i

DAC

RI

CI

VCM

RG2

RI

ext.

trig.

CCAL

input stage

charge amp.

RI

CI

VCM

RG3

I/P

RI

diff. O/P stages

gain stages

RF

CF

RFCF

VCM

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Noise Sources

common-gate

gain stage

vRf

input stage

Rf

diff.

output

stage

source

follower

Cf

iCG

RG

vFET

CIN

iRG

input stage

high Cf (low gain) to cope with large full-scale signals

=> corresponding low Rf for 40 ns time const.

=> Rf noise dominates over input FET

gain stage contribution

can’t avoid for low gain range (RG big)

but this range only used for larger signals

so signal/noise still acceptable

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Chip Layout

layout issues

gain channels

segregated as much as poss.

with separate power pads

-> try to avoid inter-channel coupling

lots of multiple power pads

die size ~ 4mm x 4mm

packaged in 100 pin TQFP (14mm x 14mm)

offset gen.

I2C

diff. O/P stage

high

gain

stage

diff. O/P stage

mid

gain

stage

1st stage

low

gain

stage

diff. O/P stage

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Test Bench

automated, controlled

by PC running LabVIEW

14-bit VME ADC

need high precision to measure

performance to 12-bit level

MGPA socketed on test board

allows chip to chip comparison

without change of external

components

prog.

attenuator

pulse

gen.

prog.

delay

MGPA

test

board

14-bit

VME

ADC

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

linear range

Measured Output Pulse Shapes

differential O/P signals

from all 3 gain ranges

0 – 60 pC, 40 steps

(logarithmic spacing)

no signs of distortion

in lower gain ranges

when higher ranges

saturate

=> effective gain channel

separation in layout

gain ratios 1 : 5.6 : 11.0

(c.f. 1 : 6 : 12)

mid gain

range

low gain

range

high gain

range

Volts

time [nsec]

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Nonlinearity

MGPA Version 2

MGPA Version 1

Nonlinearity given by:

pk.pulse height – fit (to pk.ht.)

fullscale signal

10 chips measured for each

MGPA version

v. similar results V1 cf. V2

nonlinearity within (or close to)

± 0.1% specification

high gain range

high

mid

mid

Nonlinearity [% fullscale]

Nonlinearity [% fullscale]

low

low

charge injected [pC]

charge injected [pC]

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

PSMF = Vpk-25

Vpk

Pulse Shape Matching

Output pulses spanning

full-scale range for all 3

gains (11 / range)

Vpk

high

normalise all 33

pulse shapes

to max pulse ht.

and superimpose

Vpk-25

mid

Pulse Shape Matching = (PSMF – Average PSMF)

Average PSMF

low

± 1% spec.

(Average PSMF = average over all pulse shapes for all 3 gain ranges)

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Noise

BARREL

END-CAP

high gain chan.

mid gain chan.

high gain chan.

mid gain chan.

ENC [rms electrons]

7240+5.8/pF

7870+4.9/pF

3040+4.5/pF

3270+4.5/pF

added capacitance [pF]

added capacitance [pF]

weak dependence on input capacitance as expected

within spec. for high and mid-gain ranges:

barrel < 10000 e, end-cap < 3500 e

low gain range:

barrel: 27300 e ± 12% end-cap: 8200 e ± 11%

completely dominated by gain stage noise

but signals large => electronic noise not significant (< 0.2% contribution to overall energy res’n.)

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Radiation Tests

low

mid

high

pre-rad

5 Mrads

10 keV X-rays (spectrum peak) , dosimetry accurate to ~ 10%, doserate ~ 1 Mrad/hour, no anneal

~ 3% reduction in gain after 5 Mrads (50 kGy, 2 x end-cap worst case)

no measurable effect on other performance parameters (noise, linearity, PSM ….)

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

On-chip Test Pulse

external

edge

trigger

ext.

10pF

Volts

MGPA I/P

I2C

simple DAC allows programmable (I2C)

amplitude charge injection

-> range of signal sizes

for each gain range

external trigger required

allows functional verification

during chip screening and in-system

nsec.

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Conclusion

MGPA development successful – architecture suits both barrel and end-cap detector regions

Analogue performance good

gain

linearity

pulse shape matching

noise

rad-hard as expected

power consumption 600 mW

Current status

1st barrel supermodule contructed at CERN (barrel segment, 1700 channels)

performance as expected (excellent noise uniformity)

wafer mass production complete – large nos. packaged chips already available

within (or v. close to) spec.

5 channel VFE card

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Transistor Level Schematic

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Barrel Energy Resolution

x 12

x 6

x1

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Pulse Shape Measurements

O/P signals probed

individually

0 – 60 pC, 40 steps

saturation in mid and

high gain ranges

no clamping outside

linear range

low gain range

mid gain range

high gain range

Volts

time [nsec]

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

I2C Pedestal Adjust

I2C=0

I2C=50

I2C=100

ADC

I/P

range

Volts

VCM

nsec.

High gain range, ~ fullscale signal.

I2C pedestal adjust sets offset current to diff O/P stage (one for each gain range)

I2C ~ 50 about right in this case

M. Raymond, Imperial College London IEEE NSS, Rome 2004


The mgpa ecal readout chip for cms

Linearity and Pulse Shape Matching

important for simple reconstruction of “true” pulse shape from samples coming from different gain ranges

target specifications

non-linearity <  0.1 % fullscale (each gain range)

pulse shape matching factor: Vpk-25/Vpk <  1 % within and across all 3 gain ranges

high gain

range

25 ns samples

linearize

low gain

range

12-bit

range

Vpk

Vpk-25

M. Raymond, Imperial College London IEEE NSS, Rome 2004


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