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1 AFEI shortcomings and solutions Juan Estrada 4/13/2005 • Introduction • Problems seen in AFEI • pedestal variation • SVX saturation • discriminator to ADC crosstalk • discriminator pedestal shift • Conclusion

AFEI shortcomings and solutions

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AFEI shortcomings and solutions. Introduction Problems seen in AFEI pedestal variation SVX saturation discriminator to ADC crosstalk discriminator pedestal shift Conclusion. Juan Estrada 4/13/2005. AFE and CFT. 8 photons. VLPC. 9 o K. (x512). 50 fC. - PowerPoint PPT Presentation

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Page 1: AFEI shortcomings and solutions

1

AFEI shortcomings and solutions

Juan Estrada4/13/2005

• Introduction

• Problems seen in AFEI

• pedestal variation

• SVX saturation

• discriminator to ADC crosstalk

• discriminator pedestal shift

• Conclusion

Page 2: AFEI shortcomings and solutions

2

AFE and CFT

9o KVLPC

(x512)

AFE

DISCR ADCDiscriminator output every 396 nsec for L1

Amplitude signal readout for L3 and offline

8 photons

50 fC

Central Fiber Tracker cylinder

Page 3: AFEI shortcomings and solutions

3

Occupancy in the detector

(online thresholds set to 1% occupancy between stores)

Things will get very difficult for tracking at high luminosities.

D.Lam

Page 4: AFEI shortcomings and solutions

4

Projected Occupancies

Luminosity 80E30 100E30 200E30 300E30

Zero Bias 0.14 0.17

Min Bias 0.15 0.17 0.28 0.39

JT_45TT 0.23 0.25 0.38 0.52

JT_95TT 0.23 0.25 0.37 0.50

JT_125TT 0.22 0.25 0.38 0.50

D.Lam

Page 5: AFEI shortcomings and solutions

5

Light Yield

• These data show LY v. for =0

• Average for each supersector

• “Current worst case LY” 8 pe (singlet)

– This is a lower limit due to SVX sat. & ADC dynamic range

• Expect Axial LY Stereo LY

• Axial Gain > Stereo Gain

– SVX Sat. Effects smaller in Stereo

O.Boeriu

Page 6: AFEI shortcomings and solutions

6

VLPC Luminosity Dependent Effects

• As VLPC occupancy (or luminosity) goes, up QE and gain drop

0, 10, 20, 30, 40 % occupancy(expect close to 40% on inner layers at highest RunII L)

10% drop 20% drop

Quantum Efficiency Gain

in addition: Fiber Radiation Damage induced light yield loss (10-20%)

Page 7: AFEI shortcomings and solutions

7

AFE module(analog front end)

1 64

discr. output every 396 nsec for L1

if the event is accepted at L2 send the amplitude of the signal to L3ADC

DISCR

SIFT(1..4)

(1..16)

(1..16)

(1..16)Discrim to L1

Analog signal (gain 0.5)

SVXIIe(pipeline + ADC)

L2 accept

Digitized signal to L3

AFEI is the combination of 2 custom chips (SVXIIe and SIFT).

Issues:very small signals transfered from SIFT to SVX while the discriminators are firing (baseline shift)

CFT electronics and the VLPC(introduction to the problem)

Page 8: AFEI shortcomings and solutions

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Known problems in AFEI

Page 9: AFEI shortcomings and solutions

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Tick to Tick

~20 ADC/pe

The pedestal moves up to 1 pe for the high gain pixels, this effect is much more significant in the stereo layers…

This occurs because we reset the front end once per superbunch.

data from the detectorMea

n pe

dest

al (

AD

C)

Page 10: AFEI shortcomings and solutions

10

• Tick to tick variation in pedestals.– Reset is identical

every xing, so there should be no tick to tick variations and none is observed.

xing 5, 8, 11 , 14 , 17 , 20 all 64ch from module 1

Tick to tick variations eliminated!

AFEII proto: Results

AFEII result

Page 11: AFEI shortcomings and solutions

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Channel to Channel variation

Spread with RMS=0.5 pe is typical for AFEI. With AFEI we can set only 1 zero supression threshold per SVX chip (64 channels).

Page 12: AFEI shortcomings and solutions

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• Chan to chan variation in pedestals.– RAW ADC info (10bit) is converted inside the FPGA into 8bit

SVX like data: pedestal corrected and zero suppressed. So there should be none.

xing 5, all 64ch from module 1

Chan to chan variations eliminated!

AFEII proto: Results

AFEII result

Page 13: AFEI shortcomings and solutions

13

SVX Saturation (1)

1 crossing=396 ns

Event (photons produced)

The voltage at the input amplifiers goes up, and it can hit the maximum. After this point, the electronics can not detect any more photons until after the next reset (during the next gap).Q0

Q1

crossing clock

Reset at the beginning of the superbunch.

Page 14: AFEI shortcomings and solutions

14

SVX saturation (2)

Loss of signal in main pulse vs. # pe's in early pulse

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0 20 40 60 80 100 120

N' (corrected #pe 's)

rela

tiv

e s

ign

al

The problem was studied in the 4 cassette test stand during summer 2004 (DØ Note 4495, P. Hasiakos).

At 30% occupancy (12 crossings)– 4 Hits integrated in pipeline– “Typical” integrated charge on front end » 40 pe– 40% drop in Signal for hits in “triggered” events, on average

Page 15: AFEI shortcomings and solutions

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• Biggest concern is SVX saturation• Test: inject huge LED pulse, measure small pulse in the same

superbunch

3pe in xing 20 -> readout

> 100pe in xing 5

Saturation problem solved!

AFEII proto: Results

AFEII result

Page 16: AFEI shortcomings and solutions

16

Discriminator-ADC crosstalkstudies in the test stand

Discriminator to SVX crosstalk

Discriminator firing in AFEI produce extra noise and shift of the analog readout.

0% discr occ.

32% discr occ.

Some pedestals shift to higher values (fakes).

Some pedestal shift to lower values (inefficiency).

this is an event by event effect, what matters is the discriminator occupancy for the event you triggered on. Very difficult to solve. Analog information compromised.DØ Note 4500

Page 17: AFEI shortcomings and solutions

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Discriminator-ADC crosstalkconfirmation in DØ data

This analysis was done by Avdhesh Chandra from TIFR.

Special runs were taken to study the effect seen in the test stand using real data.

Typical channel

SVX

SVXoccupancy

Chann. number

Ped

shi

ft.

Ped

shi

ft.

Page 18: AFEI shortcomings and solutions

18

Discriminator-ADC crosstalkconfirmation in DØ data (2)

• Using special runs without zero suppression, we were able to see the pedestal shift at 30% occupancy for about 11K channels.

• The effect seen in the test stand is confirmed, the pedestal shift as a function of occupancy in the event has been verified.

• The magnitude of the effect (from gaussian fit): mean -11, sigma 10• 1 pe is something between 10 to 20 ADC counts.• The tails reach very far, but the 3 pe shift seen in the test stand is not

typical… things like 1–2 pe shift are common.

Page 19: AFEI shortcomings and solutions

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• Discr to ADC xtalk is not there is AFEII. The position of the pe peaks are always in the same place, and do not depend on the discriminator occupancy (compare with slide 16).

Discr to ADC xtalk eliminated!

discr. on.

AFEII proto: Results

AFEII resultTriP results

Page 20: AFEI shortcomings and solutions

20

Discriminator pedestal shift (1)

Two discriminator scans with a fixed amplitude LED signal. In one case (RED) the LED is pulsed once per turn, in the other (BLUE) the LED is pulsed in every beam crossing.When we expect the discriminator to be firing at every crossing at 25% occupancy, we see only 15% occupancy when firing on every crossing. This is equivalent to 0.5 pe shift.

No effect in the same crossing.Threshold scan at 2 different amplitudes.

Page 21: AFEI shortcomings and solutions

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Discriminator pedestal shift (2)

This effect depends on the zero-bias occupancy, and it is smaller than what we have seen for the Discriminator-SVX crosstalk. However, it is part of our L1 trigger… 0.6 pe could be a lot.

L=200E30

L=40E30We use a 0.6 pe (instead of the 0.5 in the previous slide) to take into account the gain reduction in the VLPC.

As the occupancy goes up, so does our threshold.

Efficiency per hit to the power of eight gives the tracking efficiency for 8/8.

occupancy

Page 22: AFEI shortcomings and solutions

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Conclusion

• The current AFE has served well, but exhibits several features which limit performance:– Pedestal variation– SVX saturation– Discriminator to ADC crosstalk– Discriminator pedestal shift

• Several of these undesirable features become increasingly important and the luminosity increases.

• We understand the origin of these problems and the proposed electronics addresses and solves these problems by design.