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Calibration and performance tests of the Very-Front-End
electronics for the CMS electromagnetic calorimeter
J. Blaha, C. Combaret, J. Fay, G. Maurelli
To cite this version:
Calibration and performance tests of the Very-Front-End electronics for the CMS
electromagnetic calorimeter
J. Blaha
a,1, C. Combaret
a, J. Fay
a, G. Maurelli
aon behalf of the CMS ECAL group
a
IPNL, Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, Villeurbanne, France
j.blaha@ipnl.in2p3.fr
Abstract
The Very-Front-End electronics processing signals from pho-todetectors of the CMS electromagnetic calorimeter have been put through an extensive test programme to guarantee function-ality and reliability. The final characteristics of the VFE boards designed for the calorimeter barrel and endcaps are presented. The results, which have been also verified during test beam at CERN, confirm the high quality of the boards production and show that the CMS detector specifications are reached.
I. I
NTRODUCTIONOn detector electronics of the CMS electromagnetic calorimeter (ECAL) [1], which consist of 75,848 radiation hard scintillating crystals PbWO4, contain almost 16,000 Very-Front-End (VFE) boards that process signals from Avalanche Photodiodes (APDs) in the central barrel region and Vacuum Phototriodes (VPTs) in the forward endcaps regions, respec-tively. The VFE board was designed in two types: covering a dynamic range up to 50 pC corresponding to an incident par-ticle energy of ∼ 1.7 TeV for barrel and 16 pC for energy up to ∼ 3.5 TeV for the endcaps. Both types comprise five identi-cal and independent read-out channels. Each channel, process-ing the signal from one crystal, consists of a Multi-Gain Pre-Amplifier (MGPA), a multi-channel analogue-to-digital convec-tor (ADC) AD41240, and two level adapters LVDS-RX. The VFE ASICs are identical for both types of the board and the dif-ferent dynamic ranges are set by external electrical components. Schematic drawing of such a channel is displayed in Fig. 1.
Figure 1: Schematic representation of one channel of the Very-Front-End (VFE) architecture comprising the MGPA, multi-channel ADC AD41240 and two level adapters LVDS-RX.
The MGPA chip [2] contains a pre-amplifier and three paral-lel gain stages with nominal gain 1, 6, and 12 that shape and amplify the photodetector signal. The three analogue output
signals of the MGPA are then digitized in parallel by the multi-channel 40 MHz 12-bit ADC (AD41240) [3]. An ADC internal logic determines whether a gain is saturated and then outputs the data from the highest non-saturated channel. The Low Volt-age Differential Signal (LVDS) outputs of the ADC are adapted by two LVDS-RX to the single ended inputs of the front-end (FE) board, which thus receives 12 bit digitalized pulse infor-mation and 2 bit identification of the gain. The MGPA chip also includes a test pulse unit with an integrated digital to analogue converter (DAC) programmed via an I2C interface, which
al-lows the channel functionality to be checked by injecting test charges directly into the input of the pre-amplifier. The pedestal value can be set independently for each gain. In addition, the VFE board also incorporates a Detector Control Unit (DCU) chip for measuring the crystal temperature and, in case of barrel VFE card, also the APD leakage current. All the chips are im-plemented in 0.25 µm IBM CMOS radiation-hard technology.
To guarantee the functionality and reliability of the VFE electronics, all the VFE boards have to pass an extensive quality and assurance (Q&A) program. The program, which due to its complexity is split among several collaborating institutes (IPN Lyon, ETH Z¨urich, INFN Torino and University of Cyprus), in-cludes a number of consecutive tests beginning with an optical inspection performed by the manufacturer, followed by a power-on test - the first electrical test that measures voltages, currents and performs a basic functional test, and a burn-in for 72 hours at a temperature of 60◦C, which is finalized by a complete
cali-bration and characterization of each channel. More information about individual steps of the Q&A program can be found else-where [4, 5].
II. C
ALIBRATION AND PERFORMANCE TESTSThe main aim of the calibration and performance test of the VFE card is to fully characterize and verify all the operational parameters of the card and build a calibration database for each card which will serve for the first calibration of all the chan-nels of the CMS ECAL. For this purpose a complete calibration bench has been designed at IPN Lyon. In a 19 inch 6U crate, up to 6 test boards, which serve 6 VFE cards, can be installed and measured at the same time. The test board provides 2.5 V analogue voltage supplying the MGPA and analogue part of the ADC, and 2.5 V digital voltage for digital part of the ADC. An Altera FGPA sends data via a RS 232 bus to the master PC.
A. Calibration
The calibration procedure includes an absolute calibration of each channel for the three gains in ADC counts versus in-jected charge in pC, a channel-to-channel relative calibration, gain ratios and linearity studies. An Agilent square pulse gener-ator and an attenugener-ator are used to obtain different voltages that are applied on a calibrated 10 ± 0.01 pF capacitor which gives charges in a range from 0 up to 40 pC and up to 9 pC for the barrel and endcap, respectively. A digital output of the ADC is recorded every 25 ns and an amplitude of the pulse is computed by a 4-parameter analytical function using 5 samples around the maximum value. A pedestal value, which is estimated for each injected charge separately using a set of pre-samples, is sub-tracted. In order to check the linearity a set of pulses of precise charges are injected into the VFE card and a digital output of the card is recorded over the full dynamic range for all three gains and five channels. Injected charges were chosen to cover the whole range of each gain and to provide sufficient amount of points for the calibration as well as for linearity studies as depicted in Fig. 3. Entries 63130 Underflow 0 Overflow 0 Constant 6523 ± 32.1 Mean 64.83 ± 0.00 Sigma 0.7647 ± 0.0022 ADC Counts/pC 58 60 62 64 66 68 70 72 0 1000 2000 3000 4000 5000 6000 Entries 63130 Underflow 0 Overflow 0 Constant 6523 ± 32.1 Mean 64.83 ± 0.00 Sigma 0.7647 ± 0.0022 Gain x1, barrel Entries 63130 Underflow 0 Overflow 0 Constant 5879 ± 28.4 Mean 351.7 ± 0.0 Sigma 3.631 ± 0.010 ADC Counts/pC 320 330 340 350 360 370 380 0 1000 2000 3000 4000 5000 6000 Entries 63130 Underflow 0 Overflow 0 Constant 5879 ± 28.4 Mean 351.7 ± 0.0 Sigma 3.631 ± 0.010 Gain x6, barrel Entries 63130 Underflow 0 Overflow 0 Constant 4796 ± 23.5 Mean 685 ± 0.0 Sigma 7.332 ± 0.021 ADC Counts/pC 640 660 680 700 720 740 0 1000 2000 3000 4000 5000 Entries 63130 Underflow 0 Overflow 0 Constant 4796 ± 23.5 Mean 685 ± 0.0 Sigma 7.332 ± 0.021 Gain x12, barrel Entries 15020 Underflow 0 Overflow 0 Constant 1152 ± 11.7 Mean 307 ± 0.0 Sigma 4.13 ± 0.02 ADC Counts/pC 280 290 300 310 320 330 340 350 0 200 400 600 800 1000 1200 Entries 15020 Underflow 0 Overflow 0 Constant 1152 ± 11.7 Mean 307 ± 0.0 Sigma 4.13 ± 0.02 Gain x1, endcap Entries 15020 Underflow 0 Overflow 0 Constant 1025 ± 10.3 Mean 1616 ± 0.2 Sigma 20.29 ± 0.12 ADC Counts/pC 1500 1550 1600 1650 1700 1750 1800 0 200 400 600 800 1000 Entries 15020 Underflow 0 Overflow 0 Constant 1025 ± 10.3 Mean 1616 ± 0.2 Sigma 20.29 ± 0.12 Gain x6, endcap Entries 15020 Underflow 0 Overflow 0 Constant 964.7 ± 9.9 Mean 3121 ± 0.4 Sigma 43.16 ± 0.27 ADC Counts/pC 2900 3000 3100 3200 3300 3400 3500 0 200 400 600 800 1000 Entries 15020 Underflow 0 Overflow 0 Constant 964.7 ± 9.9 Mean 3121 ± 0.4 Sigma 43.16 ± 0.27 Gain x12, endcap
Figure 2: Slope distributions of the injected charges versus pulse height in ADC counts/pC for all the barrel (left figure) and all the endcap VFE (right figure) and the three gains 1, 6, and 12.
Charge (pC) 0 5 10 15 20 25 30 35 40 ADC counts 0 500 1000 1500 2000 2500
Amplitude vs. charge for gain x1, x6, x12
gain x1 gain x6 gain x12 barrel Charge (pC) 0 1 2 3 4 5 6 7 8 9 ADC counts 0 500 1000 1500 2000 2500 3000
Amplitude vs. charge for gain x1, x6, x12
gain x1 gain x6
gain x12
endcap
A charge of 4 pC for barrel VFE and charges 0.5 and 1 pC for endcap VFE are the only possible charges common for all three gains and which thus allow the gain ratios to be measured di-rectly. In order to suppress instrumentation effects from the test boards of the calibration system and variation during the long period of calibration of more than two years, an intercalibration between channels is applied. Since a pulse height temperature dependency and slope dependency of -0.2%/◦C were observed,
all the calibration measurements are performed in a temperature stabilized room (18±0.3◦C).
Measurements of the calibration curves, shown for one chan-nel in Fig. 3, have been completed for all VFE cards needed for barrel and endcaps. Parameters of a linear least square fit of the calibration curve fully describe the main properties of each gain channel. Average values of the slopes, which represent the gains, measured in ADC counts per pC over all calibrated chan-nels are listed together with the corresponding dispersions in Tab. 1. Very small differences (σ/mean ∼ 1%) among measured channels prove the high quality and homogeneity in the VFE board production. Slope distributions of the injected charge ver-sus pulse height in ADC counts/pC for all the barrel and endcap VFE card for three gains 1, 6, and 12 are displayed in Fig. 2.
Barrel Endcap
Gain Slope σ/mean (%) Slope σ/mean (%)
1 64.83 1.18 307 1.35
6 351.7 1.03 1616 1.26
12 685 1.07 3121 1.38
Table 1: Mean value of the slopes (ADC count/pC) of the injected charges versus pulse height and its dispersion (%) for all the barrel and endcap VFE cards for three gains 1, 6, and 12.
B. Gain ratio
The laboratory gain ratio is determined as the ratio of slopes of the calibration curve between the gains. Mean values and corresponding dispersions over all the measured channels are summarized in Tab. 2 for the barrel as well as for endcap VFEs. The gain ratio values presented in Tab. 2 show small dispersion among the channels.
Barrel Endcap
Ratio Mean σ/mean (%) Mean σ/mean (%) 12/1 10.74 1.001 10.84 1.284 12/6 1.943 0.87 1.951 1.121 6/1 5.527 0.887 5.559 1.175
Table 2: Mean values of the gain ratios and corresponding dispersions for all the barrel and endcap VFE cards.
Precision of knowledge of the gain ratio plays a very impor-tant role for higher or extremely high energies where switching to gain 6 or 1 occurs. For this purpose, it was necessary to de-velop an alternative technique for the gain ratio determination which could be used as an in situ method during CMS ECAL operations. Hence, the MGPA test pulse unit was used for gain ratio computation. By this unit it is possible to inject a test charge directly into the input of the MGPA amplifier. Because the gain path can be forced to a particular choice by stopping one
or more ADC gains, it is possible for the same injected charge to produce signals from different gain stages. Thus a ratio of the reconstructed signal amplitudes is equal to the gain ratio. The gain ratio delivered by the test pulse and extracted from the slopes were compared and it was found that both methods give comparative results, well-correlated among channels, but with a small systematic shift which can be due to different circuitry for the injected charge.
Another technique for in situ gain ratio determination uses the ECAL laser monitoring system [6], which monitors varia-tions in the light transmission of the crystals due to radiation during LHC operation. The gain ratio is measured by forc-ing gain changes for a fixed laser light pulse. The gain ratio measured by the test pulse with a charge of 4 pC and by moni-toring laser system using an infrared light, were compared and it was found that both methods give comparative results, well-correlated among channels as seen in Fig. 4.
Testpulse 1.9 1.92 1.94 1.96 1.98 2 2.02 Laser 1.9 1.92 1.94 1.96 1.98 2 2.02 Entries 1700 Mean x 1.961 Mean y 1.962 RMS x 0.01691 RMS y 0.01626 Entries 1700 Mean x 1.961 Mean y 1.962 RMS x 0.01691 RMS y 0.01626 Gain ratio x12/x6 Entries 1700 Prob 0.09215 Constant 482.3 Mean 0.0006389 Sigma 0.001847 (laser - testpulse)/laser -0.015 -0.01 -0.005 -0 0.005 0.01 0.015 0.02 0.025 # Xtals 0 100 200 300 400 500 Entries 1700 Prob 0.09215 Constant 482.3 Mean 0.0006389 Sigma 0.001847 Gain ratio x12/x6
Figure 4: Correlation (top) and difference (bottom) between gain ratio 12/6 measured by the test pulse and laser on a barrel supermodule.
Beam 1.9 1.92 1.94 1.96 1.98 2 2.02 Testpulse 1.9 1.92 1.94 1.96 1.98 2 2.02 Entries 179 Mean x 1.961 Mean y 1.957 RMS x 0.01669 RMS y 0.01663 Entries 179 Mean x 1.961 Mean y 1.957 RMS x 0.01669 RMS y 0.01663
Gain ratio x12/x6, 120 GeV
Entries 179 Prob 0.3056 Constant 35.47 Mean 0.001343 Sigma 0.001559 (beam - testpulse)/beam -0.015 -0.01 -0.005 -0 0.005 0.01 0.015 0.02 0.025 # Xtals 0 5 10 15 20 25 30 35 40 45 Entries 179 Prob 0.3056 Constant 35.47 Mean 0.001343 Sigma 0.001559
Gain ratio x12/x6, 120 GeV
Figure 5: Correlation (left) and difference (right) between gain ratio 12/6 measured by the MGPA test pulse and with an electron beam of 120 GeV at CERN for 179 channels.
C. Linearity
Tests of linearity were evaluated by help of the coefficient of determination r2delivered from the linear fit of the calibration
curve. The coefficient is better than 99.99% for all the channels for barrel VFE cards. Since the dynamic range is different for the endcap and also because of a lower precision in the charge determination for small charges, the coefficient is greater than 99.98% for the endcap VFE cards. In both cases, only very small non-linearity was found.
D. Pedestal and electronic noise
Pedestal is the electronic baseline on which an electrical sig-nal is carried. Its value is programmable using a simple DAC circuit internal to the MGPA which is controlled externally by an I2C interface. Correct functionality of the pedestal setting
was verified in two different but complementary ways. In the first case, special pedestal measurements for each gain were per-formed. The DAC value is varied from 0 up to 100 and corre-sponding pedestal value is recorded. Fig.6 shows DAC values versus pedestal for one VFE card. After setting the pedestal fluctuation was evaluated from dedicated pedestal runs when no charge was injected. In addition, the pedestal was also computed from pre-samples taken prior to the injected charge. These tests were performed for each gain separately.
DAC value 0 10 20 30 40 50 60 70 80 90 100 ADC counts 0 200 400 600 800 1000 1200 1400 1600 1800 2000
VFE Card 33152020011072, Gain x6
DAC value 0 10 20 30 40 50 60 70 80 90 100 ADC counts 0 200 400 600 800 1000 1200 1400 1600 1800 2000 DAC value 0 10 20 30 40 50 60 70 80 90 100 ADC counts 0 200 400 600 800 1000 1200 1400 1600 1800 2000 DAC value 0 10 20 30 40 50 60 70 80 90 100 ADC counts 0 200 400 600 800 1000 1200 1400 1600 1800 2000 DAC value 0 10 20 30 40 50 60 70 80 90 100 ADC counts 0 200 400 600 800 1000 1200 1400 1600 1800 2000 channel 1 channel 2 channel 3 channel 4 channel 5
Figure 6: DAC value versus pedestal for five channels of the barrel VFE card, gain 1. The channel 1 does not work correctly, hence this VFE card has been rejected.
hTotNoiseSM_0 Entries 1700 Mean 850.6 RMS 491.5 channel # 0 200 400 600 800 1000 1200 1400 1600 ADC counts 0.2 0.4 0.6 0.8 1 1.2 1.4 hTotNoiseSM_0 Entries 1700 Mean 850.6 RMS 491.5 Tot noise vs channel number, gain x12
gain x1 gain x6 gain x12 Entries 1700 Constant 363.2 Mean 0.4704 Sigma 0.03718 ADC counts 0.2 0.4 0.6 0.8 1 1.2 1.4 0 50 100 150 200 250 300 350 Entries 1700 Constant 363.2 Mean 0.4704 Sigma 0.03718
Tot Noise gain x1
Entries 1700 Constant 308.3 Mean 0.6627 Sigma 0.04329 Entries 1700 Constant 308.3 Mean 0.6627 Sigma 0.04329 Entries 1700 Constant 308.3 Mean 0.6627 Sigma 0.04329 Entries 1700 Constant 308.3 Mean 0.6627 Sigma 0.04329 Entries 1700 Constant 218.3 Mean 1.023 Sigma 0.06083 Entries 1700 Constant 218.3 Mean 1.023 Sigma 0.06083 Entries 1700 Constant 218.3 Mean 1.023 Sigma 0.06083 Entries 1700 Constant 218.3 Mean 1.023 Sigma 0.06083 gain x1 gain x6 gain x12
Figure 7: Electronic noise in ADC counts versus channel number (top) and its distribution (bottom) for a barrel supermodule (1,700 crystals).
The pedestal runs were also used for an estimation of the electronic noise as the rms of the pedestal distribution. The noise values on the calibration bench are greater than measured in ECAL which is due to the important contribution from cal-ibration system itself. VFE cards for which the pedestal could not be set correctly or the rms noise was greater than 10 σ of the noise distribution for all measured channels were rejected.
re-construction weight method [7] on the pedestal runs. A typi-cal mean value of the electronic noise for a single channel over an entire barrel supermodule (1,700 channels) is around 1 ADC count (∼37 MeV) for gain 12. This value includes all contribu-tions to the noise over the whole ECAL electronics chain. In Fig. 7 the electronics noise is plotted for all three gains for one supermodule, showing that the dispersion over the channels is very small. Sample 15 20 25 30 35 ADC counts 0 500 1000 1500 2000 2500 3000 3500 4000 Card 0 Channel 0 charge (pC) 54 56 58 60 62 64 66 ADC counts 3500 3600 3700 3800 3900 4000 4100 Card 0
Figure 8: ADC output for different injected charge in a single chan-nel (top) and linearity plot for five chanchan-nels of the barrel VFE board (bottom).
E. Electronic saturation
The gain 1 saturation effect was studied on the barrel VFE board by applying injected charges up to 66 pC (above the spec-ification and corresponding to > 2 TeV energy). It was found that the ADC saturates before the MGPA, which is linear up to levels larger than 3,900 ADC counts. If saturation occurs the ADC returns five consecutive samples with same value as can be seen in Fig. 8 top. Fig. 8 (bottom) shows that once the sat-uration is reached, it does not change with increasing value of the injected charge. It was also proven that saturation does not affect the timing corresponding to the maximum of the signal
collection needed for L1 triggering and thus does not distort the LHC bunch crossing identification.
F. Slow control
Other relevant tests, such as the calibration of leakage cur-rent measurements of the APDs and temperature read-out chan-nel on its complete dynamic range are also performed. The DCU chip placed on the VFE card collects data and sends them to the data acquisition system via the I2C interface. For the
tem-perature read-out channel, which is used for each 10th
crystal, the test board uses a digital potentiometer that changes its value to simulate a temperature variation of the crystals. The stabil-ity of the leakage current is tested using a current source which simulates a variation of the leakage current in a range between 0 – 200 nA to the input of the VFE.
III. S
UMMARY AND CONCLUSIONThe test program for all the 12,240 barrel and the ∼ 3,000 endcap VFE boards has been completed. The dispersion in the gains is found to be small (∼ 1%). Noise, linearity and gain ra-tios are complying with the CMS detector specifications. Only around 2 % of them failed the test criteria and have been re-jected, the rest have been assembled into barrel supermodules and soon will be also into endcap supercrystals. Results are reg-istered in a database and can be used for a first intercalibration of the ECAL. The results obtained during the Q&A program have been verified in summer 2006 and 2007, when several fully equipped supermodules and supercrystals were tested and cali-brated with high energy electrons.
R
EFERENCES[1] CMS Collaboration, CMS Physics: Technical Design Re-port – Detector performance and software, Vol. 1, Techni-cal Design Report CMS, CERN/LHCC 2006-001, 2006 [2] M. Raymond et al., The MGPA Electromagnetic
Calorimeter Readout Chip for CMS, IEEE Trans. Nucl. Sci., Vol. 52, 2005, pp 756–763
[3] G. Minderico et al., A CMOS Low Power, Quad Channel, 12 bit, 40MS/s Pipelined ADC for Applications in Parti-cle Physics Calorimetry, 9th
Workshop on Electronics for LHC Experiments, 2003, pp 88–91
[4] J. Blaha et al., Calibration of the very-front-end electron-ics for the electromagnetic calorimeter of the CMS exper-iment, Czech. J. Phys., Vol. 56, 2006, pp 879–881 [5] A. Nardulli et al., Performance of CMS ECAL Very Front
End Electronics, 12th
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[6] D. Bailleux et al., Performance of the Monitoring Light Source for the CMS Lead Tungstate Crystal Calorimeter, IEEE Trans. Nucl. Sci., Vol.52, 2005, pp 1123–1130 [7] R. Bruneli`ere and A. Zabi: Reconstruction of the signal
