Berkhan, K

Kolanoski, H

Pohl, M

Uwer, U

Vasilev, S

CERN Document Server

LARGE-SYSTEM EXPERIENCE WITH THE ASD-8 CHIP INTHE HERA-B EXPERIMENTK.Berkhan1, H.Kolanoski2, M.Pohl1, U.Uwer2, S.Vassiliev31DESY Zeuthen, Germany;2Humboldt Universitat Berlin, Germany;3JINR Dubna, RussiaAbstractThe amplier-shaper-discriminator chip ASD-8, developed by the University of Pennsylvania for drift chamber applica-tions in high rate environments, is used in large detector systems in the HERA-B experiment. We report on the results oftests performed on some 25 000 chips and on the design and tests of the PC-boards. The so far largest system employingASD-8 chips is the HERA-B Outer Tracker with about 120 000 drift chamber channels. We present rst experience withthe grounding scheme, noise suppression and the slow control system as well as rst measurements with the full readoutchain in the HERA-B experiment.1. INTRODUCTIONHERA-B is a xed target experiment for studying CPviolation in B-meson systems using an internal wiretarget in the proton beam of HERA [1]. To reach thenecessary production rate of b quarks an average of4 interactions per bunch at a frequency of about 10MHz (96 ns bunch separation) has to be generated.This leads to a high particle density with a radial dis-tribution of the particle ux (R is the distance fromthe beam): 1014: : : 1015R2 yearThe main detector components are a silicon vertexdetector, a magnet (2.2 Tm), a main tracker with MS-GCs in the inner and drift tubes in the outer part,`High-PT' Chambers, a RICH, an electro-magneticcalorimeter and a muon detector with drift tubes. Thedetector covers a forward angular range of 220 mradin the bending plane of the magnet and 160 mrad ver-tically.The amplier-shaper-discriminator chip ASD-8 [2],developed by the University of Pennsylvania for driftchamber applications in a high rate environment, isused in dierent detector systems of the HERA-B ex-periment (Outer Tracker, RICH, Muon System, High-PTChambers) with a total of about 200 000 channels.In the followingwe refer mainly to the Outer Trackerwhich is with about 120000 channels the largest ASD-8application in HERA-B.2. THE OUTER TRACKER SYSTEMThe Outer Tracker of HERA-B [1] consists of 13 pla-nar superlayers of drift tube modules. The detectoracceptance starts at a radial distance of 19 cm to thebeam. Together with the Inner Tracker it allows a pre-cise momentum measurement and provides fast trackrecognition on the rst trigger level.The drift tubes are folded from gold-coated poly-carbonate foil with an hexagonal cross section (honey-comb tubes). The inner diameter of the cells is 5 mmin the most exposed sections and 10 mm further out-side. As drift gas Ar/CF4/CO2(65/30/5) is chosen.Operating at a gain of about 4  104the drift velocityis about 100 m/ns which allows the track signals tobe collected within the bunch crossing time of 96 ns.The particle densities and radiation levels in theOuter Tracker are comparable to those in similar de-tectors for LHC. In the hottest area the particle uxis about 2  103mm 2s 1. The drift tubes are longi-tudinally subdivided to limit the single channel occu-pancy to about 20% (the shortest segmentation nearthe beam is 20 cm). Because of the planar detectorgeometry the front-end ampliers can be placed awayfrom the beam pipe yielding a radiation load belowabout 50 Gy per year at the location of the ASD-8boards.Pattern recognition in a dense particle environmentand a precise momentum measurement require a po-sition resolution of about 200 m. The correspondinggood time resolution can be achieved by triggering onthe rst electron cluster arriving at the anode. At again of 4 104a cluster results in about 1.5 fC after fastshaping. Such a low threshold requires low noise andlow crosstalk in the system. The rst level trigger re-quires a high hit eciency and a fast signal collectionwithin the bunch separation of 96 ns.In order to full the requirements on tracking andtriggering in a large system of about 120000 channelsthe tracker front-end electronics should have the fol-lowing characteristics:- High integration density at low power consumption;- low per-channel costs;- fast signal shaping and precise timing;- low noise: threshold sensitivity of about 2 fC;Table 1. Performance gures of the ASD-8 chipintegration density 8 ch. on 2:7 4:3mm2diepower consumption 0.3 W/chip (incl. output)cost about 4 DM/channelsignal shaping time about 10 nstail cancellation t0 1:5nsdouble pulse resol. 25 nsintrinsic noise (900 + 70/pF) electronson-chip crosstalk analog:  0.1%threshold  2 fCbaseline shift 0.5 - 1.0 fC/MHzDISTRIBUTION BOARDGAS BOXCHAMBERLV SUPPLYTEST PULSEHV BOARDASDASD BOARDHVFLTTDC   DSP  Figure 1. The front-end electronics of the OuterTracker.- low crosstalk, required dynamic range:total charge / threshold charge = 40;- small rate dependence of the threshold up to about2 MHz;- radiation hardness up to a level of about 1 kGy.These requirements were found to be met by theASD-8 chip as shown by the actually achieved perfor-mance gures listed in table 1.3. THE OUTER TRACKER FRONT-END ELECTRONICSFor the Outer Tracker drift tubes the anode wires areon high voltage and the cathode foils on ground. Theanode signals are AC coupled to the amplier via cou-pling capacitors which are placed inside the gas vol-ume (discharge protection). For better accessibilitythe ASD-8 board is mounted outside the gas box.The whole readout chain of the Outer Tracker(Fig. 1) consists of an HV board with coupling ca-pacitors for the chamber signals, a connecting twistedpair cable, a feed-through board to transfer the signalthrough the wall of the gas box, the ASD-8 board, atwisted pair cable and TDCs. The TDCs are read outby digital signal processors. In addition the hit in-formation of 4 selected superlayers (trigger layers) aretransferred to trigger link boards.FEE Components:HV board: The HV boards are printed circuit boardsconnecting the chamber wires to the high voltage andto the pre-ampliers. The anode signals are AC cou-PREAMP. 1.SHAPER 2.SHAPER TAIL CANCEL. DISCR.Analog OutTest Select2. ShaperTest SelectTail Cancel.ThresholdMultiplexerFigure 2. Principal functions of the ASD-8 chip.pled to the amplier via 330 pF capacitors. The boardsare located inside the gas volume.Feed-through board: The signal cables (twistedpair, length 25 or 50 cm) from the HV boards areplugged from the inside of the gas-box to the feed-through boards. On their outside the feed-throughboards hold the ASD-8.ASD-8 board: On the ASD-8 board the drift cham-ber signals are transformed to a digital output whichis transmitted via shielded twisted pair cables to theTDC's. The board is described in detail below.Time-to-digital converter: The customized 8-channel TDC chip digitizes the time in 0.5 ns binswith an 8 bit output. Each TDC channel has a GTLreceiver on chip. The dierential, open collector out-put of the ASD-8 is terminated at the receiver side(see sect. 5).The crates housing the TDC boards are mountedon the cable frame of the detector superlayers. Themaximal cable length between the ASD-8 boards andthe TDC crates is 5 to 10 m.ASD-8 power distribution board: These boardsserve three purposes: distribution of the +/- 3Vpower, slow control of thresholds, currents, voltagesand switching of the test pulser.A distribution board supplies 6 groups with 8 ASD-8boards each. The threshold is the same for all ASD-8 boards in a group and can be selected out of twothreshold values provided on the distribution board.Tests pulses can be sent to selected groups.The boards are controlled by a SLIO processor andare connected to the slow control system via CAN bus.4. THE ASD-8 CHIP4.1. Technical DataThe amplier-shaper-discriminator chip ASD-8 [2](g. 2) has been developed by the University of Penn-sylvania for drift chamber applications under high ra-diation (specically for SSC). The 8-channel chip is de-signed in a bipolar process which combines high speedwith a low noise level and low power consumption.Amplier: The ASD-8 input is a preamplier with asensitivity of 2.5 mV/fC, a bandwidth of 100 MHz andan input impedance of 115 . The input is dierentialTable 2. Technical data of the ASD-8 boardchannels 16 (2 chips)dimensions 4 layers, 67  56 mm2pickup suppression all supply voltages RC lteredspark protection diode protection 3 kVcross talk (analog) < 0:5%gain uniformity 15% per boardoutput 2 mA into 62 voltage supply +3 V, 100 mA 3 V, 100 mApower 600 mWand symmetric for positive and negative pulses.Shaper: The two-stage shaper with tail cancellationyields a double pulse resolution of 25 ns. The tail can-cellation compensates the ion tail of the drift chamberpulses. Analog outputs are provided for 3 channels perchip, selectable after the 2nd shaper or after the tailcancellation. The output pulse is proportional to theinput signal up to about 50 fC (20 mV/fC).Discriminator: The discriminator is a two-stage dier-ential amplier with positive feedback. The thresholdis voltage programmable for each channel (with 250mV/ fC, maximal about 1.4 V). The dierential, opencollector output is current programmable to adjust theswing.4.2. Series chip tests and selectionHERA-B ordered a total of 90 wafers, each with about335 chips. The yield per wafer was on average 77%.Each chip was tested according to a scheme which eval-uated the general functioning, the threshold behaviourand the noise level.For each channel the threshold Ueffto record astandard 4 fC test pulse with 50% eciency was de-termined. The noise behaviour of a chip was char-acterized by the threshold Unoisefor which the noiserate exceeded 2 kHz. The dierence Ueff  Unoiseisa measure for the signal-to-noise distance and thus ofthe quality of a channel.The chip tests revealed an appreciable range for theUeffthreshold from about 900 to 1400 mV (g. 3).With this variation it is not possible to dene a com-mon threshold for the whole system if a homogeneoussensitivity to the rst arriving electron cluster shouldbe obtained. On the other hand, to keep the front-endelectronics simple and compact, individual thresholdsettings for each channel should be avoided. There-fore 4 categories of threshold ranges were dened:Ueff= 860:::950; 960:::1050; 1060:::1150; > 1150mV:To account for the signal-to-noise variation for a givenUeff(g. 3) in each of the 4 gain categories, 3 noisecategories corresponding to noise distances had to bedened:Ueff  Unoise= 550; 500 50; 400 50mV;Correlation of signal and noise thresholds (channels)80010001200140016000 200 400 600 800 1000Unoise [mV]          Ueff [mV]Figure 3. Threshold Ueffplotted against the noisethreshold Unoisefor each channel of the tested chips.Signal threshold variation on a board02004006000 100 200 300 400 500Ueff max - Ueff min  [mV]Number of boardsFigure 4. Threshold uniformity on an ASD-8 board:distribution of the dierence between maximal andminimal reference threshold Ueffon a board.where Ueffare the mean values of the signal cate-gories (Ueff= 900; 1000; 1100; 1200mV). A chip wasassigned to one of the 12 categories according to itsminimal Ueffand maximal Unoisevalues. The assign-ment was used to mount similar chips on a board (car-rying two chips) and combine similar boards to groupswhich are supplied with the same threshold. Chipswith worse noise performance are used in less sensitiveareas (lower rates, 10 mm cells).5. THE ASD-8 BOARD5.1. The Board DesignDesign considerations: The analog inputs of theASD-8 chips have a very high sensitivity in the orderof 1 fC which makes them susceptible to noise and RFpickup. Because of the combination of analog inputsand digital outputs on the chip one has in particu-lar to worry about feedback from the output to theDiode protectionRC filter forpower supplythreshold filterwith possiblediode shiftwireCouplingcapacitanceto reducepickupTest pulsecapacitance10 : 1 dividerfor test pulseTest pulseThresholdPower supply(digital andanalog separated)Differential outputs( current~ 2 mA)Figure 5. Schematic of the ASD-8 board.input. In the design of the printed circuit boards car-rying the ASD-8 chips special care was taken for agood grounding scheme, decoupling of the analog anddigital parts, noise rejection from power sources andnoise and crosstalk separation of dierent channels ina densely packed environment.The board design: Two ASD-8 chips with 8 chan-nels each are mounted on a multilayer board (table 2,g. 5).The chip has a fully dierential input: the anodesignal is fed into the negative input and the positiveinput is connected via 10 pF to the chamber groundwhich reduces the RF pickup.All supply voltages (3V) are separated into ana-log, digital and output drive supplies and have RC l-ters at the input. Between the analog and digital partof the chip is a gap in the groundplane. Both groundsare connected via an inductance at the input connec-tor. The analog ground is extended to rails along thesides of the board which slide into the holding brack-ets on the chambers. The brackets are made of CuBesprings providing a ne-mashed shielding between theboards in the densely packaged front-end electronicson the chambers.Since it was found that the ASD-8 inputs survivevoltage spikes only up to about 300 V a diode protec-tion was added. Tests have shown that the diode incombination with an input resistor of 50 Ohms pro-tected the input transistors for discharges of 3000 Vfed into the input via a 1 nF capacitor.Only one voltage level for the thresholds is providedper board. The two chips on each board were thereforematched according to their gain and noise quality class(see chip tests, sect. 4.2). The maximum dierence ofthe reference thresholds Ueffof two channels of a sin-gle board should be less than 300 mV. In some casesSchottky diodes ( Uf= 200 or 380 mV ) were used toshift the threshold of chips or single channels to avoidmore categories or to reduce the rejects. The varia-tions of the threshold sensitivities for dierent boardswill be compensated by a threshold correction table.The dierential open collector output is currentprogammable. Pull-up resistors on the TDC boardsof 62  yield at the chosen current of 2 mA a swing of120 mV (an oset of 1.25 V is added by the receiverson the TDC board). The analog outputs of the chipsare not enabled.Testpulses can be coupled to the positive input viaa 1:10 divider (reduction of noise pickup via the testpulse system) and a 1 pF capacitance. The pulses reall channels on a board at the same time.5.2. Board TestsThe 11000 produced boards had to undergo qualitytests and were then sorted according to 12 categoriesas done for the single chips: 4 categories (1, 2, 3, 4)according to the threshold of 50% eciency for 4 fCUeffand 3 categories (green, blue, red) of signal-to-noise distance Ueff  Unoise. A board enters into acategory according to the channels with minimal Ueffand maximal Unoise.Table 3 shows the distribution of the boards in dif-ferent categories. For each board the two chips be-long to the same category. The remaining variationsof Ueffwithin the 16 channels can be seen in g. 4which shows the dierence between the maximal andminimal Ueffon the boards. Dierences larger than300 mV have been decreased by Schottky diodes asdescribed above. With this procedure the thresholduniformity on the bords is about 15%. The vari-ations between dierent boards are accounted for bythe threshold setting on the distribution boards.The board quality is mainly determined by theUeff Unoisecategory. Table 3 shows that more than80% are in the two better categories `green' and `blue'.6. ELECTRONICS INSTALLATIONAND COMMISSIONINGThe ASD-8 boards of the dierent categories were in-stalled such that for the high-occupancy, innermost de-tector parts ampliers with a large signal-noise dier-ence (e.g green) are chosen. The outer detector parts,contributing less to the acceptance, are equipped withboards which have a small signal-noise dierence (e.g.red). The CAN bus controlled distribution cards al-low to set individual thresholds for groups of 8 ASD-8boards (see sect. 3).Cu-Be rails soldered to the gas box are used to pro-vide good ground connection between the gas box andthe individual ASD-8 boards. In addition shielding ofthe output signal cables is necessary to minimize thecapacitive feedback of the digital output signals to theamplier inputs. Care is taken that the shielding has avery good, low impedance contact to the digital groundpotential of the ASD-8 board. Bad ground connectionsbetween either amplier and gas box or amplier andcable shielding leads to oscillations of the amplierswith their characteristic frequency of about 40 MHz.Table 3. Distribution of the tested ASD-8 boards in signal-noise categories, 4 categories (columns) for dierentgains, 3 categories (rows) for dierent signal-noise distance (Ueff= 900; 1000; 1100; 1200mV).Ueff  Unoise[mV] Ueff[mV]# 850{950 960{1050 1060{1150 >1150 whole range> 550 (green) 4.6 % 12.6 % 20.3 % 4.4 % 42.0 %500 50 (blue) 4.2 % 18.0 % 12.1 % 4.4 % 39.0 %400 50 (red) 4.0 % 6.1 % 8.2 % 0.6 % 19.0 %whole range 12.8 % 36.7 % 30.6 % 9.4 %10-410-310-210-110 1 2 3 4 55mm cells, ASD-8: green 210mm cells, ASD-8: red 2threshold [fC]noise occupancyFigure 6. Noise occupancies for two dierent sectors ofa superlayer after installation in HERA-B: The 5mmcells are equipped with ASD-8 boards from the bestnoise category (green) while the 10 mm cells are con-nected to boards of the worst noise category (red).024681012140 10 20 30 40 50 60 70 80Occupancies in MC2 -x100902010605Target Rate (MHz)Average Occupancy (%)Figure 7. Hit occupancies per channel as a function ofthe primary interaction rate in sectors of a superlayerat dierent distances to the proton beam. In the upperleft corner the position of the sectors is indicated; theproton beam goes through the cut-out on the right.The above measures lead to very low noise of thereadout chain. Operated at the threshold values Unoisefrom the laboratory tests the complete chain consist-ing of ASD-8 boards, plugged to the gas box, 5 m longtwisted-pair cable for the connection to the TDC, andTDC boards for the digitization, showed noise occu-pancies of less than 1% (96 ns large readout windows).The connection of the honeycomb modules to theamplier leads to a higher noise level. To compen-sate the antenna eect of the up to 2m long wires, anincrease of the threshold voltage by 150 to 200 mV,corresponding to about 0.8 fC, is necessary.The noise occupancies (readout window of 96 ns)for two sectors of a superlayer after installation inHERA-B are shown in g. 6. The plot shows a sec-tor with 5 mm cells equipped with ASD-8 boards withlow noise threshold (green) and a sector with 10 mmcells and ASD-8 boards from the worst noise class (red)is shown. For the `green' ASD-8 boards the thresholdcan be set as low as 2 fC, while for the `red' categorythe threshold has to be increased to about 3 fC toachieve a similar noise level (< 10 2).The well-functioning of the readout chain for largeparticle uxes is illustrated in g. 7. The hit occu-pancy for the dierent sectors of a superlayer (magnetchamber) is shown for dierent target interaction rates(primary proton-nucleus interaction rate). The mea-sured occupancies depend up to 80 MHz linearly onthe interaction rate, well above the nominal interac-tion rate of 40 MHz. There is no saturation eect athigh occupancies observed (the osets at zero targetrate results from electronic noise and background fromthe HERA accelerator)REFERENCES1. E.Hartouni et al., HERA-B Design Report,DESY-PRC 95/01 (1995).2. M.Newcomer et al., IEEE Trans.Nucl. Sci.NS-40(1990) 690;H.H.Williams et al., Nucl. Instrum.Meth.A360(1995) 146.

Large-System Experience with the ASD-8 Chip in the HERA-B Experiment

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