An optically coupled charge injection system for ionization based radiation detectors which allows a test charge to be injected without the creation of ground loops has been developed. An ionization like signal from an external source is brought into the detector through an optical fiber and injected into the electrodes by means of a photodiode. As an application example, crosstalk measurements on a liquid Argon electromagnetic calorimeter readout electrodes were performed

Chen, H

Citterio, M

Lanni, F

Leite, M A L

Radeka, V

Rescia, S

Takai, H

Chen, H.

Citterio, M.

Lanni, F.

Leite, M. A. L.

Radeka, V.

Rescia, S.

Takai, H.

UNT (University of North Texas) Digital Library

Pres. 7iJ’FVorkslIop on Electronics for LHC ExperimentsStoclchobn, Swedent 11-14 September (2001)BNL-68716OPTICALLY BASED CHARGE INJECTION SYSTEMFOR IONIZATION DETECTORS*H. Chenl, M. Citterio2, F. Lannil, M.A.L. Leitel,V. Radekal, S. Rescial, and H. TakaillBrookhaven National Laboratory, Upton, NY 1197321NFN,Via Celoria, 1620100 Milano, ItalySeptember, 2001“Work supported in part by the U.S. Department of Energy Contract No. DE-ACO2-98CH1O886.Optically Based Charge Injection System for Ionization Detectors”.H. Chenl, M. Citterio2, F. Larmil, M.A.L. Leitel,”, V. Radekal. S. Rescial and H. Takail(1) Brookhaven National Laboratory, Upton, NY -11973. USA(*j leite@bnl.gov(2) INFN, Via Celoria, 1620100 Milano, ItalyAbstractAn optically coupled charge injection system forionization based radiation detectors which allows a testcharge to be injected without the creation of ground loopshas been developed. An ionization like signal from anexternal source is brought into the detector through anoptical fiber and injected into the electrodes by means of aphotodiode. As am application example. crosstalkmeasurements on a liquid Argon electromagnetic calorimeterreadout electrodes were performed.L INTRODUCTIONFor the performance tests of ionization based radiationdetectors it is desirable to have a system which is capable ofinjecting a charge of known value in a condition that is asclose as possible to the operating environment, where chargeis locally generated by the ionization of the sensitive mediain the detector. One of the main problems with theconventional approach of the direct injection through anelectrical cable connected to the detector electrodes is thechange of the detector electrical characteristics[ l]. Inparticular, the grounding configuration of the system can becompletely modified by the introduction of the injectioncircuit new ground path. The use of an optically coupledinjection, in which a light to current converter is placed onthe electrodes of the detector to generate the ionizationsignal, allows for a full galvanic isolation between thedetector and the test pulser (Fig. 1).BiasT–’m’’’”optical fiber~—Figure 1: Principle of the optically c;upled injection of a testsignal to an ionization detector. The capacitance added by thephotodiode is negligible.An optical fiber carries a light pulse, modulated to thesame waveshape of a physical signal, to a photodiodeconnected in parallel to the detector. The photodiode isbiased using the same voltage distribution network for thedetector bias, using an appropriate voltage, typically 30 V – .50 V. Since no additional electrical connection to thedetector is necessary, the electrical environment of thedetector, including grounding, is leit undisturbed. Thismakes possible to study on a test bench issues likecrosstalk[l ] or electromagnetic interference [2] whereadditional ground loops might taint the results of bench tests.A photodiode installed on the electrodes and biased bymeans of the high voltage system achieves the light tocurrent conversion. It has a capacitance of only a fewpicofarads, small if compared with detector capacitances ofthe order of nanofarads. The photodiode should also have afast time response and low dark current. Size is also an issue,as this device may needs to fit in spaces of only a fewmillimeters. The light, generated by a laser diode stimulatedto produce a suitable signal for the detector, is brought to thephotodiode using a multimode optical fiber. Fig. 2 and Fig. 3show the characteristics of two commercial devices whichcan be used for this application. The laser diode is a VCSEL(Vertical Cavity Surface Emitting Laser) device, capable ofgenerating up to 3 mW of optical power, with a peakwavelength of 860 nm, matching the photodiode maximumsensitivity of 850 nm. The use of a PIN photodiode allowsthe generation of signals with a rise time of 2 ns, adequatefor this application.25 1 I I \:?.120 –~I1r .;~ 15 –{8: : {,,=g ,0 \\G R‘.*L -m5----“—-.-.+ . . . . . . . . . . . . ...+L_—----=- . . . . .00 . k.~. ~;10 20 30 40Bias Voltage [VlFigure 2: PIN diode capacitance. Capacitance of a PIN diode(Panasonic PNZ334) as a function of the bias voltage. At theoperating voltage (28 V) the capacitance is 3.6 pF.~ Work supported by the U.S. Department of Energy:Contract No. DE–ACO2–98CH1O886..3 – — MI .,/.M2--- M3 /J ,.,’-:-- M4 ,../’ ,,;-”; .?--M5--- M6 /’ .,-”:-’ “’~2 - M7 /’” /.’”-’:.,/,” “ ‘.E_ ---’ M8 /-/.,. -,fl ., ,, ~c..,.z ,/-’ ,:, . / ., ,,... /: ,.$ &“- ‘“ ~::...,-..” ‘“--.,. ,.1 –,$~j$:~’~:’””-””’///>&-’(o.- -.I Io 5 10 15 20Sias Current [mA]Fignre 3: VCSEL power. Power output as a function of the biascurrent for VCSEL laser diode (MITEL 1A444). Eight differentdevices (Ml – M8) have been measured.II. EXAMPLEOFAPPLICATION:CROSSTALKSTUDYAs an example of this method, the ATLASElectromagnetic Liquid Argon Calorimeter[3] test stand atBNL has been modified by adding a photodiode on a fewcalorimeter channels (Fig. 5). The optical signal is createdusing the VCSEL modulated to produce the same triangularshaped pulse generated by an ionization signal. Crosstalkstudies were performed by injecting a signal in one channelat a time and recording the crosstalk pattern on neighborchannels. The output signal is processed using an identicalsegment of the readout chain of the ATLAS barrelcalorimeter a transimpedance preamplifier (BNL 25S210824 [4]) and a CR–RC2 shaper, using a time constant of15 ns (Fig. 4)...............Figure 4: Pulse generation and readout signal circuit. The opticalsignal is generated by LD1 (MITEL IA444 VCSEL), biased by acurrent source (ILX Lightwave LDX–3630) and driven directly byan arbitrary signal generator (Arralogic Model 2040) programmedto give out a triangular pulse. The light signal is transmitted bythe optical fiber to PD1 (Panasonic PNZ 334). A current is theninjected in one channel of the calorimeter test module(capacitance Cd)j which is AC coupled by G+cdirectly to thereadout chain.,/_-=.:__=. -.--__#(c)Figure 5: Experimental setup for optically coupled charge injectionof the ATLAS barrel liquid Argon electromagnetic calorimeter. APIN photodiode is connected (a) within the gap from the absorber@round) to the high voltage layer. Ml through M4 and Bl, B2 arereadout channels. The si~mal ground is provided by two points(GND Spring 1 and 2). The optical fiber is run along the tip of thebending of an electrode (b), and brought to the side of the module,where it is coupled to a laser source. The photodiode is mounted ona carrier PC board (c), soldered to the electrode (anode) andcontacting the absorber (cathode). An iridium foil is used to assurea low resistance electrical connection.~OizcI=lInjected Signal (channel Ml)0 200 400 6;0 8001 />,,1 I7 ,,/ \Injected Signal (channel M2)1p ~ / ‘,,.-:0 — )’ , ‘~.>_..., ,-,.... ....Io 200 400 600 800Figure 6: Measured crosstalk. A signal is injected in one channel(Fig. 5a Ml or M2) and the crosstalk is observed in the closest andin the farthest neighboring channels with ground springs both tothe left (GND spring 2) and to the right (GND spring 1) of theconnector (continuous trace). Removal of the ground springs to thelefi of the connector increases the distant crosstalk (dashed traces).In this example, one of the ground connecticms betweenthe electrodes and the absorbers on the left side of the signalconnector (GND Spring 2 in Fig. 5a) was removed, and thechange in the crosstalk pattern observed by injecting a signalin two different neighbor channels (M1 and M2). The results(Fig. 6) show that the nearest neighbor crosstalk isunchanged. The distant crosstalk (channel M4) is indeedaffected by the removal of the ground springs, but remainsalways less than 0.2°/0. The optical injection allows toreliably detect differences in crosstalk signals of less than 1mV and to rule out any ground loop effect as the cause ofthese small differences.III. CONCLUSIONSWe showed that an ionization–like signal can be injectedby optical means, allowing measurements of small amplitudesignals in large systems without disturbing the groundingconfiguration of the detector. Since the capacitance of thephotodiode is much smaller than the detector capacitance,there is no change in the electrical characteristics of thesystem. Most important, the undisturbed groundconfiguration makes possible to systematically study smallamplitude effects that otherwise would be masked byinterferences caused by differences in the ground path.This setup can be expanded for injecting signals inmultiple channels simultaneously. This may beaccomplished by the use of a direct current injection of thelaser (Fig. 7) and direct coupling to the fiber without opticalconnectors, allowing injection and inter–calibration ofseveral channels. With this method it is possible, forexample, to reproduce the charge distribution of an EMshower over many cells of a calorimeter, thus allowing amore complete study of the crosstalk of the system.Figure 7: Fast voltage-to-current converter for direct lasermodulation. This simple driver circuit allows to build manycompact driver–laser–photodiode assemblies which can be inter-calibrated before installation on the electrodes. The opticalconnections could be simply made with optical glue, thus avoidingoptical connectors and the variation in attenuation inherent in themating of optical connectors.IV. ACKNOWLEDGEMENTSWe would like to express our gratitude to Dr. T. Tsang,from the Instrumentation Division of the BrookhavenNational Laboratory, for the many helpfid discussions aboutthe optical setup.V. ReferenCeS[1] M. Citterio, M. Delmastro, M. Fanti. A study of theelectrical properties and of the signal shapes in the ATLASliquid Argon accordion calorimeter using a hardware model.ATLAS Larg Internal Note, May 2001.[2] B. Chase. M. Citterio, F. Lanni, D. Makowiecki, V.Radeka, S. Rescia, H. Takai, J. Bin. J. Parsons, W.Sippach. Characterization of the coherent noise,electromagnetic compatibility and electromagneticinterference of the ATLAS EM calorimeter Front End Board.5th Conference on Electronics for LHC Experiments,Snowmass, CO, USA, 20 – 24 Sep 1999. CERN–99-09 ;CERN–LHCC–99–033 – pp.222–226.[3] ATLAS Collaboration. ATLAS liquid argoncalorimeter technical desig report. CEIUWLHCC196-41,CERN (1996).[4] R. L. Chase and S. Rescia. A linear low powerremote preamplifier for the ATLAS liquid argon EMcalorimeter. IEEE Trans. Nucl. Sci., 44:1028, 1997.

Optically Based Charge Injection System for Ionization Detectors

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Optically Based Charge Injection System for Ionization Detectors.H. Chen1, M. Citterio2, F. Lanni1, M.A.L. Leite1,*, V. Radeka1, S. Rescia1 and H. Takai1 (1) Brookhaven National Laboratory, Upton, NY − 11973, USA(*) leite@bnl.gov(2) INFN, Via Celoria, 16 20100 Milano, ItalyAbstractAn optically coupled charge injection system forionization based radiation detectors which allows a testcharge to be injected without the creation of ground loopshas been developed. An ionization like signal from anexternal source is brought into the detector through anoptical fiber and injected into the electrodes by means of aphotodiode. As an application example, crosstalkmeasurements on a liquid Argon electromagnetic calorimeterreadout electrodes were performed.I. INTRODUCTIONFor the performance tests of ionization based radiationdetectors it is desirable to have a system which is capable ofinjecting a charge of known value in a condition that is asclose as possible to the operating environment, where chargeis locally generated by the ionization of the sensitive mediain the detector. One of the main problems with theconventional approach of the direct injection through anelectrical cable connected to the detector electrodes is thechange of the detector electrical characteristics[1]. Inparticular, the grounding configuration of the system can becompletely modified by the introduction of the injectioncircuit new ground path. The use of an optically coupledinjection, in which a light to current converter is placed onthe electrodes of the detector to generate the ionizationsignal, allows for a full galvanic isolation between thedetector and the test pulser (Fig. 1). Figure 1: Principle of the optically coupled injection of a testsignal to an ionization detector. The capacitance  added by thephotodiode is negligible.An optical fiber carries a light pulse, modulated to thesame waveshape of a physical signal, to a photodiodeconnected in parallel to the detector. The photodiode isbiased using the same voltage distribution network for thedetector bias, using an appropriate voltage, typically 30 V −50 V. Since no additional electrical connection to thedetector is necessary, the electrical environment of thedetector, including grounding, is left undisturbed. Thismakes possible to study on a test bench issues likecrosstalk[1] or electromagnetic interference[2] whereadditional ground loops might taint the results of bench tests.A photodiode installed on the electrodes and biased bymeans of the high voltage system achieves the light tocurrent conversion. It has a capacitance of only a fewpicofarads, small if compared with detector capacitances ofthe order of nanofarads. The photodiode should also have afast time response and low dark current. Size is also an issue,as this device may needs to fit in spaces of only a fewmillimeters. The light, generated by a laser diode stimulatedto produce a suitable signal for the detector, is brought to thephotodiode using a multimode optical fiber. Fig. 2 and Fig. 3show the characteristics of two commercial devices whichcan be used for this application. The laser diode is a VCSEL(Vertical Cavity Surface Emitting Laser) device, capable ofgenerating up to 3 mW of optical power, with a peakwavelength of 860 nm, matching the photodiode maximumsensitivity of 850 nm. The use of a PIN photodiode allowsthe generation of signals with a rise time of 2 ns, adequatefor this application.Figure 2: PIN diode capacitance. Capacitance of a PIN diode(Panasonic PNZ334) as a function of the bias voltage. At theoperating voltage (28 V) the capacitance is 3.6 pF.    Figure 3:  VCSEL power. Power output as a function of the biascurrent for VCSEL laser diode (MITEL 1A444). Eight differentdevices (M1 − M8) have been measured. II. EXAMPLE OF APPLICATION: CROSSTALK STUDY As an example of this method, the ATLASElectromagnetic Liquid Argon Calorimeter[3] test stand atBNL has been modified by adding a photodiode on a fewcalorimeter channels (Fig. 5). The optical signal is createdusing the VCSEL modulated to produce the same triangularshaped pulse generated by an ionization signal. Crosstalkstudies were performed by injecting a signal in one channelat a time and recording the crosstalk pattern on neighborchannels. The output signal is processed using an identicalsegment of the readout chain of the ATLAS barrelcalorimeter: a transimpedance preamplifier (BNL 25ΩIO824 [4]) and a CR−RC2 shaper, using a time constant of15 ns (Fig. 4).Figure 4: Pulse generation and readout signal circuit. The opticalsignal is generated by  LD1 (MITEL 1A444 VCSEL), biased by acurrent source (ILX Lightwave LDX−3630) and driven directly byan arbitrary signal generator (Analogic Model 2040) programmedto give out a triangular pulse. The  light signal is transmitted  bythe optical fiber to PD1 (Panasonic  PNZ 334). A current is theninjected  in one channel of the  calorimeter test module(capacitance Cd), which is AC coupled by CAC directly to thereadout chain.    (b)  (c)Figure 5: Experimental setup for optically coupled charge injectionof the ATLAS barrel liquid Argon electromagnetic calorimeter.   APIN photodiode  is connected (a) within the gap from the absorber(ground) to the high voltage layer. M1 through M4 and B1, B2 arereadout channels. The signal ground is provided by two points(GND Spring 1 and 2). The  optical fiber is run along the tip of thebending of an electrode (b), and brought to the side of the module,where it is coupled to a laser source. The photodiode is mounted ona carrier PC board (c), soldered to the electrode (anode) andcontacting the absorber (cathode). An indium foil is used to assurea low resistance electrical connection.  Figure 6: Measured crosstalk. A signal is injected in one channel(Fig. 5a  M1 or M2) and the crosstalk is observed in the closest andin the farthest neighboring channels with ground springs both tothe left   (GND spring 2) and to the right (GND spring 1) of theconnector (continuous trace). Removal of the ground springs to theleft of the connector increases the distant crosstalk (dashed traces).In this example, one of the ground connections betweenthe electrodes and the absorbers on the left side of the signalconnector (GND Spring 2 in Fig. 5a) was removed, and thechange in the crosstalk pattern observed by injecting a signalin two different neighbor channels (M1 and M2). The results(Fig. 6) show that the nearest neighbor crosstalk isunchanged. The distant crosstalk (channel M4) is indeedaffected by the removal of the ground springs, but remainsalways less than 0.2%. The optical injection allows toreliably detect differences in crosstalk signals of less than 1mV and to rule out any ground loop effect as the cause ofthese small differences.  III. CONCLUSIONSWe showed that an ionization−like signal can be injectedby optical means, allowing measurements of small amplitudesignals in large systems without disturbing the groundingconfiguration of the detector. Since the capacitance of thephotodiode is much smaller than the detector capacitance,there is no change in the electrical characteristics of thesystem. Most important, the undisturbed groundconfiguration makes possible to systematically study smallamplitude effects that otherwise would be masked byinterferences caused by differences in the ground path.  This setup can be expanded for injecting signals inmultiple channels simultaneously. This may beaccomplished by the use of a direct current injection of thelaser (Fig. 7) and direct coupling to the fiber without opticalconnectors, allowing injection and inter−calibration ofseveral channels. With this method it is possible, forexample, to reproduce the charge distribution of an EMshower over many cells of a calorimeter, thus allowing amore complete study of the crosstalk of the system. Figure 7:  Fast voltage−to−current converter for direct lasermodulation. This simple driver circuit allows to build manycompact driver−laser−photodiode assemblies which can be inter−calibrated before installation on the electrodes. The opticalconnections could be simply made with optical glue, thus avoidingoptical connectors and the variation in attenuation  inherent in themating of optical connectors.IV. ACKNOWLEDGEMENTSWe would like to express our gratitude to Dr. T. Tsang,from the Instrumentation Division of the BrookhavenNational Laboratory, for the many helpful discussions aboutthe optical setup.V. REFERENCES[1] M. Citterio, M. Delmastro, M. Fanti. A study of theelectrical properties and of the signal shapes in the ATLASliquid Argon accordion calorimeter using a hardware model.ATLAS Larg Internal Note, May 2001.[2] B. Chase,  M. Citterio,  F. Lanni,  D. Makowiecki, V.Radeka,  S. Rescia, H. Takai,  J. Bán,  J. Parsons,  W.Sippach.  Characterization of the coherent noise,electromagnetic compatibility and electromagneticinterference of the ATLAS EM calorimeter Front End Board.5th Conference on Electronics for LHC Experiments,Snowmass, CO, USA, 20 − 24 Sep 1999. CERN−99−09 ;CERN−LHCC−99−033 − pp.222−226. [3] ATLAS Collaboration. ATLAS liquid argoncalorimeter technical desig report. CERN/LHCC/96−41,CERN (1996).[4] R. L. Chase and S. Rescia. A linear low powerremote preamplifier for the ATLAS liquid argon EMcalorimeter. IEEE Trans. Nucl. Sci., 44:1028, 1997.

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Optically Based Charge Injection System for Ionization Detectors

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