de Mul, F.F.M.

Nieland, J.

Serov, A.

Steenbergen, W.

van Kranenburg, H.

Wallinga, H.

English

A method to measure the blood perfusion of tissue is to apply photons to tissue and measure the frequency shift of the Doppler shifted photons.. To avoid the use of fibers, a chip was designed which contains photodetectors and electronic circuitry to amplify the signal. This IC serves as an interface between the tissue and the signal processor, which transforms the electrical data to medical information about the perfusion of tissue. To be able to measure at various tissue depths, the die contains photodiodes at various distances from the laser To make the photo current less susceptible to noise and disturbances, the current is amplified. The circuit features very low noise electronic circuitry. The electrical input noise current of the circuit is comparable to the noise current of a resistor of 400 kΩ. The die area is 4.5 by 5 millimeter. The total area for photodiodes is about 0.93µm2. The electronics, which is largely analog, require almost the rest of the available area

University of Twente Research Information

Designof an integrated photo detectorcircuit for laserDoppler blood flowmonitoringJ.Nielanda, H. vanKranenburg,H. Wallinga,Universityof TwenteMESAResearch InstituteP. O. Box217,7500AEEnschedetheNetherlandstel. +31 (0)534892841H.vanKranenburg@el.utwente.nlais at PhilipsSemiconductorsNijmegennow.A. Serov, W. Steenbergen,F. F. M. deMulUniversityof TwenteP. O. Box217,7500AEEnschedetheNetherlandstel. +31 (0)534893870A.Serov@tn.utwente.nlAbstractA methodto measurethebloodperfusionof tissueis toapplyphotonsto tissueandmeasurethefrequencyshiftoftheDopplershiftedphotons.To avoid the useof fibers, a chip wasdesignedwhichcontainsphotodetectors and electronic circuitry to am-plify the signal. This IC servesas an interfacebetweenthe tissueand thesignalprocessor, which transformstheelectricaldatato medicalinformationabouttheperfusionof tissue.To beableto measure at varioustissuedepths,thediecontainsphotodiodesat variousdistancesfromthelaser.To makethephotocurrentlesssusceptibleto noiseanddisturbances,thecurrentis amplified.Thecircuit featuresvery low noiseelectronic circuitry. Theelectrical inputnoisecurrentof thecircuit is comparableto thenoisecur-rentof a resistorof 400k .Thedie area is 4.5 by 5 millimeter. Thetotal area forphotodiodesis about0.93 mm2. The electronics, whichis largely analog, require almosttherestof theavailablearea.1. Intr oductionLaserDopplerbloodflowmetryis anoninvasivewaytomeasurethebloodperfusionof humantissueupto adepthof about1 mm. It is usedin plastic surgery and othermedicalfields.The principle of laser Doppler blood flowmetry isdrawn in figure1 [1]. A lasertransmitsphotonsinto thetissue.Thephotonsarescatteredandreflectedin the tis-sue. Every photon that meetsa moving (blood) parti-cle getsa shift in frequency. A photodiodereceivesthephotonsleaving the tissueandconvertstheminto a cur-rent.TheDopplershiftedphotonscauseanAC currentontop of the DC currentfrom the nonshiftedphotons.Thefrequency spectrumof the AC currentgivesinformationabouttheDopplershift andthusaboutthebloodperfusionof thetissue. 	 	 ! "	#%$&' 	#)(*	! +,	(*	#-. /PresentlaserDopplerbloodflowmetersuseanopticalfiberto transporthephotons.A disadvantageof thissetupis thatmovementsof thefibercausea faulty readout.In this paper, a circuit is describedwhich is to be in-tegratedin a probefor laserDopplerbloodflow measure-ments.Thechipcontainsfive pairsof photodiodesat var-iousdistancesfrom the laser, analogcircuitry to amplifythesignalanddigital gatesto selecta sourcedetectordis-tance.Dueto thecoppercablebetweentheprobeandthesignalprocessor, this setupis immuneto movementsofthecable.2. SystemsetupAn overview of the systemis given in figure 2. Thesystemcanbedividedin threeparts,thephotodiodes,theamplifierandthedigital circuitry whichmakesit possibleto selectanothersourcedetectordistance.The laserwillbemountedonaseparatesubstrateandis not treatedhere.The photodiodesareplacedin pairsat five distancesfrom thelaser. As theDC valueof thephotocurrentis re-&& & &data5A2A115A2A11clock 	102%34!	#,4# 	657#& .8! .-.	#(9quiredasanoutputsignal,eachsignalis treatedassingleended.To decreasetheinfluenceof disturbances,thesignalisamplifiedby a currentamplifier. The probewill be cali-brated,so the linearity doesn’t have to be high. The ACcurrentcontainsfrequenciesup to 40 kHz, whichdictatesthebandwidthof thesystem.Themostimportantpointofconcernis thenoisecontributionof thesystem,whichhasto be assmall aspossible,the AC signal is rathersmallandit is easilydrownedin thenoise.3. PhotodiodeA crosssectionof a photodiodeis given in figure 3.The photodiodehasto be madein a CMOS process,sooptimizationof thephotodiodeis not reallypossible.: ,	<;2)=) > ? "	/+-@ A#B-DC	9C-.E :E	Thephotocurrentcanbewrittenas:F@GIHKJ&LMG+N.O(1)whereF@Gis the photocurrent,JBLPG+Nis the optical powerof the light on the photodiodeandOis the responsitiv-ity of thediode.Theresponsitivity dependson thewave-lengthof the photons. Accordingto a simulationof thephotodiode,the responsitivity peaksat a wavelengthof800nm. In figure4 thesimulatedcathodecurrentis plot-ted againstthe optical wavelengthat a constantopticalpower(35.68 Q W/mm2). Thewavelengthof thelaserwillbe 780 nm. Themeasuredsensitivity at a wavelengthof675nmis 0.3A/W [2] and0.32A/W at780nm.Due to the discretenatureof photons(andelectrons)theconversionfrom light to currentproducesshotnoise.Thesquarednoisecurrent,of thephotodiodeis givenby:RPST HVU#WF GXZY(2)whereWis the elementarycharge,F@Gis the currentgen-eratedby photonsandXZYis the bandwidth. As the ACATLASData from diepdiod.log0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 102e-094e-096e-098e-091e-081.2e-08Optical wavelength (um)cathode current [A/um]cathode current (A/um) '	\[]Plotof cathodecurrentof thephotodiodeagainstopticalwavelength.currentwill besmallcomparedto theDC current,theDCvaluecanbeusedto calculatethenoisecurrent.To increasethe signal to noiseratio, the photodiodesat a larger distancefrom the laser have a larger area.The smallestdiode is 100̂ 100 Q m and the largest is1200̂ 100 Q m. Thediodesareshapedasa partof a ring.(Seefigure12 in section6.)The graphon the left in figure5 shows the currentofthediodeasfunctionof thereversebiasvoltagein severalcases.Thegraphon theleft shows theresultof measure-mentswith a light sourceascloseaspossibleto a diode(light), with the light source4 mm above a diode (twi-light) andwithoutany light. Thegraphontheright showsthebreakdown behaviour of thediodes.Thediodeshavea breakdown voltagearound16 V. Thehigh currentbe-tween6 and16V canbeexplainedby theforwardbiasingof thepnjunctionof thePMOStransistorswhichareusedto connectthe diodeto oneof the amplifiers. The bumparound5.4V couldbeexplainedby thepresenceof apar-asiticdevice.1101001000100001000001e+061e+071e+080 1 2 3 4 5current [pA]_reverse bias [V]darknesslighttwilight010020030040050060070080090010000 5 10 15 20diode current [uA]`reverse bias [V] '	ba2Currentof photodiodeas function of reversebias.4. Setupof AmplifiersIn figure6 theschematicof theanalogcurrentamplifieris given. The photocurrentI is convertedinto a voltageby transimpedanceR1. Thenthevoltageis amplifiedandconvertedinto a currentagain.Theresistorsin thecircuit havea parasiticcapacitanceto thebulk. Thiscapacitancereducesthebandwidthof thesystem.To maximizethebandwidth,theareaof theresis-torshouldbeassmallaspossible.Thevalueof theresistorshouldalsobeassmallaspossiblebut thatconflictswiththenoiserequirement.Neglectingnoisein theamplifier,the resistornoisecanbe modelledasan additionalinput: ,	Ac'\dC!	#(e-@ #B-DC!	1#f	#-g(<  h	#Pcurrentsource: R ST HjiklO XZY (3)whereR Tis the noisecurrent, k is the Boltzmanncon-stant, l is the absolutetemperature,O is the resistanceandX9Yis the bandwidth. Taking into accounta band-width of 100kHz,variationsin processingandtheamountof noise,a valueof 400 k waschosenfor the resistor.By dimensioningthe otherstagesin the properway, thenoiseperformanceof thecircuit is mainly determinedbythefirst stage.The opampsin figure 6 are actually commonsource(CS)amplifiers.TheCSamplifierhadto bedimensionedin suchawaythatits noisecontributionis smallcomparedto thenoisecurrentof theresistor.Thewidth over lengthof theamplifyingPMOSdeviceof theCSamplifierwassetto 2000micronover5 micronandits biasingNMOS transistorhasa width over lengthratio of 600 over 50 micron. The bias currentis set to125 Q A.A disadvantageof theCSamplifieris thelow openloopgain. Theopenloopvoltagegainis determinedby the mnof thePMOStransistorof theCSamplifierandtheoutputresistorsof bothtransistors.In formula:oep H m n ^rqts LMGuvu s L Tw (4)whereo*pis thevoltageamplification,m n is thesmallsig-nal transconductanceand s LPG and s L T aretheoutputresis-torsof thetransistors.Accordingto simulations,the m n is1.7mSandtheoutputresistanceis 282k . Theresultingvoltageamplificationis about480. Loadingwith 10 k theamplifierreducesit to 31in thefirst amplifierandwith250k to 270in thevoltageamplifier(seebelow). Thesevaluesareabsolutelysufficient for thisapplication.The secondstage,the block in the middle in figure 6is aninvertingvoltageamplifier. Dueto therelatively lowopenloopgain, theamplificationwill besomewhat lowerthan R3 divided by R2 (both of figure 6). R4 and theswitchmake it possibleto changetheamplificationfrom24 to 8.The CS amplifiersare biasedby a current referencesource.This currentsourceis a x N -referencedself-biasedcircuit andcanbefoundin [3].Theoutputstageis a sourcefollowerusedasa voltageto currentconverter(the drain of the PMOStransistorisusedascurrentoutput).Thetransconductanceis:y pDz,{ H m n}|~O (5)wherey pDz{is thetransconductanceof thevoltageto cur-rentconverter, mn is thesmallsignaltransconductanceofthe transistorandOis valueof the resistor. The mn willvary with theDC valueof thephotocurrent.Theeffect isreducedby theresistor.01234560 200 400 600 800 1000 1200 1400 1600DC Voltage [V] / DC current [mA]Input current [nA]IVC, simulatedVoltage amplifier, simulatedVIC, simulatedIVC, measuredVoltage amplifier, measuredVIC, measured: '<')2< ./*!+,+.#v . Figure7 gives the simulatedandmeasuredDC char-acteristicof the amplifier (with the switch to R4 in fig-ure 6 closed). The measuredgain is higher for the firsttwo stagesandlower at the outputof the last stagethanthe simulationpredicted. It can be explainedby higherresistorvaluesthanexpected.In figure8 the frequency responseof theamplifiersisgiven.On theleft thesimulatedandon theright themea-sured. The capacitanceof the photodiodeis assumedtobe70 pF, which is a worstcasevalue. A smallercapaci-tancemakesthebandwidtha little higher. Themeasuredbandwidthis higherthansimulated.1e-111e-101e-091e-081e-071e-061e-050.00010.0010.010.1110 100 1000 10000 100000 1e+06 1e+07 1e+08AC Voltage [V] / AC current [A]Frequency [Hz]IVCVoltage amplifierVIC0.0010.010.111000 10000 100000 1e+06Voltage [V] / Current [mA]Frequency [Hz]IV2IV4IVV2IVV4VI2VI4: '<'/'!+6+#'"1#D!<<  #/0.20.250.30.350.40.450.50.5510 100 1000 10000 100000Input Referred Noise Current [pA/Hz^0.5] Frequency [Hz]01e-072e-073e-074e-075e-076e-077e-078e-079e-071e-060 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000Input referred noise current [uA/Hz^0.5]Frequency [Hz]"ccc.txt": <¡'¢/e,6t#f,!: >A#f,/g#£D<I  ,Theinputreferrednoisecurrentof theamplifierfor var-iousvaluesof thediodecapacitanceis shown in theleft offigure9. Thediodecapacitancewasvariedfrom 4.7pF to70pF. Thehigherthecapacitance,thehigherthenoiseat100kHz. Thetemperatureis 298Kelvin. Ontheright, themeasuredinput referrednoisecurrentwithout any diodeconnectedto theamplifier. The~ u Ynoiseis a lot higherthanexpecteddueto poormodellingof thenoiseandbe-causethefinal H2-annealstepfor reducinginterfacestatesin theoxidewasnotyetdone.5. Digital partThefunctionof thedigital partis to selectandconnecta photodiodeto anamplifier. It consistsof switches,seefigure10,somebooleanlogic in theblock‘switchcontrol’andtwo flip-flops.A1A2switch controlD DCLK CLKQ Q0100 0110: ,	¤'Schematicof photodiodeselectionprinciple.Theswitchesconnectthephotodiodeto oneof theam-plifiersor to noneof them.Theswitchesareactuallyusedasanalogswitches.As theswitcheshave to passcurrentinsteadof avoltageandthevoltageat theterminalsof theswitchesremainsrelatively constant,they canbe imple-mentedby PMOStransistors.The block ‘switch control’ in figure 10 performsthedecodingof thestatestoredin theflip-flops to controlthesignalsof the actualswitches. The decodingis doneinsuchawaythatonly oneswitchcanbeclosedata time. Itis possibleto connectmultiple photodiodesto oneampli-fier input at thesametime. This is not very useful.How-ever additionalcircuitry to preventthis would requiretoomuchareaandit doesnoharmeither.Thestateof thesystem,theselectedamplifier, is storedin a shiftregister. Theusercanclock datainto thechip tochangethe state. The shiftregisterconsistsof a chainofflip-flops. Theoutputof thefirst flip-flop is connectedtothe input of the secondflip-flop andso on. A schematicdiagramof a flip-flop is givenin figure11.During measurementsit wasdiscoveredthat the usedflip-flop changesa0100datasequencein a0110sequenceundercertaincircumstances.So theshiftregisteris unre-liable. The problemis causedby the delaybetweentheclock signalandits inverse. It caneasilybe solved in anext realisation.ClkClkClkClk QClkClkD: ,	Z)d:C!	#(-@ e&(e! .-.	 + 4!	1¥]¦v¥'6. LayoutThechip is 5.0mmwide and4.5mm high. It contains4 amplifiers,10 photodiodes,24 flip-flops andsomedig-ital gates,seefigure12. Thetotal areafor photodiodesisabout0.93mm2. Theelectronics,which is largelyanalog,requirealmosttherestof theavailablearea.Theamplifiersareconcentratedin theupperpartof thechip, thedigital part in the lower/middlepartof thechip(it looks like a § ) and the diodesare the small squareswhich togetherform partsof circles. '	/02&¨?C!	© 686'/-67. ConclusionSimulationsand measurementsindicate that an inte-gratedphotodetectorcircuit was successfullydesigned.The simulated performanceis excellent for the laserDopplerblood perfusionprobe. The circuit hasa band-width of 100 kHz and a quiescentcurrentconsumptionof 1.4 mA. The input referrednoisecurrentis lessthan250fA/ ª Hz at a temperatureof 298Kelvin. Thecircuitmakesit possibleto measureat two laserdiodedistancessimultaneously. Theoutputcurrentcontainsboth theACandDC componentsof the photocurrent.The measuredperformanceis largelyasexpected.Thephotodiodesper-form well, exceptfor thebehaviour abovethesupplyvolt-age.Theamplifiershavealargerbandwidththanexpectedbut also producemore noise. The digital part howeverneedssomeredesigning.8. AcknowledgementsSupportof theTechnologyFoundation(STW)is kindlyacknowledged.[1] Perimed, “Principle of laser Doppler measurements,”www.perimed.se.[2] Arthur Kimmels, “Designof an IntegratedPhotoDetectorfor aLaserDopplerBloodFlow Monitor,” Tech.Rep.,Uni-versityof Twente,Enschede,Feb. 1995.[3] P. R. GrayandR. G. Meyer, AnalysisandDesignof AnalogIntegratedCircuits, JohnWiley & Sons,Inc., New York,third edition,1993.

Design of an integrated photo detector circuit for laser doppler blood flow monitoring

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Design of an integrated photo detector circuit for laser doppler blood flow monitoring

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