芯片及器件ad603應用和使用說明

于RF/IF自............ 增益控制電

提供了精確

(SimplestGain

90時，增益為Interface).............

寬為9特 應

(MinimumError)................. 到

個外部電阻來調(diào)節(jié)任何概

中間范圍

的參考噪聲頻譜密度僅

有1.3 5V

注

125mW

益分貝為線

溫度和供電防靜

電源影響下 可以穩(wěn)定工

輪

于一個高阻

（兆歐 低偏置

NA）

為25毫伏/增益控制接

1伏增益控制電壓來覆蓋中央增益為

AGC電路是一款40分貝的范低功耗，電壓控制的圍。無論選定140500Ω10MHz的正弦波的總諧波失真通常是-60Db。在負載為500Ω±2.5AD603X-AMP，X-AMP0?42.14

1分貝壓縮點3分貝帶寬

f10MHz,gainV=R≥LR≥L37R-2R6.021AD603規(guī)定工作在-40°C至+85°C。

tof≤

f=3MHz;V=0V;f=Gf=40MHz,2 ?500mVTA=25°C,VS=±5V,–500mV+500mVGNEG0V,–10dBto+30dBgainrange,RL500Ω,andCL=5pF條件下。

f=100kHz;Gain(dB)=(40V+10)f10.7

源（3

7.5V=-GGV=G（50mV≤

條 最

S腳S典 最 單

3腳、4

2

pF

部功S部功率耗散00

dB作溫度范40° 圖 455kHz,MHz,4570MHz時的增益控制電未特殊指明時均圍+85C55°65°儲溫度范圍導溫度出說明操作章程時并沒有指明在長時間的絕對最大額定表 熱特型位55403CC85

在TA=25°C,VS±5V,–500mV≤VG≤+500mV,GNEG=0V,–10to+30dBgainrange,RL=500Ω,andCL5pF Gain=?10dB,PIN=dBm時的增圖4100kHzand10.7圖 Gain=?10dB,PIN=?30dBm時的增益幅頻特 Gain=?10dB,PIN=?30dBm時的增益幅頻特

1455kHz時的三級交 MHz三級交越失

了任何情況1.07分貝的欠量程和過量程。例益為-11.07分貝到分貝（5腳和7腳連接在一這種X-結(jié)構(gòu)比以前基于非線性因素為基礎(chǔ)的方法有很圖15輸入阻抗與頻率要的是固定（增益=10dB

增益放大器可以利用負大的輸入首

AD603包括一個固定器的輸入始增益放大器，前置有終是小信號。帶寬為0DB到例如，在42.14DB的衰減dB/+41dB器，該衰減器具有增式下驅(qū)動±140DB/V輸出（也就伏。固定增益激光微是說，使用固調(diào)在兩個范圍，要么定增益為31.07分貝（×35.8）41.07dB358，它的也可能被設(shè)置為任何輸入僅為范圍內(nèi)使用5腳和78.84V，因此腳之間的外部電阻。失真非常小。有些更高增益可通過同樣重要的5連接共同的是，小信號增電阻，但增加的輸出益和相位響失調(diào)電壓限制了最大應，以及脈沖60分貝。對于響應，本質(zhì)上任何給定的范圍，帶是獨立的增寬是獨立于電壓控制益。增益的，該系統(tǒng)提供297R-2R梯形網(wǎng)絡(luò)，利用阻R62.5Ω125Ω20%。分流號VINP是1（±1.4V峰值。雖然在高頻失真和饋通增長6.020dB,6.0212.04dB,18.06dB24.08dB,30.1dB36.12dB,and42.14dB29

AD603AD603基本的增益可以通過下式計Gain(dB)=40VG+10(1)VG單位是福57腳連接在一起（參見使用

管腳捆扎對AD603進行判斷。事實上，他的位置是受到在引0.2dB峰值。X-AMP

衰減器受控于差分，固定增益放使用激光微調(diào)因子調(diào)益變?yōu)椋?0Db/Gain(dB)=說，25mV/dB。一個40VG+20for內(nèi)部帶隙基準，可以0to+40dB確保由于電源和溫度以及變化帶來的穩(wěn)定性方Gain(dB)=（125Ω的三分之一并表現(xiàn)出0.83nV/√Hz的頻譜噪聲密

40VG+30=+10to+50當在多通道1nV/√Hz1.3nV/√Hz。很明顯，100ΩAD603Ω輸入電阻分壓器。在某些AD603的低阻抗。最大增益噪聲取決于輸入是短路還是開路，當短路時，最小的1nV/√Hz100ΩAD603900Ω的電阻，將會使輸入提高到rms×

0V中或者級聯(lián)器置于中間，提供應用時驅(qū)動21.07dB衰減。對于多個放大器最大帶寬范圍，這個時，高阻抗輸入增益控制10dB(=?21.07dB+31.07dB)。當控制輸確保最小的入是?500mV,增益減負載。差分輸?shù)?20dB(=0.500V入能力提供×40dB/V)to?10了在選擇合dB，當控制輸入是適信號水平+500mV，增益增加到+20dB到+30dB。當方案極性的接口在任意方向過載靈活性。例時，增益分別接近?11.07dB(=?42.14受控于正向dB++31.07dB)或者的，以地面為是31.07dB(=0+參考輸出地31.07dB)

關(guān)于增益控制電壓唯（GNEG一的限制是，它必須引腳應該偏保持在增益控制接口向一個固定的共模范圍內(nèi)（+5V500mV電源時?1.2Vto+2.0調(diào)設(shè)置增益 為-10當高增益控制（GPO）的是零，1.00V30dB8DAC2.55V(10mV/bit)。2分壓比（55（FDBK）可以問反饋網(wǎng)絡(luò)。用戶可以使用該引腳對放大30、31、32所示。有三種模式：有默認模FDBK-11分貝/31分貝;VOUTFDBK間時，任何中間增益可以實現(xiàn)，例如，?1dB/+4130顯示了在此模式下標稱的最大增益與外部電阻之圖 ?10dB到+30 90MHz帶

310dBto40dB30

kΩ±20%的。。發(fā)生在2kΩ

圖34誤差時的情況增益誤

LowNoise,

a“LinearindB”GainControlPinProgr bleGainRanges–11dBto+31dBwith90MHz+9dBto+51dBwith9MHzAnyIntermediateRange,e.g.,–1dBto+41dBwith30MHzBandwidthBandwidthIndependentofVariable1.3nV/HzInputNoiseSpectral0.5dBTypicalGainMIL-STD-883CompliantandDESCVersionsGainControlA/DRangeExtensionSignalMeasurementPRODUCTTheAD603isalownoise,voltage-controlledamplifierforuseinRFandIFAGCsystems.Itprovidesaccurate,pinselectablegainsof–11dBto+31dBwithabandwidthof90MHzor+9dBto+51dBwithabandwidthof9MHz.Anyintermediategainrangemaybearrangedusingoneexternalresistor.Theinputreferrednoisespectraldensityisonly1.3nV/Hzandpowerconsumptionis125mWatthe mended5V Thedecibelgainis“l(fā)inearindB,”accuraycalibrated,andstableovertemperatureandsupply.Thegainiscontrolledatahighimpedance(50M),lowbias(200nA)differentialinput;thescalingis25mV/dB,requiringagain-controlvoltageofonly

1Vtospanthecentral40dBofthegainrange.Anover-andunder-rangeof1dBisprovidedwhatevertheselectedrange.Thegain-controlresponsetimeislessthan1sfora40dBchange.Thedifferentialgain-controlinterfaceallowstheuseofeitherdifferentialorsingle-endedpositiveornegativecontrolvoltages.Severaloftheseamplifiersmaybecascadedandtheirgain-con-trolgainsoffsettooptimizethesystemS/Nratio.TheAD603candrivealoadimpedanceaslowas100withlowdistortion.Fora500loadinshuntwith5pF,thetotalharmonicdistortionfora1Vsinusoidaloutputat10MHzistypically–60dBc.Thepeakspecifiedoutputis2.5Vmini-mumintoa500load,or1Vintoa100load.TheAD603usesaproprietarycircuittopology—theX-TheX-AMPcomprisesavariableattenuatorof0dB–42.14dBfollowedbyafixed-gainamplifier.Becauseoftheattenuator,theamplifierneverhastocopewithlargeinputsandcanusenegativefeedbacktodefineits(fixed)gainanddynamicperformance.Theattenuatorhasaninput of100,lasertrimmedto3%,andcomprisesaseven-stageR-2Rladdernetwork,resultinginanattenuationbetweentappointsof6.021dB.Aproprietaryinterpolationtechniqueprovidesacontinuousgain-controlfunctionwhichislinearindB.TheAD603Aisspecifiedforoperationfrom–40Cto+85Candisavailableinboth8-leadSOIC(R)and8-leadceramicDIP(Q).TheAD603Sisspecifiedforoperationfrom–55Cto+125Candisavailableinan8-leadceramicDIP(Q).TheAD603isalsoavailableunderDESCSMD5962-94572.FUNCTIONALBLOCK

PRECISIONPASSIVE

FIXEDGAIN6.44k

–6.02dB–12.04dB–18.06dB–24.08dB–30.1dB–36.12dB–42.14dB R=2RLADDERNETWORK

69420X-AMPisatrademarkofogDevices,InformationfurnishedbyogDevicesisbelievedtobeaccurateandreliable.However,noresponsibilityisassumedbyogDevicesforitsuse,norforanyinfringementsofpatentsorotherrightsofthirdpartieswhichmayresultfromitsuse.NolicenseisgrantedbyimplicationorotherwiseunderanypatentorpatentrightsofogDevices.

OneTechnologyWay,P.O.Box9106,Norwood,MA02062-9106,:781/329- WorldWideWebSite:Fax:781/326- ?ogDevices,Inc.,PeakOutputRL500VOutputf102OutputShort-CircuitGroupDelayChangevs.f=3MHz;FullGainGroupDelayChangevs.VG=0V;f=1MHzto10Differential%DifferentialTotalHarmonicf=10MHz,VOUT=1V3rdOrderf=40MHz,Gain=max,RS=50G1TMINtoOutputOffset3VG=0MINtoTOutputOffsetVariationvs.–500mVVG+500TMINtoGAINCONTROLGainScaling toGNEG,GPOSVoltage4VInputBiasInputOffsetDifferentialInputPins1toResponseFull40dBGainPOWERSpecifiedOperatingVQuiescentTMINto(@(@TA=+25C,VS5V,–500mVVG+500mV,GNEG=0V,–10dBto+30 SRange,RL=INPUTInputInputNoiseSpectralDensityNoiseFigure1dBCompressionPointPeakInputVoltage1Pins3toAD6032nV/InputShortf=10MHz,Gain=max,RS=10f=10MHz,Gain=max,RS=10OUTPUT–3dBBandwidthSlewRateVOUT=100mVrmsRL5001Typicalopenorshort-circuitedinput;noiseislowerwhensystemissettoumgainandinputisshort-circuitedThisfigureincludestheeffectsofbothandcurrentnoise2Usingresistiveloadsof500orgreater,orwiththeadditionofa1kpull-downresistorwhendrivinglower3ThedcgainofthemainamplifierintheAD603is357;thus,aninputoffsetof100 esa357mVoutput SpecificationsshowninboldfacearetestedonallproductionunitsatfinalelectricaltestResultsfromthosetestsareusedtocalculateoutgoingqualitylevelsAllminandmaxspecificationsareguaranteed,althoughonlythoseshowninboldfacearetestedonallproductionunitsSpecificationssubjecttochangewithout+500 PINFUNCTION UMPINFUNCTIONPinGain-ControlInputPinGain-ControlInputPinGain-ControlInputPinAmplifierPinAmplifierPinConnectiontoFeedbackPinNegativeSupplyPinAmplifierPinPositiveSupply 2V VSfor10GPOS,GNEG(Pins1, InternalPower

400 40Cto 55Cto 65CtoLeadTemperatureRange(Soldering60 1StressesabovethoselistedunderAbsolute umRatingsmaycauseperma-nentdamagetothedeviceThisisastressratingonly;functionaloperationofthedeviceattheseoranyotherconditionsabovethoseindicatedintheoperationalsectionofthisspecificationisnotimpliedExposuretoabsolute umratingconditionsforextendedperiodsmayaffectdevice

CONNECTION sticSOIC(R)2Thermal8-LeadSOICPackage:JA=155C/W,JC=33 8-LeadCeramicDIP(Q)8-LeadCeramicPackage:JA=140C/W,JC=15GPOSGNEG VINP3(NottoCOMM

876ORDERINGPart

–40Cto–40Cto–55Cto–40Cto–40Cto–40Cto

8-Lead8-LeadCeramicDIP8-LeadCeramicDIPEvaluationBoard13"7"

SO-*RefertoAD603MilitarydatasheetAlsoavailableas ESD(electrostaticdischarge)sensitivedevice.Electrostaticchargesashighas4000Vreadilyaccumulateonthehumanbodyandtestequipmentandcandischargewithoutdetection.AlthoughtheAD603featuresproprietaryESDprotectioncircuitry,permanentdamagemayoccurondevicessubjectedtohighenergyelectrostaticdischarges.Therefore,properESDprecautionsare mendedtoavoidperformancedegradationorlossoffunctionality.

ESDSENSITIVE TheAD603comprisesafixed-gainamplifier,precededbyabroadbandpassiveattenuatorof0dBto42.14dB,havingagain-controlscalingfactorof40dBpervolt.Thefixedgainislaser-trimmedintworanges,toeither31.07dB(35.8)or50dB(358),ormaybesettoanyrangeinbetweenusingoneexternalresistorbetweenPins5and7.SomewhathighergaincanbeobtainedbyconnectingtheresistorfromPin5tocom-mon,buttheincreaseinoutputoffsetvoltagelimitstheumgaintoabout60dB.Foranygivenrange,theband-widthisindependentofthevoltage-controlledgain.Thissystemprovidesanunder-andoverrangeof1.07dBinallcases;forexample,theoverallgainis–11.07dBto31.07dBintheum-bandwidthmode(Pin5andPin7strapped).ThisX-AMPstructurehasmanyadvantagesoverformermethodsofgain-controlbasedonnonlinearelements.Mostimportantly,thefixed-gainamplifiercanusenegativefeedbacktoincreaseitsaccuracy.Sincelargeinputsarefirstattenuated,theamplifierinputisalwayssmall.Forexample,todelivera1Voutputinthe–1dB/+41dBmode(thatis,usingafixedamplifiergainof41.07dB)itsinputisonly8.84mV;thusthedistortioncanbeverylow.Equallyimportant,thesmall-signalgainandphaseresponse,andthusthepulseresponse,areessentiallyindepen-dentofgain.Figure1isasimplifiedschematic.Theinputattenuatorisaseven-sectionR-2Rladdernetwork,usinguntrimmedresistorsofnominallyR=62.5,whichresultsinacharacteristicresis-tanceof12520%.Ashuntresistorisincludedattheinputandlasertrimmedtoestablishamoreexactinput 1003%,whichensuresaccurateoperation(gainandHPcornerfrequency)whenusedinconjunctionwithexternalresistorsorcapacitors.Thenominal umsignalatinputVINPis1Vrms(1.4Vpeak)whenusingthe mended5Vs,althoughoperationto2VpeakispermissiblewithsomeincreaseinHFdistortionandfeedthrough.Pin4(SIGNALCOMMON)mustbeconnecteddirectlytotheinputground;significantimpedanceinthisconnectionwillreducethegainaccuracy.Thesignalappliedattheinputoftheladdernetworkisattenu-atedby6.02dBbyeachsection;thus,theattenuationtoeachofthetapsisprogressively0dB,6.02dB,12.04dB,18.06dB,24.08dB,30.1dB,36.12dBand42.14dB.Auniquecircuittechniqueisemployedtointerpolatebetweenthesetap-points,

indicatedbythe“slider”inFigure1,thusprovidingcontinuousattenuationfrom0dBto42.14dB.Itwillhelp,inunderstandingtheAD603,tothinkintermsofamechanicalmeansformovingthissliderfromlefttoright;infact,its“position”iscontrolledbythevoltagebetweenPins1and2.Thedetailsofthegain-controlinterfacearediscussedlater.Thegainisatalltimesveryexactlydetermined,andalinear-in-dBrelationshipisautomaticallyguaranteedbytheexponentialnatureoftheattenuationintheladdernetwork(theX-AMPprinciple).Inpractice,thegaindeviatesslightlyfromtheideallaw,byabout0.2dBpeak(see,forexample,Figure16).NoiseAnimportantadvantageoftheX-AMPisitssuperiornoiseper-formance.Thenominal seenatinnertappointsis41.7(onethirdof125),whichexhibitsaJohnsonspectraldensity(NSD)of0.83nV/Hz(thatis,4kTR)at27C,whichisalargefractionofthetotalinputnoise.Thefirststageoftheamplifiercontributesafurther1nV/Hz,foratotalinputnoiseof1.3nV/Hz.Itwillbeapparentthatitisessentialtousealow intheladdernetworktoachievetheverylowspecifiednoiselevel.Thesignal’ssourceimpedanceformsavoltagedividerwiththeAD603’s100input .Insomeapplications,theresultingattenuationmaybeunaccept-able,requiringtheuseofanexternalbufferorpreamplifiertomatchahighimpedancesourcetothelowimpedanceAD603.Thenoiseat umgain(thatis,atthe0dBtap)dependsonwhethertheinputisshort-circuitedoropen-circuited:whenshorted,theminimumNSDofslightlyover1nV/Hzisachieved;whenopen,the of100lookingintothefirsttapgenerates1.29nV/Hz,sothenoiseincreasestoatotalof1.63nV/Hz.(ThislastcalculationwouldbeimportantiftheAD603wereprecededby,forexample,a900resistortoallowoperationfrominputsupto10Vrms.)Astheselectedtapmovesawayfromtheinput,thedependenceofthenoiseonsourceimpedancequicklydiminishes.Apartfromthesmallvariationsjustdiscussed,thesignal-to-noise(S/N)ratioattheoutputisessentiallyindependentoftheattenuatorsetting.Forexample,onthe–11dB/+31dBrangethefixedgainof35.8raisestheoutputNSDto46.5nV/Hz.Thus,forthe umundistortedoutputof1Vrmsanda1MHzbandwidth,theoutputS/Nratiowouldbe86.6dB,thatis,20log(1V/46.5V).

PRECISIONPASSIVE

FIXEDGAIN

6.44k

–6.02dB–12.04dB–18.06dB–24.08dB–30.1dB–36.12dB–42.14dB R=2RLADDERNETWORK

69420Figure SimplifiedBlockDiagramofthe WhenthedifferentialinputvoltageVG=0V,theattenuator“slider”iscentered,providinganattenuationof21.07dB.For umbandwidthrange,thisresultsinanoverallgainof10dB(=–21.07dB+31.07dB).Whenthecontrolinputis–500mV,thegainisloweredby20dB(=0.500V40dB/V),to–10dB;whensetto+500mV,thegainisincreasedby20dB,to30dB.Whenthisinterfaceisoverdrivenineitherdirection,thegainapproacheseither–11.07dB(=–42.14dB+31.07dB)or31.07dB(=0+31.07dB),respectively.Theonlyconstraint

COMMa.–10dBto+30dB;90MHz thegain-controlvoltageisthatitbekeptwithinthecommon-moderange(–1.2Vto+2.0Vassuming+5Vs)ofthegaincontrolinterface.ThebasicgainoftheAD603canthusbecalculatedusingthefollowingsimpleexpression:Gain(dB)=40VG+ whereVgisinvolts.WhenPins5and7arestrapped(seenextsection)thegain Gain(dB)=40VG+20for0to+40Gain(dB)=40VG+30for+10to+50 Thehighimpedancegain-controlinputensuresminimalloadingwhendrivingmanyamplifiersinmultiplechannelorcascaded

COMMCOMM

applications.Thedifferentialcapabilityprovidesflexibilityinchoosingtheappropriatesignallevelsandpolaritiesforvariouscontrolschemes.Forexample,ifthegainistobecontrolledbyaDACprovidingapositiveonlyground-referencedoutput,the“GainControlLO”(GNEG)pinshouldbebiasedtoafixedoffsetof+500mV,tosetthegainto–10dBwhen“GainControlHI”(GPOS)isatzero,andto30dBwhenat+1.00V.Itisasimplemattertoincludeavoltagedividertoachieve

c.+10dBto+50dB;9MHzFigureFigure2.Pin toSet–1:VdBVdB–2:VdBscalingfactors.Whenusingan8-bitDAChavinganFS of+2.55V(10mV/bit),adividerratioof2(generating5wouldresultinagain-settingresolutionof0.2dB/bit.TheofsuchoffsetsisvaluablewhentwoAD603sarecascaded,whenvariousoptionsexistforoptimizingtheS/Nprofile,aswillbeshownlater.ProgrammingtheFixed-GainAmplifierUsingPinStrapAccesstothefeedbacknetworkisprovidedatPin5(FDBK).TheusermayprogramthegainoftheAD603’soutputamplifierusingthispin,asshowninFigure2.Therearethreemodes:

B42

R

thedefaultmode,FDBKisunconnected,providingthe+9dB/+51dB;whenVOUTandFDBKareshorted,thegain

Figure3.Gainvs.

,ShowingWorst-CaseTheGain-ControlTheattenuationiscontrolledthroughadifferential,high-impedance(50M)input,withascalingfactorwhichislaser-trimmedto40dBpervolt,thatis,25mV/dB.TheGain-ControlTheattenuationiscontrolledthroughadifferential,high-impedance(50M)input,withascalingfactorwhichislaser-trimmedto40dBpervolt,thatis,25mV/dB.Aninternalbandgapreferenceensuresstabilityofthescalingwithrespecttosupplyandtemperaturevariations.

AssumingInternalResistorsHavea of20% Optionally,whenaresistoriscedfromFDBKtoCOMM,highergainscanbeachieved.Thismodeisoflimitedvaluebecauseofthelowbandwidthandtheelevatedoutputoff-sets;itisthusnotincludedinFigure2.–1:VdB(OUT)–(–1):VdBVdB(OUT)–VdBThegainofthisamplifierinthefirsttwomodesissetbytheratioofon-chiplaser-trimmed–1:VdB(OUT)–(–1):VdBVdB(OUT)–VdBS0.4BD Figure4.Worst-CaseGainError,AssumingInternalResis-torsHavea umToleranceof–20%(TopCurve)or+20%(BottomWhilethegain-bandwidthproductofthefixed-gainamplifierisabout4GHz,theactualbandwidthisnotexactlyrelatedtotheumgain.Thisisbecausethereisaslightenhancingoftheacresponsemagnitudeonthe umbandwidthrange,duetohigherorderpolesintheopen-loopgainfunction;thismildpeakingisnotpresentonthehighergainranges.Figure2showshowoptionalcapacitorsmaybeaddedtoextendthefrequencyresponseinhighgainmodes.CASCADINGTWOTwoormoreAD603scanbeconnectedinseriestoachievehighergain.Invariably,accouplingmustbeusedtopreventthedcoffsetvoltageattheoutputofeachamplifierfromoverload-ingthefollowingamplifierat umgain.Therequiredhighpasscouplingnetworkwillusuallybejustacapacitor,chosentosetthedesiredcornerfrequencyinconjunctionwiththewell-defined100input ofthefollowingamplifier.FortwoAD603s,thetotalgain-controlrange es84dB(twotimes42.14dB);theoverall–3dBbandwidthofcascadedstageswillbesomewhatreduced.Dependingonthepin-strap,thegainandbandwidthfortwocascadedamplifierscanrangefrom–22dBto+62dB(withabandwidthofabout70MHz)to+22dBto+102dB(withabandwidthofabout6

Thereareseveralwaysofconnectingthegain-controlinputsincascadedoperation.Thechoicedependsonwhetheritisimpor-tanttoachievethehighestpossibleInstantaneousSignal-to-NoiseRatio(ISNR),or,alternatively,tominimizetherippleinthegainerror.ThefollowingexamplesfeaturetheAD603programmed umbandwidth;theexnationsapplytoothergain/bandwidthcombinationswithappropriatechangestothearrange-mentsforsettingthe umgain.SequentialMode(OptimalS/NInthesequentialmodeofoperation,theISNRismaintainedatitshighestlevelforasmuchofthegaincontrolrangepossible.Figure5showstheSNRoveragainrangeof–22dBto+62dB,assuminganoutputof1Vrmsanda1MHzbandwidth;Figure6showsthegeneralconnectionsto plishthis.Here,boththepositivegain-controlinputs(GPOS)aredriveninparallelbyapositive-only,ground-referencedsourcewitharangeof0V+2V,whilethenegativegain-controlinputs(GNEG)arcbiasedbystablevoltagestoprovidetheneededgain-offsets.Thesevolt-agesmaybeprovidedbyresistivedividersoperatingfromacommonvoltagereference. Figure5.SNRvs.ControlVoltage—SequentialControl(1MHzBandwidth)Thegainsareoffset(Figure7)suchthatA2’sgainisincreasedonlyafterA1’sgainhasreachedits umvalue.Notethatforadifferentialinputof–600mVorless,thegainofasingleamplifier(A1orA2)willbeatitsminimumvalueof–11.07dB;foradifferentialinputof+600mVormore,thegainwillbeat umvalueof31.07dB.Controlinputsbeyondtheselimitswillnotaffectthegainandcanbetoleratedwithoutdam-ageorfoldoverintheresponse.ThisisanimportantaspectoftheAD603’sgain-controlresponse.(SeetheSpecificationssec-tionofthisdatasheetformoredetailsontheallowablevoltagerange)ThegainisnowGain(dB)=40VG+ whereVGistheappliedcontrolvoltageandGOisdeterminedbythegainrangechosen.Intheexnatorynotesthatfollow,weassumethe um-bandwidthconnectionsareused,forwhichGOis–20dB. VC=

VO1= VO2= VC=

V=

VO2=

V

VG1VO1=VC=

VO2=Figure6.AD603GainControlInputCalculationsforSequentialControl

*

WhenVG=+2.0V,thegainofA1ispinnedat31.07dBandthatofA2isnearitsumvalueof28.93dB,resultinginanoverallgainof60dB(seeFigure6c).ThismodeofoperationisfurtherclarifiedbyFigure8,whichisaplotoftheseparategainsofA1andA2andtheoverallgainversusthecontrolvoltage.Figure9isaplotofthegainerrorofthecascadedamplifiersversusthecontrolvoltage.Figure10isaplotofthegainerrorofthecascadedstagesversusthecontrol0(dB)0*GAINOFFSETOF1.07dB,ORFigure7. nationofOffsetCalibrationforSequentialVWithreferencetoFigure6,notethatVG1referstothedifferen-tialgain-controlinputtoA1andVG2referstothedifferentialgain-controlinputtoA2.WhenVGiszero,VG1=–473mVandthusthegainofA1is–8.93dB(recallthatthegainofeachindi-vidualamplifierinthe um-bandwidthmodeis–10dBforVG=–500mVand10dBforVG=0V);meanwhile,VG2=–1.908VsothegainofA2is“pinned”at–11.07dB.Theover-allgainisthus–20dB.ThissituationisshowninFigure6a.WhenVG=+1.00V,VG1=1.00V–0.473V=+0.526whichsetsthegainofA1toatnearly umvalue31.07dB,whileVG2=1.00V–1.526V=0.526V,whichsetsA2’sgainatnearlyitsminimumvalue–11.07dB.Close

O

showsthatthedegreetowhichneitherAD603iscompleypushedtoits umorminimumgainexactlycancelsintheoverallgain,whichis

Figure8.PlotofSeparateandOverallGainsinSequentAI30ARV B

–0.2 0.8

1.82.0

Figure9.SNRforCascadedStagesSequential

–0.5

–0.20.01.01.82.00 Figure10.GainErrorforCascadedStages—SequentialParallelMode(SimplestGain-ControlInthismode,thegain-controlofvoltageisappliedtobothinputsinparallel—theGPOSpinsofbothA1andA2areconnectedtothecontrolvoltageandtheGNEWinputsaregrounded.Thegainscalingisthendoubledto80dB/V,requiringonlya1.00Vchangeforan80dBchangeofgain:Gain(dB)=80VG+ where,asbeforeGOdependsontherangeselected;forexample,inthe um-bandwidthmode,GOis+20dB.Alternatively,theGNEGpinsmaybeconnectedtoanoffsetvoltageof+0.500V,inwhichcase,GOis–20Theamplitudeofthegainrippleinthiscaseisalsodoubled,asshowninFigure11,whiletheinstantaneoussignal-to-noiseratioattheoutputofA2nowdecreaseslinearlyasthegainincreased(Figure12).

Figure12.ISNRforCascadedStages–ParallelLowGainRippleMode(MinimumGainAscanbeseenfromFigures9and10,theerrorinthegainisperiodic,thatis,itshowsasmallripple.(Notethatthereisalsoavariationintheoutputoffsetvoltage,whichisduetothegaininterpolation,butthisisnotexactinamplitude.)ByoffsettingthegainsofA1andA2byhalftheperiodoftheripple,thatis,by3dB,theresidualgainerrorsofthetwoamplifierscanbemadetocancel.Figure13showsthatmuchlowergainripplewhenconfiguredinthismanner.Figure14plotstheISNRasafunctionofgain;itisverysimilartothatinthe“ParallelMode.”N B–O–3.0–0.1 0.4

ALowNoiseAGCFigure15showstheeasewithwhichtheAD603canbeconnectedasanAGCamplifier.Thecircuitillustratesmanyofthepointspreviouslydiscussed:Itusesfewparts,haslinear-in-dBgain,operatesfromasinglesupply,usestwocascadedamplifiersinsequentialgainmodefor umS/Nratio,andanexternalresistorprogramseachamplifier’sgain.ItalsousesasimplepensatedThecircuitoperatesfromasingle10Vsupply.ResistorsR1,R2,R3,andR4biasthecommonpinsofA1andA2at5V.Thispinisalowimpedancepointandmusthavealowimpedancepathtoground,hereprovidedbythe100Ftantalumcapacitors0.91.0 andthe0.1FceramicThecascadedamplifiersoperateinsequentialgain.Here,theoffsetvoltagebetweenthepins2(GNEG)ofA1andA2isFigure13.GainErrorforCascadedStages–Low–S Figure14.ISNRvs.ControlVoltage–LowRipple

1.05V(42.14dB25mV/dB),providedbyavoltagedividerconsistingofresistorsR5,R6,andR7.Usingstandardvalues,theoffsetisnotexact,butitisnotcriticalforthisThegainofbothA1andA2isprogrammedbyresistorsR13andR14,respectively,tobeabout42dB;thusthe gainofthecircuitistwicethat,or84dB.Thegain-controlrangecanbeshiftedupbyasmuchas20dBbyappropriatechoicesofR13andR14.Thecircuitoperatesasfollows.A1andA2arecascaded.CapacitorC1andthe100of attheinputofA1formatime-constantof10s.C2blocksthesmalldcoffsetvoltageattheoutputofA1(whichmightotherwisesaturateA2atits umgain)andintroducesahigh-passcorneratabout16kHz,eliminatinglowfrequencynoise.Ahalf-wavedetectorisused,basedonQ1andR8.TheintocapacitorCAVisjustthedifferencebetweenthecollectorcurrentofQ2(biasedtobe300Aat300K,27C)andthecol-lectorcurrentofQ1,whichincreaseswiththeamplitudeoftheAGCTIME

0.1

0.1

0.1+ 100F

1002

AGC

1VOFFSET

12C3ANDC5ARE

Figure15.ALowNoiseAGC outputsignal.Theautomaticgaincontrolvoltage, ,is ThisresistoralsoservestolowerthepeakcurrentinQ1time-integralofthiserrorcurrent.Inorderfor

(and moretypicalsignals(usually,sinusoidal)areinvolved,andthegain)toremaininsensitivetoshort-termamplitudefluctuations 1.8kHzLPfilteritformswithCAV helpstominimizedistortionintheoutputsignal,therectifiedcurrentinQ1must,onaverage, duetorippleinVAGC.NotethattheoutputamplitudeunderexactlybalancethecurrentinQ2.IftheoutputofA2istoosmalltodothis,VAGCwillincrease,causingthegaintoincrease,untilQ1conductssufficiently.ConsiderthecasewhereR8iszeroandtheoutputvoltageVOUTisasquarewaveat,say,455kHz,whichiswellabovethecornerfrequencyofthecontrolloop.DuringthetimeVOUTisnegativewithrespecttothebasevoltageofQ1,Q1conducts;whenVOUTispositive,itiscutoff.SincetheaveragecollectorcurrentofQ1is dtobe300A,andthesquarewavehasaduty-cycleof1:1,Q1’scollectorcurrentwhenconductingmustbe600A.WithR8omitted,thepeakamplitudeofVOUTis dtobejusttheVBEofQ1at600A,typicallyabout700mV,or2VBEpeak-to-peak.Thisvoltage,hencetheamplitudeatwhichtheoutputstabilizes,hasastrongnegativetemperaturecoefficient(TC),typically–1.7mV/C.Althoughthismaynotbetroublesomeinsomeapplications,thecorrectvalueofR8willrendertheoutputstablewithtemperature.Tounderstandthis,firstnotethatthecurrentinQ2ismadetobeproportionaltoabsolutetemperature(PTAT).Forthemoment,continuetoassumethatthesignalisasquarewave.WhenQ1isconducting,VOUTisnowthesumofVBEandavoltagethatisPTATandwhichcanbechosentohaveanequalbutoppositeTCtothatoftheVBE.Thisisactuallynothingmorethananapplicationofthe“bandgapvoltagereference”WhenR8ischosensuchthatthesumofthevoltageacrossitandtheVBEofQ1isclosetothebandgapvoltageofabout1.2V,VOUTwillbestableoverawiderangeoftemperatures,provided,ofcourse,thatQ1andQ2sharethesamethermalenvironment.Sincetheaverageemittercurrentis600Aduringeachhalf-cycleofthesquarewavearesistorof833wouldaddaPTATvoltageof500mVat300K,increasingby1.66mV/C.Inprac-tice,the