芯片及器件ad603应用和使用说明

于RF/IF自............ 增益控制电

提供了精确

(SimplestGain

90时，增益为Interface).............

宽为9特 应

(MinimumError)................. 到

个外部电阻来调节任何概

中间范围

的参考噪声频谱密度仅

有1.3 5V

注

125mW

益分贝为线

温度和供电防静

电源影响下 可以稳定工

轮

于一个高阻

（兆欧 低偏置

NA）

为25毫伏/增益控制接

1伏增益控制电压来覆盖中央增益为

AGC电路是一款40分贝的范低功耗，电压控制的围。无论选定140500Ω10MHz的正弦波的总谐波失真通常是-60Db。在负载为500Ω±2.5AD603X-AMP，X-AMP0−42.14

1分贝压缩点3分贝带宽

f10MHz,gainV=R≥LR≥L37R-2R6.021AD603规定工作在-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条件下。

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

源（3

7.5V=-GGV=G（50mV≤

条 最

S脚S典 最 单

3脚、4

2

pF

部功S部功率耗散00

dB作温度范40° 图 455kHz,MHz,4570MHz时的增益控制电未特殊指明时均围+85C55°65°储温度范围导温度出说明操作章程时并没有指明在长时间的绝对最大额定表 热特型位55403CC85

在TA=25°C,VS±5V,–500mV≤VG≤+500mV,GNEG=0V,–10to+30dBgainrange,RL=500Ω,andCL5pF Gain=−10dB,PIN=dBm时的增图4100kHzand10.7图 Gain=−10dB,PIN=−30dBm时的增益幅频特 Gain=−10dB,PIN=−30dBm时的增益幅频特

1455kHz时的三级交 MHz三级交越失

了任何情况1.07分贝的欠量程和过量程。例益为-11.07分贝到分贝（5脚和7脚连接在一这种X-结构比以前基于非线性因素为基础的方法有很图15输入阻抗与频率要的是固定（增益=10dB

增益放大器可以利用负大的输入首

AD603包括一个固定器的输入始增益放大器，前置有终是小信号。带宽为0DB到例如，在42.14DB的衰减dB/+41dB器，该衰减器具有增式下驱动±140DB/V输出（也就伏。固定增益激光微是说，使用固调在两个范围，要么定增益为31.07分贝（×35.8）41.07dB358，它的也可能被设置为任何输入仅为范围内使用5脚和78.84V，因此脚之间的外部电阻。失真非常小。有些更高增益可通过同样重要的5连接共同的是，小信号增电阻，但增加的输出益和相位响失调电压限制了最大应，以及脉冲60分贝。对于响应，本质上任何给定的范围，带是独立的增宽是独立于电压控制益。增益的，该系统提供297R-2R梯形网络，利用阻R62.5Ω125Ω20%。分流号VINP是1（±1.4V峰值。虽然在高频失真和馈通增长6.020dB,6.0212.04dB,18.06dB24.08dB,30.1dB36.12dB,and42.14dB29

AD603AD603基本的增益可以通过下式计Gain(dB)=40VG+10(1)VG单位是福57脚连接在一起（参见使用

管脚捆扎对AD603进行判断。事实上，他的位置是受到在引0.2dB峰值。X-AMP

衰减器受控于差分，固定增益放使用激光微调因子调益变为：40Db/Gain(dB)=说，25mV/dB。一个40VG+20for内部带隙基准，可以0to+40dB确保由于电源和温度以及变化带来的稳定性方Gain(dB)=（125Ω的三分之一并表现出0.83nV/√Hz的频谱噪声密

40VG+30=+10to+50当在多通道1nV/√Hz1.3nV/√Hz。很明显，100ΩAD603Ω输入电阻分压器。在某些AD603的低阻抗。最大增益噪声取决于输入是短路还是开路，当短路时，最小的1nV/√Hz100ΩAD603900Ω的电阻，将会使输入提高到rms×

0V中或者级联器置于中间，提供应用时驱动21.07dB衰减。对于多个放大器最大带宽范围，这个时，高阻抗输入增益控制10dB(=−21.07dB+31.07dB)。当控制输确保最小的入是−500mV,增益减负载。差分输低+20dB(=0.500V入能力提供×40dB/V)to−10了在选择合dB，当控制输入是适信号水平+500mV，增益增加到+20dB到+30dB。当方案极性的接口在任意方向过载灵活性。例时，增益分别接近−11.07dB(=−42.14受控于正向dB++31.07dB)或者的，以地面为是31.07dB(=0+参考输出地31.07dB)

关于增益控制电压唯（GNEG一的限制是，它必须引脚应该偏保持在增益控制接口向一个固定的共模范围内（+5V500mV电源时−1.2Vto+2.0调设置增益 为-10当高增益控制（GPO）的是零，1.00V30dB8DAC2.55V(10mV/bit)。2分压比（55（FDBK）可以问反馈网络。用户可以使用该引脚对放大30、31、32所示。有三种模式：有默认模FDBK-11分贝/31分贝;VOUTFDBK间时，任何中间增益可以实现，例如，−1dB/+4130显示了在此模式下标称的最大增益与外部电阻之图 −10dB到+30 90MHz带

310dBto40dB30

kΩ±20%的。。发生在2kΩ

图34误差时的情况增益误

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“linearindB,”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