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LTC6257CMS-PBF Datasheet(PDF) 13 Page - Linear Technology |
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LTC6257CMS-PBF Datasheet(HTML) 13 Page - Linear Technology |
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13 / 24 page  LTC6255/LTC6256/LTC6257 625567f operAtion ApplicAtions inForMAtion TheLTC6255familyinputsignalrangeextendsbeyond thenegativeandpositivepowersupplies.Theoutputcan evenextendallthewaytothenegativesupplywiththe properexternalpull-downcurrentsource.Figure1depicts aSimplifiedSchematicoftheamplifier.Theinputstage iscomprisedoftwodifferentialamplifiers,aPNPstage Q1/Q2andNPNstageQ3/Q4thatareactiveoverdifferent rangesofcommonmodeinputvoltage.ThePNPstage isactivebetweenthenegativepowersupplytoapproxi- mately1Vbelowthepositivesupply.Astheinputvoltage approachesthepositivesupply,transistorQ5willsteerthe tailcurrentI1tothecurrentmirrorQ6/Q7,activatingthe NPNdifferentialpairandthePNPpairbecomesinactive fortheremaininginputcommonmoderange.Alsoforthe inputstage,devicesQ17,Q18andQ19acttocancelthe biascurrentofthePNPinputpair.WhenQ1/Q2isactive, thecurrentinQ16iscontrolledtobethesameasthe currentQ1/Q2.Thus,thebasecurrentofQ16isnormally equaltothebasecurrentoftheinputdevicesofQ1/Q2. Similarcircuitry(notshown)isusedtocancelthebase currentofQ3/Q4.Thebufferandoutputbiasstageuses aspecialcompensationtechniquetotakefulladvantage oftheprocesstechnologytodrivehighcapacitiveloads. ThecommonemittertopologyofQ14/Q15enablesthe outputtoswingfromrailtorail. LowSupplyVoltageandLowPowerConsumption TheLTC6255familyofoperationalamplifierscanoperate withpowersupplyvoltagesfrom1.8Vto5.25V.Eachampli- fierdrawsonly65µA.Thelowsupplyvoltagecapabilityand lowsupplycurrentareidealforportableapplications. HighCapacitiveLoadDrivingCapabilityandWide Bandwidth The LTC6255 family is optimized for wide bandwidth low power applications. They have an extremely high gain-bandwidthtopowerratioandareunitygainstable. Whentheloadcapacitanceincreases,theincreasedca- pacitanceattheoutputpushedthenon-dominantpole tolowerfrequencyintheopenloopfrequencyresponse, worseningthephaseandgainmargin.Theyaredesigned todirectlydriveupto100nFcapacitiveloadinunitygain configuration(seeTypicalPerformanceCharacteristics, Capacitive Load Handling). Higher gain configurations tendtohavebettercapacitivedrivecapabilitythanlower gainconfigurationsduetolowerclosedloopbandwidth andhencehigherphasemargin. LowInputReferredNoise TheLTC6255familyprovidesalowinputreferrednoise of20nV/√Hzat1kHz.Thenoisedensitywillgrowslowly withthefrequencyinwidebandrange.Theaveragenoise voltagedensityover3MHzrangeislessthan24nV/√Hz. TheLTC6255familyisidealforlownoiseandlowpower signalprocessingapplications. LowInputOffsetVoltage TheLTC6255familyhasalowoffsetvoltageof350μV maximumwhichisessentialforprecisionapplications.The offsetvoltageistrimmedwithaproprietarytrimalgorithm toensurelowoffsetvoltageovertheentirecommonmode voltagerange. LowInputBiasCurrent TheLTC6255familyusesabiascurrentcancellationcircuit tocompensateforthebasecurrentoftheinputtransistors. Whentheinputcommonmodevoltageiswithin200mV ofeitherrail,thebiascancellationcircuitarenolongerac- tive.Forcommonmodevoltagesrangingfrom0.2Vabove |
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