ADC分类及全参数.docx
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ADC分类及全参数.docx
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ADC分类及全参数
ADC分类及参数
ADC分类
∙直接转换模拟数字转换器(Direct-conversionADC),或称Flash模拟数字转换器(FlashADC)
∙循续渐近式模拟数字转换器(SuccessiveapproximationADC)
∙跃升-比较模拟数字转换器(Ramp-compareADC)
∙威尔金森模拟数字转换器(WilkinsonADC
∙集成模拟数字转换器(IntegratingADC)
∙Delta编码模拟数字转换器(Delta-encodedADC)
∙管道模拟数字转换器(PipelineADC)
∙Sigma-Delta模拟数字转换器(Sigma-deltaADC)
∙时间交织模拟数字转换器(Time-interleavedADC)
∙带有即时FM段的模拟数字转换器
∙时间延伸模拟数字转换器(Timestretchanalog-to-digitalconverter,TS-ADC
1、闪速型
2、逐次逼近型
3、Sigma-Delta型
1.闪速ADC
闪速ADC是转换速率最快的一类ADC。
闪速ADC在每个电压阶跃中使用一个比较器和一组电阻。
2.逐次逼近ADC
逐次逼近转换器采用一个比较器和计数逻辑器件完成转换。
转换的第一步是检验输入是否高于参考电压的一半,如果高于,将输出的最高有效位(MSB)置为1。
然后输入值减去输出参考电压的一半,再检验得到的结果是否大于参考电压的1/4,依此类推直至所有的输出位均置“1”或清零。
逐次逼近ADC所需的时钟周期与执行转换所需的输出位数相同。
3.Sigma-deltaADC
Sigma-deltaADC采用1位DAC、滤波和附加采样来实现非常精确的转换,转换精度取决于参考输入和输入时钟频率。
Sigma-delta转换器的主要优势在于其较高的分辨率。
闪速和逐次逼近ADC采用并联电阻或串联电阻,这些方法的问题在于电阻的精确度将直接影响转换结果的精确度。
尽管新式ADC采用非常精确的激光微调电阻网络,但在电阻并联中仍然不甚精确。
sigma-delta转换器中不存在电阻并联,但通过若干次采样可得到收敛的结果。
Sigma-delta转换器的主要劣势在于其转换速率。
由于该转换器的工作机理是对输入进行附加采样,因此转换需要耗费更多的时钟周期。
在给定的时钟速率条件下,Sigma-delta转换器的速率低于其它类型的转换器;
或从另一角度而言,对于给定的转换速率,Sigma-delta转换器需要更高的时钟频率。
Sigma-delta转换器的另一劣势在于将占空(dutycycle)信息转换为数字输出字的数字滤波器的结构很复杂,
但Sigma-delta转换器因其具有在IC裸片上添加数字滤波器或DSP功能而日益得到广泛应用。
AtmelAVR127:
UnderstandingADCParameters
Thisapplicationnoteisaboutthebasicconceptsofanalog-to-digitalconverter(ADC)andtheparametersthatdeterminetheperformanceofanADC.
TheseADCparametersareofgoodimportancesincetheydeterminetheaccuracyoftheADC’soutput.
Theparameterscanbebroadlyclassifiedintostaticperformanceparametersanddynamicperformanceparameters.
Staticperformanceparameters arethoseparametersthatarenotrelatedtoADC’sinputsignal.
TheseparametersaremeasuredandanalyzedforalltypesofADCs
(ADCsintegratedwithinthemicrocontrollerorstandaloneADCswhoseoperatingfrequencyareusuallyhigher).
Instead, dynamicperformanceparameters arerelatedtoADC’sinputsignalandtheireffectsaresignificantwithhigherfrequencies.
Majorstaticparameters includegainerror,offseterror,fullscaleerrorandlinearityerrors
whereassomeimportantdynamicparameters includesignal-to-noiseratio(SNR),totalharmonicdistortion(THD),
signaltonoiseanddistortion(SINAD)andeffectivenumberofbits(ENOB).
BasicConcepts
AnADCisanelectronicsystemoramodulethathasanaloginput,referencevoltageinputanddigitaloutputs.
TheADCconverttheanaloginputsignaltoadigitaloutputvaluethatrepresentsthesizeoftheanaloginputcomparingtothereferencevoltage.
Itbasicallysamplestheinputanalogvoltageandproducesanoutputdigitalcodeforeachsampletaken.
Figure1-1.BasicdiagramofADC
TogetabetterpictureabouttheADCconcepts,letusfirstlookintosomebasicADCtermsused.
1.1InputVoltageRange
TheinputvoltagerangeofanADCisdeterminedbythereferencevoltage(VREF)appliedtotheADC.
Areferencevoltagecanbeeitherinternalvoltageorexternalvoltagebyapplyingavoltageonanexternalpinofthemicrocontroller.
Generallyreferencevoltagecanbeselectedbyconfiguringthecorrespondingregister’sbitfieldofthemicrocontroller.
ADCwillsaturatewithaanalogvoltagehigherthanthereferencevoltage,
sothedesignermustmakesurethattheanaloginputvoltagedoesnotexceedthereferencevoltage.
Theinputvoltagerangeisalsocalledas conversionrange.
IfADCrunsinsignedmode(themodeproducessignedoutputcodes),itallowsnegativeanaloginputvoltages.
Insuchcasestheanaloginputrangeisfrom–VREFto+VREF.
AnADCwhichacceptsbothpositiveandnegativeinputvoltagesiscalledas bipolarADC
whereasanADCthatacceptsonlypositiveinputvoltageiscalledas unipolarADC.
1.2Resolution
Theentireinputvoltagerange(from0VtoVREF)isdividedintoanumberofsub-ranges.
Eachsubrangeisassignedasingleoutputdigitalcode.
AsubrangeisalsocalledLSB(leastsignificantbits)andthenumberofsubrangesisusuallyinpowersoftwo.
ThetotalnumberofsubrangesiscalledtheresolutionoftheADC.
Foranexample,anADCwitheightLSBshastheresolutionofthreebits(2^3=8).
IfanADC’sresolutionisthreebitsthenitalsomeansthatthecodewidthoftheoutputisthreebits.
1.3Quantization
TheLSBisdeterminedifinputanalogvoltageliesinthelowestsub-rangeoftheinputvoltagerange.
Forexample,consideranADCwithVREFas2Vandresolutionasthreebits.
Nowthe2Visdividedintoeightsub-ranges,sotheLSBvoltageiswithin250mV.
Nowaninputvoltageof 0Vaswellas250mV isassignedtothesameoutputdigitalcode 000.
Thisprocessiscalledasquantization.
1.4ConversionMode
AconversionmodedetermineshowtheADCprocessestheinputandperformstheconversionoperation.
AstandardADChasbasicallytwotypesofconversionmodes.
1.Singleendedconversionmode.
2.Differentialconversionmode.
1.4.1SingleEndedConversionMode
Insingleendedconversion,onlyoneanaloginputistakenandtheADCsamplingandconversionisdoneonthatinput.
InsingleendedconversionADCcanbeconfiguredtooperateinunsignedorsignedmode.
TheanalogisconnectedtoADChasnon-inverting(+)inputandinverting(-)input
whichshouldbedifferentlyconnectedundersignedorunsignedmode.
Forexampleinsignedmodeofoperation,thesingle-endedinputmaybegiventothenon-invertinginputoftheADC
andtheinvertinginputoftheADCisgrounded.
ThisisdepictedinFigure1-2.Inthiscasethereferencevoltageisfrom–VREFto+VREF,
whichmeansitallowsnegativeinputvoltages.
Inunsignedsingle-endedmode,thesingle-endedinputisgiventothenon-invertinginputoftheADC
asbeforeandtheinvertinginputoftheADCissuppliedwithsomefixedvoltagevalueVFIXED
whichisusuallyhalfofthereferencevoltageminusafixedoffset)asshowninFigure1-3.
Inthiscasetheinputvoltagerangeisfrom0VtoVREF,whichmeansitdoesnotallowsnegativeinputvoltages.
1.4.2DifferentialConversion
Indifferentialconversionmode,twoanaloginputsaretakenandappliedtotheinvertingandnon-invertinginputsoftheADC,
eitherdirectlyorafterdoingsomeamplificationbyselectingsomeprogrammableamplificationstages(gainamplifierstage).
Differentialconversionsareusuallyoperatedinsignedmode,wheretheMSBoftheoutputcodeactsasthesignbit.
Alsothereferencevoltageisfrom-VREFto+VREFforsignedmode.ThisisshownintheFigure1-4
1.5IdealADC
AnidealADCisjustatheoreticalconcept,andcannotbeimplementedinreallife.
Ithasinfiniteresolution,whereeverypossibleanaloginputvaluegivesauniquedigitaloutput
fromtheADCwithinthespecifiedconversionrange.
AnidealADCcanbedescribedmathematicallybyalineartransferfunction,asshowninFigure1-5andFigure1-6.
1.6PerfectADC
TodefineaperfectADC,theconceptofquantizationmustbeused.
DuetothedigitalnatureofanADC,continuousoutputvaluesarenotpossible.
TheperfectADCperformsthequantizationprocessduringconversion.
ThisresultsinastaircasetransferfunctionwhereeachsteprepresentsoneLSB.
Ifthereferencevoltageis2V,say,andtheADCresolutionisthreebits,
thenthestepwidthbecomes250mV(1LSB).
Theinputanalogvoltagerangefrom0Vto250mVwillbeassignedthedigitaloutputcode000
andtheinputanalogvoltagerangefrom251mVto500mVwillbeassignedthedigitalcode001andsoon.
ThisisdepictedinFigure1-7whichshowsthetransferfunctionofaperfect3-bitADCoperatinginsingleendedmode.
Figure1-8,givenbelow,showsthetransferfunctionofaperfect3-bitADCoperatingindifferentialmode.
NOTEThereferencevoltageisfrom-1Vto+1VinthiscaseandtheMSBactsassignbit.
FromtheFigure1-7,itisobviousthataninputvoltageof0Vproducesanoutputcode000.
Atthesametime,aninputvoltageof250mValsoproducesthesameoutputcode000.
Thisexplainsthequantizationerrorduetotheprocessofquantization.
Astheinputvoltagerisesfrom0V,thequantizationerroralsorisesfrom0LSB
andreachesamaximumquantizationerrorof1LSBat250mV.
Againthequantizationerrorincreasesfrom0to1LSBastheinputrisesfrom250mVto500mV.
Thismaximumquantizationerrorof1LSBcanbereducedto±0.5LSBbyshiftingthetransferfunctiontowardsleftthrough0.5LSB.
Figure1-9depictsthequantizationadjustedperfecttransferfunctiontogetherwiththeidealtransferfunction.
Asseenonthefigure,theperfectADCequalstheidealADContheexactmidpointofeverystep.
ThismeansthattheperfectADCessentiallyroundsinputvaluestothenearestoutputstepvalue.
SimilarlyFigure1-10isfordifferentialADC.
QuantizationerroristheonlyerrorwhenperfectADCisconsidered.
ButincaseofrealADC,thereareseveralothererrorsotherthanquantizationerrorasexplainedbelow.
1.7OffsetError
TheoffseterrorisdefinedasthedeviationoftheactualADC’stransferfunction
fromtheperfectADC’stransferfunctionatthepointofzerotothetransitionmeasuredintheLSBbit.
Whenthetransitionfromoutputvalue0to1doesnotoccurataninputvalueof0.5LSB,
thenwesaythatthereisanoffseterror.
Withpositiveoffseterrors,theoutputvalueislargerthan0whentheinputvoltageislessthan0.5LSBfrombelow.
Withnegativeoffseterrors,theinputvalueislargerthan0.5LSBwhenthefirstoutputvaluetransitionoccurs.
Inotherwords,iftheactualtransferfunctionliesbelowtheidealline,thereisanegativeoffsetandviceversa.
PositiveandnegativeoffsetsareshowninFigure1-11andFigure1-12respectivelymeasuredwithdoubleendedarrows.
InFigure1-11,thefirsttransitionoccur
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- ADC 分类 参数