A Switched-Capacitor Multiplier/Divider with Digital and Analog

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A Switched-Capacitor Multiplier/Divider with Digital and
Analog Outputs
Watanabe, Kenzo; Temes, Gabor, C.
IEEE Transactions on Circuits and Systems. 31(9), p. 796-800
1984
http://hdl.handle.net/10297/3447
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lEEJ:T≠ANSACnONSONCI CUITSANDSYmNS,VOL・C^S−31,NO.9,SEPTuEJL1984
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Fig・1・Anm−bitprogrammablebinary−Weightedcapacitorarray(a)andits
Symbol(b).
Finally,it canreadily be modified to act as a high−aCCuraCy
CaPaCitanCebridgewithdigitalread−Out,aSwillbedescribedina
forthcomngpublication.
Itishopedthatthiscircuitwi11alsoserveasanillustrationof
thepotentialsofhigh−aCCuraCyMOSanalogcircuitryingeneral
Signalprocesslng,analog,digital,Orhybrid.
DAAD DA
SS
SS
朝1
(b)
II.CIRCUITDESCRIPTION
Inwhatfollows,Xandyarebinarynumbersm−bitandn−bit
long,reSPeCtively,lntheforms
Fig・2・(a)The schematic diagramOfthe switched−CapaCitordigitalmulti−
plier/divider,WhereAD,DA,and SSarethestatesignalscontrollingits
Operation.nLeirtimingsequenceisshownin(b).
J=ズ1・2−1+ズ2・2 ̄2+…+ズ椚・2 ̄椚 (1)
γ=八・2 ̄1+γ2・2 ̄2+…十九・2 ̄〃. (2)
Thesignsofxandyarerepresentedbytheadditionalbitsxm+1
and凡.1,reSPeCtively,Whichassumethevalue“1’’whenxand
yarenegativeand“0’’whentheyarepositive・Tbeoperation
the RESETsignalsent fromaneXternaldevice toinformthe
Circuitofthereceiptoftheprevi0usreSult・UponrecelVlngthis
Signal,thecircuitclearSthez reglSterand,attherisingedgeof
the¢clocksignalimmediatelyfollowingit,temiatestheOUT
State andimitiates the DA substate of the XEC state.This
executedisx・yOrX/y.Whenmultiplicationiscarriedout,Xis
Substatelastsonlyoneclockperiod,tObefollowedbytheAD
themultiplicandandyisthemultiplier・Tbeproductzisstored Substate.TheADstatelasts m+n+lclockperiod.Afterone
in’aregisterof(m+n)−bitlength.Whendivisionisperformed,X
isthedividendand yisthedivisor.ToavoidanOVermow,the
diviSorylSnOrmalizedsothatitsMSB,yl,isl.Tberatiois
trunCatedto(m+n)bits,andisstoredinthez register.Each
numbercanbeconvertedtotheappropnatechargebymeanSOfa
ClockperiodintheDAstate,thecircuitreturnsagalntOtheOUT
State・Thisstatecontinuesunti1thenextRESETsignalstartsa
newcycleofoperation.
TbemodecontroIMC signalin Fig.2(a)selects the actual
Operationperformed・WhenMC=1,thecircuitexecutesmultipli−
binary−Weightedcapacitorarrayll】,aSShoYninFig・1・Ths
COnVerSionisthebasisofthedigitaloperationlnVOIvedhere.
Fig・2(a)showstheschematicdiagramOftheoverallcircuit.It
CPnSistsbasicallyofthreepartS;theD/Aconverter,thesucces−
SIVe−q)PrOXimation A/D converter,andthe samPle and hold
Cation,and
when
MC=1,it
performS
division.Tbe
detailed
descnptlOnOftheoperationisglVenneXt.
」.肋〟¢〟C加わ乃
(S/H)circuit・nLeD/Aconverterisformedbythecapacitor
Sincenow雨空=0,thecapacitorsClandC2(Fig.2(a))are
arraySCr,q,andC;,thefeedbackcapacitorsClandCi,and COnneCtedinparalleltoformafeedbackcq)aCitorCFOfvalue
the op−amP Al.Ths converter also formS the successive−
2mC.TbecircuitconngurationintheDAandADstatesarethus
approximationA/DconverterincoQiunctionwithop−amPA2 asshowninFig.3(a)and(b),reSPeCtively.
andthe(m+n)−bitshiftregister(Zregister).
Thecircuitcanbeinoneoftwostates;theXEC(execute)state
duringwhichthedigitaloperationisbeingexecuted,OrtheOUT
(OutPut)state dming whichthe resultis availableinthe z
reglSter.TbeXECstateisfurtherdividedintotheDAandAD
Sub−StateSduringwhichtheD/Aconversionandthesuccessive−
approximationA/DconversionarePerformed,reSPeCtively.The
dJ.(わemJfo〃山鹿以gJdJe
Tbemultiphcandx programsthearrayq tosetitsvalueto
2mCx・Op−amPAlwithcapacitorsqandCFformSanOffset−free
D/Aconverterl2],【3]・Itconvertsthevalueofxintotheanalog
VOltage fl by charglng q tO the scaled reference voltage
CkV/(CLtCb)inthe¢=lphaseandthendischargingitinto
CFinthe¢=1phase・TheoutputvoltageofAlisthen
COrreSPOnding“state’’signalSandtheselectionsignalSSofthe
reference sources controlthe operation ofthe circuit・Tbeir
timingisshowninFig.2(b).Tもeoperationcycleisimitiatedby
q Gり Gり
K=石●百7㍍=ズ石了石
(3)
798
IEEETRANSACTIONSONCIRCUITSANDSYSTEMS,VOL.CAS−31,NO.9,SEPTEMBER1984
CF
幽W
(b)
(a)
(b)
(a)
Fig・3・Circuitconfiguratipnsformultiplication:(a)TbeD/Aconverter(DA
State).O))TbesuccessIVe−aPprOXi皿ationA/Dconverter(ADstate)・
Fig.4.Circuit con丘gurations for division:(a)ne D/A converter(DA
State)・(b)Tbesuccessive−aPprOXimationA/Dconverter(ADstate).
ftisappliedtothetopterminaloftheparal1el−COnneCtedcapaci−
holds,Where
torsGandCか
Op−ampA2isconnectedasavoltagefollowerwithagrounded
lnput・Itsoutputisthusitsownlnput−referredoffsetvoltageIて2
andthisisappliedtothebottomtemialsofGand Cb.The
VOltage鴨acrossCbisTt一巧2,Whichwi11beapplied,inseries
withtheoffsetvoltageofA2,tOthenoninvertinglnPutteminal
OfA2inthesubsequentADstate.
d−2.(わemJわ乃血血dpgねJe
Themuldphery programSthearrayq tOSetitsvalueto
2mq′.Tbea∫rayGisnowaccessedbythe
m
MSB’softhe
eJ=Gり
(7)
ez=G〟り+G/り/2〝
=(zl・2 ̄1+Z2・2 ̄2+…十g朋.〃・2 ̄(∽+〝))2椚C・り
=G巧.
(8)
The(m+n)−bitshiftregisterthusstorestheproductz三=X・y.
月.且肋血血椚
z
register.Tberema皿ngnbitsofthezregisterprogramthearray
C;/.TbesetwoaLraySformincombinationthearrayC;forthe
PrOductz・Op−amPAlandthearraySGandq,nOWforman
Offset−freeD/Aconverter.Tbec叩aCitorsGandCbareinthis
StateCOnneCtedinseries;theirinitialvoltageisI左,aSaCquiredin
thepreやusDAstate・Op−amPA20PerateSaSaCOmparatOr・・The
WholeclrCuit,hcludingthe z register,nOWformSaSuCCeSSive−
approximation.mitiplyingA/Dconverter(Fig・3(b))・
neconverslOnbeginsafterCFisdischargedinthe7もperiod
Inthisoperation,MC=1and,therefore,thecapacitorCiis
Short−Circuited.The circuit configurationsinthe DAand AD
StateSarethenasshowninFig.4(a)and(b),reSpeCtively.During
theDAstate,thearraysClandqaresettothevalues2mCX
and2mCy,reSpeCtively.TbesetwoarraySandop−amPAlforman
Offset−freeD/Aconverterwhichconvertstheratiox/yintothe
analog voltage ft/,by charglng q tOthe scaled reference
VOltageCJft/(Ci+Cb)inthe¢=lphaseandthendischarging
itintoqinthe¢=1phase・Tbeoutputvoltageofop−ampAlis
(Fig・2(b))・IntimeslotTl,thecapacitanCeOftheamayqisset
to2m.lcbyassumngtemporarilyzl=landischargedtothe
referencevoltageりinthe¢=lphase.Theinputvoltagennof
Kル=器・号・ (9)
thecomparatoristhentheweightedsumOfthepresentoutputq
OfAl,thevoltageacrossCb,andtheoffsetvoltageofA2:
nLuSIも=ft/y一巧2isgeneratedasdescribedinSectionII−A−1・
InthesubsequentAI)state,theanalogvoltagefl/,lSCOn−
巧n=藷が+…2=盈(云一号)・(4)
Tbe offsetvoltage of A2does not appearin(4)becauseitis
CanCelled.TbeoffsetvoltageofAlisnotincludedeither,because
Ofthe offset−free D/Aconversion.Tbecomparisonprocessis,
therefbre,Offset free.Depending onthe polarity of nn,the
COmParatOrA2keepszlaSl(ifnnispositive)orresetszltOO
(if nnisnegadve).
Inthenextti皿eSlot T;,thecapacitoroftheamyC;corre−
SPOnding to22lS aCCeSSed simi1ady,Whlethe value of zlis
StOred.nnisnow
Vertedtoan(m+n)−bitnumberzbythesuccessivF−aPPrOXimチー
tionA/DconvertershowninFig・4(b)・TもeconverslOnPrOCeSSIS
thesameaSthatdescribedinthemultiplierandisagalnPraCti−
Callyoffset−freeiftheopen−loopgalnSOfAland A2areVery
high.TbechargeonCbisnowapproximatelybalanCedsothat
臥 eZ
■■■l
q
q
(10)
holds,WhereCF=Cl=2m−lcandQ,andQzaregivenby(7)
and(8),reSPeCtively.The ratio z rounded to(m+n)bitsis
StOredinthez reglSter.
ThecircuitinFig.4canaccomplishotherfunctionsaswell.If,
e・g・,ananalog signal巧is apphed at the temialwhere the
SCaled reference voltage CkTt/(G+Cb)was previously con−
q zl・2椚 ̄lc+2椚 ̄2C
CF
q
nected,then nominvertlnganPlificationwitha programmable
)・(5)
gainx/yisobtainedbythecircuitofFig.4(a).nesuccessive一
A2keepsz2aSlif nnispositiveandresetsz2tOOotherwise.
叩PrOXimationA/DconvertershowninFig.4仲)thenprovidesa
digitaloutputzsuchthat
Thsprocessisrepeated(m+n)times,unti1achargebalanCeis
reachedonCbSOthat
可…・器巨 (11)
eJ eZ
CF q
(6)
holds.Therefore,ifxand y aresetsothattheparenthesized
799
IEEETRANSACTlONSONCIRCUlTSANDSYSTEMS,VOL.CAS−31,NO.9,SEPTEMBER1984
Fig・6・The schematic diagramOfthe multiplier circuitincluding theim−
portantParaSiticelements.
\血
(b)
Fig・5・CircuitconEigurationsforanalogoutput:(a)TbeD/Aconverterin
the7L汁n+lperiod・(b)Tbesampleandholdcircuit.
factorFqualsl,thedivideraccomplishesan(m+n)−bitA/D
COnVerSlOn.
III.ANALOGOUTPUT
Ineithermultiplicationordivisionoperation,the circuitre−
turnstotheDAstateattheonsetofthe Tn+n+l Period.The・
CircuitdiagramOfthestagecontalnmgAl,Validinthisperiod,is
Shownin Fig.5(a).ItisanOffset−free(m+n)−bitI)/A con−
isalso true forthose gate−SOurCeand gate−drain feedthrough
CaPaCitancesoftheMOSswitcheswhichareCOnneCtedbetween
VOltage sourcesand ground only.FurthemOre,SOme Ofthese
feedthroughcapacitanCeSareCOnneCtedtothesamenOdesand
drivenbythecomplementaryclocksignalS,SuChasCLandC;in
Fig.6 These paired capacitanCeS Can bealso neglected to a
且rst−Orderapproximation.Onthebasisoftheabove,WeObtain
thecircuitinFig.6asthepractiCalcircuitmodelofFig.2(a).
ReferringtoFig.6,WeCanObtaintheexpandedchargebalanCe
equation
Cり・G/(G+Cぁ)+q均+(q+q+q)巧1/(1+dl)
q(1+叶等))
G ez+Aez−q均一(C+q+q)巧l/(1+dl)
=0.(12)
耶+G q(1・叶等))
Verter,PrOvidingthe analog voltageI乞=lzIV which canbe
Here,TtistheamPlitudeofthecl∝ksignalappliedtothereset
SチmPledandheldbytheaddedcircuitshowninFig・5(b)・Ths
ClrCuitformSaVOltagefollowerwhenthesignoftheproductor
therati0,Sgn Z,ispositive.Whensgn zisnegative,itformSa
SWitch・TheprimedcapacitanでeS(G,C;,etC・)denotetheactual
ValuesincludingtheinaccuracleS,WhileAQzrepresentstheerror
VOltageinverterswitchl4]andprovidestheoutputwithanega−
tivepolarity.Ineitherconnection,theoutputisoffset−freebe−
CauSetheoffsetvoltageofA3isdetectedandheldintheholding
inQzandhenceintheoutputzduetothenomidealfactors.
AdetailedanalysisofthetermSCOntributingtoAQz,0mitted
intheinterestofbrevity(but availablefrom the authorsupon
request),reVealSthattheexpectedcircuityield foralO−bit
CaPaCitorChinthe“Sample”period,andthencanCelledinthe
accuracylS about38percent;for a9−bit accuracy,about68
Subsequent“hold’’period.Tbeclockfeedthroughhasnoeffect
PerCent;andfbran8−bit accuracy,about95percent.Identical
Onthe output either,Since eachnodeis connected either to a
VOltagesourceortoground.
COnClusionsholdfortheaccuracywhenthecircuitisusedasa
divider.
ⅠV.ExpERIMENTALVERIFICATION
ThechargebalancerelatioTSgivenin(6)and(10)donottake
into account nOnidealcircult COnditions such as capacitanCe
mismatch,theoffsetvoltagesandthefimitegalnSOftheop−amPS,
ParaSiticcapacitanCeS,andthefeedthroughfrom theclocksig−
nalS.In this section,the charge balance equation valid under
PraCtiCalconditionsisglVen,tOObtainaneStimateoftheaccu−
racyoftheoperation.
ThecircuitshowninFig.2(a)contains,infact,manyparaSitic
CaPaCitanCeS nOt Showninthe ngure.Tbose paraSitic capaci−
tances,however,Whichareconnectedorswitchedonlybetweena
VOltagesourceandgroundhavenoeffectontheoperation.Ths
A4×4−bit multiplier/divider,based onthe scheme of Fig.
2(a),hasbeenconstruCtedushgCMOStranSmiSsiongates,an
LM347JFETop−amP,and discretecapacitors.Allarrayscon−
Sistedof2,1,0.5,andO.25nFcapacitors.AllothercapacitanCeS
WereChosento2nF.Thesupplyvoltageswere±6V.
Fig.7showsthewaveformSObservablewhenthedigitalmulti−
Plication(−0111)×(1100)wasexecuted.Tbezerolevel,thecir−
Cuitstates,andthetimlngarealsoindicatedinthefigure・Tbe
referencevoltageりWaS3.6V.Tbeuppertraceshowsthevoltage
fLatnode④duringtheXECstatefFig・2)・TbeinidalValueis
thesmalloffsetvoltageofAlaPPeanngatnOde④duringthe
800
IEEETRANSACTIONSONCIRCUITSANDSYSTEMS,VOL.CAS−31,NO.9,SEPTEMBER1984
Fig・7・Waveforms appeanng Whenthe digital
du血gtheXECstate. F霊ut芝a芸荒£駕豊霊悪邑芯三豊三豊霊
笥慧
Lowertmce:rmeOutPutVOltageofS/Hcircuit.(Theactualverticalscaleis
(一0111)×(1100)
WaSeXeCuted.LWertrace:nLeVOltageatnode
tentimesthatindicatedinthefigure,SincealO:loscilloscopeprobewaLS
trace:TbeoutputvoltageofS/Hcircuit.(Tbeactualverticalscaleis ten
timesthatindicatedinthe且gure,SincealO:loscilloscopeprobewasused.)
used.)
¢=1phaseintheDAsubstateofXEC.Inthenext¢=lphase,
themultiphcand(7/16)isconvertedtotheanalogvoltageTt=
(7/16)×(3.6/2)=0.79V.NoticethatduringthesubsequentAD
Substate,thisvoltageisheldinCbandIちisthesumof fLand
halfoftheoutputvoltageofop−amPAl.Duringthe¢=1phase
in7も,0.5Vcanbeseensuperposedonflduetothedischargeof
CFCauSedbyP2=1.Inthe¢=lphasein71,Zlistested.The
VOltage yLinthisphaseisO.79−(2/3)×3.6/2三一0.41V,thus
thevalue“0”isasslgnedtozl.Inspectlngthevoltagelevelsinthe
Subsequent seven ¢=l phases,We Can read the product
(01010100),aSindicatedintheFigure・Thsisthecorrectaneyer・
The circuit now returns tothe DA substate and,inthe¢=1
phase,COnVertSthe product(21/64)intothe analog voltage
fL=1.18V.Thelowertraceistheoutputy;ofop−amPA3(Fig.
5),Showing how y:is sampled,inverted sincethe sgn zis
negative,andheldbytheS/Hcircuit.
Fig.8showsthewaveformsobservablewhenthedigitaldiviー
Sion(1000)÷(1111)wasexecuted.Indivision,theoutputvoltage
OfAICanbeashighas2Tl becausetheratiozcanaSSumethe
maximumvalue2.Therefore,the referencevoltage f;was re−
ducedto2Vtopreventtheop−amPAlfrombeingsaturated.The
timingofthetraceisthesameasthatinFig.7.Theuppertrace
(thevoltageIl atnode④)showstheprocessproducingthe
ratio(01000100).Thsratiois correctup to8bits.Tbelower
tracdshowshowtheratioTiinanalogformissamPledandheld
bytheS/Hcircuit.
bythemismatchesinthecapacitorarrayS・Numericalcalculations
uslng aVai1able dataon MOScapacitorsindicate that8,9,Or
lO−bitaccuracylSObtainablewith95,68,Or38percentyields,
respectively.
The main advantageOftheproposed circuit overaconven−
tionaldigitalmultiplierisitsversatility:ItcanPerform,besides
digitalandhybridmultiplicationanddivision,alsoprogramm−
ableamPlificationaswellasD/AandA/Dcopversion・Tもe
maindisadvantageistheloweroperatingspeed,eStumatedas2to
3times slower thanthat of a serial−Parallel CMOS digital
multiplier・Inadditiontoitsversatility,theproposedcircuitcan
beimplementedbyuslngarelativelysmallnumberofcompo−
nents・Countlng amit capacitor as being equivalent to a
MOSFET,a4×4−bit multiplier canbe built by about500
MOSFET’S・Aconventional4×4−bitserial−Parallelmultiplier,On
theotherhand,requiresabout700devicesl7],【8].Tbeproposed
Circuit,however,may require a slightlylargerarea Whenin−
tegratedonSichipbecausetheanalogMOSFET,sinvoIvedinthe
OP−amPSrequlrelargergateareaSthandigitaldevices.Thscan
beremediedbyuslngthemuchsimplerdynamicop−ampSfirst
SuggeStedbyCopelandandRabaeyl9].
AcKNOWLEDGMENT
The authors are gratefulto Prof.Ken Martin ofUCLAfor
usefuldiscussions・TbenrstauthoralsothankstheMimistryof
Education,Japan,forsupportlnghisstayatUCLA.
REFERENCES
Ⅴ.CoNCLUSION
ll】J・LMcCrearyaLndP・R・Gray,“Al1−MOSchargeredistributionanalog−
Aswitched−CaPaCitorcircuitwaspresentedforthemultiplica−
tionordivisionoftwodigitalnumbers.Theoutputisavai1ablein
bothanaloganddigitalforms.TheprlnCiplesofoperationhave
beenconLirmedexperimentally.TbedesigncriteriaforICrealiza−
tionaresummarizedasfollows:
1)Thegainsoftheop−anPSdonotdirectlyaffecttheaccu−
racy.Thsis becausethe charge balanceis achieved by the
iterativeuseofoneop−amP.Therefbre,thereductionoftheoffset
VOltage should be emphasizedinthe design of the op−amPS,
ratherthanaveryhighgainwhichwouldalsodegradetheslew
rate.
2)TomaketheunitcapacitorsinthearraySSmal1,andthereby
to−digitalconversion
teclmiques−PartI,”IEEEJ.SoHdStateCircuits,
VOl.SC−10,pp.37ト379,Dec.1975.
[2]R,GregorianandG・Amk,‘‘Anintegrated,Single−Chip,Switched−CapaCi−
torspeechsynthesizer,”hProc・Int・勘mp・OnCircuiLfandSysteJW,Pp.
733−736,(Houston,TX),1980.
[3]R・GregorianandS.Fan,“Offsetfreehigh−reSOlutionD/Aconverter,”in
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