ACSP · Analog Circuits And Signal Processing Taimur Rabuske Jorge Fernandes Charge-Sharing SAR ADCs for Low-Voltage Low-Power Applications Analog Circuits and Signal Processing Serieseditors MohammedIsmail,Dublin,USA MohamadSawan,Montreal,Canada Moreinformationaboutthisseriesathttp://www.springer.com/series/7381 Taimur Rabuske • Jorge Fernandes Charge-Sharing SAR ADCs for Low-Voltage Low-Power Applications 123 TaimurRabuske JorgeFernandes INESC-ID INESC-ID InstitutoSuperiorTécnico InstitutoSuperiorTécnico UniversidadedeLisboa UniversidadedeLisboa Lisboa,Portugal Lisboa,Portugal ISSN1872-082X ISSN2197-1854 (electronic) AnalogCircuitsandSignalProcessing ISBN978-3-319-39623-1 ISBN978-3-319-39624-8 (eBook) DOI10.1007/978-3-319-39624-8 LibraryofCongressControlNumber:2016942569 ©SpringerInternationalPublishingSwitzerland2017 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped. Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthispublication doesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevant protectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthorsandtheeditorsaresafetoassumethattheadviceandinformationinthisbook arebelievedtobetrueandaccurateatthedateofpublication.Neitherthepublishernortheauthorsor theeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinorforany errorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAGSwitzerland To ourfamilies Preface The integrated successive-approximation-register(SAR) analog-to-digital con- verter (ADC) is known to present a remarkable energy efficiency. Additionally, the SAR ADC is a veryscaling-friendlyarchitecture,due to its highlydigital and switching-intensive nature and its ability to effortlessly accommodate rail-to-rail signals without resorting to precision amplifiers. These characteristics have been boosting the popularity of SAR ADCs as process scaling reduces the transistor’s intrinsic gainand supply voltages. The mostbasic formof an SAR ADC requires a voltage comparator, a digital controller, a track-and-hold (TH) circuitry, and a digital-to-analogconverter(DAC). Inmostofthereporteddesigns,theDACofanSARADCiscapacitiveandrelies onthecharge-redistribution(CR)principle.Alternatively,anSARADCthatrelies on the charge-sharing(CS) principle has been proposed more recently. The main advantages of the CS-ADC are that it is immune to inaccuracies in the reference bufferduringtheconversionandrequireslessdemandingbuffersfortheinputsignal and the reference voltage. Additionally, the CS-ADC allows the use of nonlinear capacitors. On the other hand, the CS-ADC is less tolerant to noise and to the comparatoroffset,whichlimitsthecompetitivenessofthearchitecture.Finally,the literaturelacksacomprehensiveanalysisofthelimitingfactorsofCS-ADCs. The aim of this book is to fill the gap in knowledge that exists for CS-ADCs by providing an in-depth analysis of the architecture and quantifying its limiting factors.Finally,theinsightsgainedthroughthisanalysisareappliedtodevisetech- niquesthatmitigatethecriticaldrawbacksofthearchitectureandallowthedesign of energy-efficientSAR ADCs for low-voltageapplications.These techniquesare validated through the design of two SAR ADCs that operate at supply voltages down to 0.35 and 0.4V. Experimental results show that the two reported ADCs presentthe bestenergyefficiencyamongADCs designedin technologiesdownto 0.13µm. Additionally, one of the presented prototypes is the first demonstration of SAR ADC that uses a DAC with very-nonlinear capacitors, benefitting from the improved capacitance density and matching characteristics of metal-oxide- semiconductor(MOS)capacitors. vii viii Preface Much of the material presented in this monograph originates from the work carried out by the first author for his Ph.D., at Universidade de Lisboa, Portugal. Thisworkincludesoriginalresearchresultsthathavebeenpresentedatinternational conferences(ESSCIRC,ISCAS,andICECS,amongothers),publishedinSpringer AnalogIntegratedCircuitsandSignalProcessing,IEEETransactionsonVeryLarge ScaleIntegration(VLSI)SystemsandIEEEJournalofSolid-StateCircuits. Theresearchreportedinthisbookreceivedcontributions frommanygenerous and gifted people. The authors would like to thank all colleagues and friends at INESC-IDLisboa,Portugal,andUFSM,Brazil. Finally,thisworkwasfinanciallysupportedby: (cid:129) FCT, Fundação para a Ciência e a Tecnologia (Portugal), under projects UID/CEC/50021/2013andDISRUPTIVE(EXCL/EEI-ELC/0261/2012) (cid:129) CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazil),underthePh.D.grant201887/2011-8 Lisboa,Portugal TaimurRabuske Lisboa,Portugal JorgeFernandes April2016 Contents 1 Introduction .................................................................. 1 1.1 Background.............................................................. 1 1.2 ResearchGoals.......................................................... 6 1.3 BookOutline............................................................ 6 1.4 OriginalContributions.................................................. 7 1.5 FinalRemarks........................................................... 7 References..................................................................... 8 2 ADCsforLow-VoltageLow-PowerApplications ........................ 11 2.1 Introduction ............................................................. 11 2.2 SamplingandQuantization............................................. 14 2.3 SearchMethodsforNyquistADCs .................................... 15 2.3.1 DirectSearch(FlashADCs).................................... 17 2.3.2 BinarySearch(SARADCs).................................... 18 2.3.3 PipelinedBinarySearch(PipelineADCs)..................... 19 2.3.4 Summary........................................................ 21 2.4 TheSARADC .......................................................... 21 References..................................................................... 23 3 ReviewofSARADCSwitchingSchemes ................................. 25 3.1 Introduction ............................................................. 25 3.2 OperationModesofCharge-BasedSARADCs....................... 26 3.2.1 TheChargeRedistributionPrinciple........................... 26 3.2.2 TheChargeSharingPrinciple.................................. 27 3.3 ChargeRedistributionSwitchingSchemes ............................ 28 3.3.1 ConventionalSwitching........................................ 28 3.3.2 Monotonicor“Set-and-Down”Switching .................... 30 3.3.3 V -BasedCapacitorSwitching............................... 32 CM 3.3.4 Tri-LevelCapacitorSwitching................................. 34 3.4 ChargeSharingSwitchingScheme .................................... 36 3.5 ComparisonofReviewedSwitchingSchemes ........................ 42 3.6 StateoftheArtinCS-ADCs ........................................... 46 ix