Power-Efficient Continuous-Time Incremental Sigma-Delta Analog-to-Digital Converters

201 Pages·2015·14.45 MB·English

Power-Efficient Continuous-Time Incremental
Sigma-Delta Analog-to-Digital Converters
SHA TAO
Doctoral Thesis
Information and Communication Technology
KTH Royal Institute of Technology
Stockholm, Sweden 2015
TRITA-ICT/MAP AVH Report 2015:03 KTH School of Information and
ISSN 1653-7610 Communication Technology
ISRN KTH/ICT-MAP/AVH-2015:03-SE SE-164 40 Kista
ISBN 978-91-7595-507-0 SWEDEN
Akademisk avhandling som med tillstånd av Kungl Tekniska högskolan framlägges
tilloffentliggranskningföravläggandeavteknologiedoktorsexameniInformations-
och Kommunikationsteknik måndag den 18 maj 2015 klockan 13.00 i Sal C, Elect-
rum 229, Isafjordsgatan 22, Kungl Tekniska högskolan, Kista 164 40, Stockholm.
© Sha Tao, May 2015
Tryck: Universitetsservice US AB
iii
For my family

v
Abstract
Over the past decade, there has been a growing interest in the devel-
opment of integrated circuits (ICs) for wearable or implantable biosensors,
aimingatprovidingpersonalizedhealthcareservicesandreducingthehealth-
care expenses. In biosensor ICs, the analog-to-digital converter (ADC) is a
key building block that acts as a bridge between analog signals and digital
processors. Since most of the biosensors are attached to or implanted in hu-
man bodies and powered by either portable batteries or harvested energy,
ultra-low-power operation is often required. The stringent power budget im-
poseschallengesindesigningpower-efficientADCs,especiallywhentargeting
high-resolution. Among different ADC architectures, the Sigma-Delta (Σ∆)
ADC has emerged as the most suitable for low-power, high-resolution appli-
cations. This thesis aims to enhance the power efficiency of continuous-time
(CT) incremental Σ∆ (IΣ∆) ADCs by exploring design techniques at both
architectural and circuit levels.
The impact of feedback DACs in CT IΣ∆ ADCs is investigated, so as
to provide power-efficient feedback DAC solutions, suitable for biosensor ap-
plications. Different DAC schemes are examined analytically considering the
trade-off between timing error sensitivity and power consumption. The an-
alytical results are verified through behavioral simulations covering both the
conventionalandincrementalΣ∆modes. Additionally,byconsideringatypi-
calbiosensorapplication,differentfeedbackDACsarefurthercompared,aim-
ing to offer a reference for selecting a power-efficient DAC scheme.
Atwo-stepCTIΣ∆ADCisproposed,analyzed,implementedandtested,
with the objective of offering flexible and power-efficient A/D conversion
in neural recording systems. By pipelining two CT IΣ∆ ADCs, the pro-
posed ADC can achieve high-resolution without sacrificing the conversion
rate. Power-efficient circuits are proposed to implement the active blocks of
the proposed ADC. The feasibility and power efficiency of the two-step CT
IΣ∆ADCarevalidatedbymeasurementresults. Furthermore,enhancement
techniques from both the architecture and circuit perspectives are discussed
and implemented, which are validated by post-layout simulations.
A comparative study of several CT IΣ∆ ADC architectures is presented,
aimingtoboostthepowerefficiencybyreducingthenumberofcyclespercon-
version while benefiting from the advantage of CT implementation. Five CT
IΣ∆ADCarchitecturesareanalyzedandsimulatedtoevaluatetheireffective-
ness under ideal conditions. Based on the theoretical results, a second-order
CTIΣ∆ADCandanextended-rangeCTIΣ∆ADCareselectedasimplemen-
tation case studies together with the proposed two-step CT IΣ∆ ADC. The
impact of critical circuit non-idealities is investigated. The three ADCs are
thenimplementedandfabricatedonasinglechip. Experimentalresultsreveal
that the three prototype ADCs improve considerably the power efficiency of
existing CT IΣ∆ ADCs while being very competitive when compared to all
types of the state-of-the-art IΣ∆ ADCs.
Keywords: analog-to-digital conversion, sigma-delta modulation, incre-
mental ADC, two-step ADC, continuous-time, biosensor applications.

Acknowledgments
"Education is what remains after one has forgotten what one has learned in school."
- Albert Einstein -
Iwouldliketoexpressmyfirstandforemostgratitudeforthisworktomyadvi-
sor,Prof. AnaRusu. Sheintroducedmetothemagicworldofsigma-deltawithher
extraordinarypassionandprofoundunderstandingofit. Iamsincerelyhonoredfor
havingthechancetoworkonthistopicunderhersupervision. Itreasureandenjoy
all the inspiring discussions with her, from which I gradually learn to appreciate
the beauty of sigma-delta data converters that were "intimidating" and complexed
to me. I am deeply indebted to her for many things: her generosity in devoting
her valuable time to discuss my immature ideas, her patience in correcting my im-
mature manuscripts, and her acceptance to my immaturity. Life is filled with ups
and downs, and so is the journey of pursuing a Ph.D. She always has confidence
in me even sometimes I lost it myself. She cheered me up when I was trapped
with setbacks and obstacles, and gave her best to suggest me possible solutions.
On the other hand, I am especially grateful for her frank comments and criticisms
that helped me improving my work and growing up. I could write the entire thesis
about my appreciation and respect towards her. To make it short, without her ex-
traordinary guidance and endless support that have been encouraging, motivating,
and inspiring me all along the journey, this thesis work would not be possible.
I would like to express my gratitude to my co-advisor, Dr. Saul Rodriguez, for
mentoring and taking care of me ever since I did my Master thesis in 2009. During
the past six years, he has been always backing me up and providing me continuous
supportandincredibleguidance,bothtechnicalandpersonal. Iwouldnot"survive"
thePh.Dwithouthishelp. Myappreciationalsogoestomyco-advisorDr. Martin
Gustafsson from Maxim Integrated, who gave me many valuable advices regarding
chipimplementationandmeasurementissues. Martinalsohelpedmegoingthrough
themostdifficultperiodduringthemeasurement. Iamespeciallythankfulforthat.
I am indebted to Dr. Julian Garcia for letting me standing on the shoulders of
giants. Julian guided me at the start-up and gave me numerous advices/tips after.
Ihavealsobenefiteda lotfromthe"legacy"heleft. Withoutthem, thereisno way
that I can finish my Ph.D. within three years. A special thank to Jiazuo Chi. I am
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very lucky to be his Master thesis supervisor, and it is my great pleasure to work
with him. I wish Jiazuo all the best for his Ph.D. study at the University of Ulm.
Iwouldliketothankthecurrentmembersoftheintegratedcircuitsandsystems
groupfortheircontinuousencouragementandsupport. Icouldn’tbeluckiertohave
JankoKaticasmyofficemateinthepast2-year-and-half. Iamverygratefulforhis
supportbothtechnicalandemotionalthathelpedandencouragedmeincompleting
this work. Tingsu Chen, for her daily support and chats as well as her help and
encouragement all these years. Nikola Ivanišević, Muhammad Waqar Hussain, and
PanagiotisChaourani,forthegoldentimewespenttogetherandfortheirkindhelp
in proofreading my thesis and manuscript. I would also acknowledge the former
membersandvisitingresearchersofthegroupwithwhomIhadthechancetowork
andsharedgoodmoments,especially,myformerofficemateVasileiosManolopoulos,
Prof. Mohammed Ismail, Prof. Panagiotis Papadimitratos, Prof. Eduard Alarcon,
Dr. Raul Onet, Ying Song, and Milad Razzaghpour.
My appreciation goes to Prof. Mikael Östling and Prof. Carl-Mikael Zetterling
(Bellman) for their excellent management, which creates an enjoyable working en-
vironment in the department. I would also like to thank Gunilla Gabrielsson, our
departmentadministrator,forallthehelpandsupportsheprovidedtome. Special
thanks to Dr. Shi Cheng, for providing me many career advices and other interest-
ing conversations. My gratitude goes to our "neighbor" Doc. Henry Radamson for
his gentleness and many good talks with us. My appreciation also goes to Bengt
Molin for his advice and help in PCB fabrication and component orders.
I would like to extend my gratitude to my other colleagues at EKT who made
mytimeatworkmoreenjoyable. YeTian(William),forsharinglotsofopinionsand
discussions on continuous-time sigma-delta designs and his daily support. Babak
Taghavi,RahelehHedayati,andSalehKargarraziforthediscussionsontheCadence
tools,analogcircuitdesign,andotherconversations. Dr. JiantongLi,Dr. Yonhbin
Wang, Anders Eklund, Ganesh Jayakumar, Maryam Olyaei, Dr. Maziar Naiini,
and Dr. Luiga Lanni (Gina) for their support during my stay at the department.
Gracious appreciation goes to Dr. Håkan Bengtsson, Mikael Pettersson, and
GitiAmozandehfromEricssonABfortheusefuldiscussionsandconstructivecom-
ments during my design review. I am especially thankful to Giti for her valuable
suggestions on continuous-time sigma-delta design and measurement. I would like
tothankUweKoppfromTexasInstrumentfortheinterestingdiscussionsonband-
pass continuous-time sigma-delta modulators and his kind help in providing the
evaluation boards and components for the measurement. Agilent Technologies is
also acknowledged for supplying the lab equipment for the chip characterization.
It is a great honor for me to have Prof. Franco Maloberti from the University
of Pavia as the opponent at my defense. I am also grateful to the members of the
grading committee, Prof. Susana Patón Álvarez from the University Carlos III of
Madrid,Prof. AtilaAlvandpourfromLinköpingUniversity,andProf. KjellJeppson
from Chalmers University of Technology. I would like to thank Prof. Ahmed
Hemani from KTH for kindly accepting the role of substitute committee member.
Special thanks to Prof. Gunnar Malm, for his valuable comments and constructive
ix
suggestionsduringmyPh.D.proposalseminaraswellaswhenreviewingthisthesis.
I gratefully acknowledge the Swedish Research Council (VR) for funding this
research. I would like to thank the Hans and Nils Backmark travel stipend for
the financial support of my conference trip. I am very grateful to the EURO-
DOTs program under the EC Framework 7 Support Action for granting me the
scholarships to attend two fantastic summer courses at EPFL, Lausanne. Thanks
to EURO-DOTs, I got the great chance to listen to the lectures given by several
top-class experts in the field, such as Prof. Willy Sansen and Dr. Richard Schreier
and meet many analog IC designers working in the industry.
IwouldliketothankthepeopleiniPackcenter,fortheirkindsupportduringmy
measurement, particularly, Dr. Qiang Chen, Dr. Zhuo Zou, and Dr. Geng Yang.
Also, I would like to thank Zhuo Zou for sharing with us many great moments
playing the PES game and dining together.
Many thanks to Prof. Mark Smith for his patient help in using the PCB proto-
typingmachine. MyappreciationalsogoestoDr. FredrikJonssonforsharingwith
us his tape-out and engineering experience during his Ph.D. course on EDA tools.
Special thanks to my friends at Electrum and in Stockholm for their friendship
andthegreatmomentswesharedtogether,particularly,Dr. XiChen,Dr. Xiaodan
Pang, Qin Zhou, Jue Shen and Dr. Liang Rong, Ning Ma, Chuanying Zhai, and
Dr. Yi Feng. Anja Djuric, Janko’s sweet girlfriend, thank you for being supportive
and sharing a lot of good moments with us during the leisure time. My friend
since childhood Ran Bi, and her husband Yu Wang, for the relaxing time we spent
together. Dr. Yeyu Fang and Chen Hu, for sharing the two-year joyful lunch time.
IwouldliketopaymytributetothemagnificentSystem-on-Chip(SoC)Master
program that brought me to Stockholm in 2007 and changed my life trajectory.
Duringthepastdecade, ithasnurturedgenerationsofexcellentengineersanddoc-
toralstudents. Iamsoproudofbeingpartofit. ManythankstomySoCclassmates
for making me feel at home since the first day I came to Sweden, especially, my
roommates Ye Xu and Yichan Lu, as well as Huayu Yang and Yan Bao.
My appreciation goes to the people from my home university BJTU in Beijing.
Particularly,myBSc. thesisadvisor,Prof. ZhangfaLiuwhoshowedmethebeauty
of RFIC design and has been supportive all along this journey, Prof. Zheying Li
for introducing me to the fascinating world of IC design, and Prof. Yuntong Wang
whoencouragedmetoenjoygivingpresentations. Manythankstomyfriends/allies
from my BSc. study who have also been supporting me during these years.
My deepest and heartfelt gratitude goes to my family members who love me
unconditionally. Mom and dad, who raise me up and root for me, for their endless
love. My parents-in-law and my uncle and aunt, for their continuous support. My
twograndmas,fortheirloveandcaring. Mytwograndpas,whopassedawayduring
myPh.D.study,maytheyrestinpeace. Thisthesisisespeciallydedicatedtothem.
Ultimately, I would like to express my love to my husband Jia Mao, who has
been standing by me in the past eight years. You shared your shoulders for me to
cry on during those tough moments and provided me the strength to endure the
difficulties. You are the reason I am. With you, I will never walk along.