A HIGH CMRR INSTRUMENTATION AMPLIFIER FOR BIOPOTENTIAL .

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A HIGH CMRR INSTRUMENTATION AMPLIFIER FORBIOPOTENTIAL SIGNAL ACQUISITIONA ThesisbyReza Muhammad AbdullahSubmitted to the Office of Graduate Studies ofTexas A&M Universityin partial fulfillment of the requirements for the degree ofMASTER OF SCIENCEMay 2011Major Subject: Electrical Engineering

A HIGH CMRR INSTRUMENTATION AMPLIFIER FORBIOPOTENTIAL SIGNAL ACQUISITIONA ThesisbyReza Muhammad AbdullahSubmitted to the Office of Graduate Studies ofTexas A&M Universityin partial fulfillment of the requirements for the degree ofMASTER OF SCIENCEApproved by:Chair of Committee,Committee Members,Head of Department,Edgar Sanchez-SinencioHamid A. ToliyatSamuel PalermoDuncan Henry M. WalkerCostas N. GeorghiadesMay 2011Major Subject: Electrical Engineering

iiiABSTRACTA High CMRR Instrumentation Amplifier for BiopotentialSignal Acquisition. (May 2011)Reza Muhammad Abdullah, B.Sc., Kwame Nkrumah University of Science & TechChair of Advisory Committee: Dr. Edgar Sanchez-SinencioBiopotential signals are important to physicians for diagnosing medical conditions inpatients. Traditionally, biopotentials are acquired using contact electrodes together withinstrumentation amplifiers (INAs). The biopotentials are generally weak and in thepresence of stronger common mode signals. The INA thus needs to have very goodCommon Mode Rejection Ratio (CMRR) to amplify the weak biopotential whilerejecting the stronger common mode interferers. Opamp based INAs with a resistorcapacitor feedback are suitable for acquiring biopotentials with low power and low noiseperformance. However, CMRR of such INA topologies is typically very poor.In the presented research, a technique is proposed for improving the CMRR of opampbased INAs in RC feedback configurations by dynamically matching input and feedbackcapacitor pairs. Two instrumentation amplifiers (one fully differential and the other fullybalanced fully symmetric) are designed with the proposed dynamic element matchingscheme.Post layout simulation results show that with 1% mismatch between the limitingcapacitor pairs, CMRR is improved to above 150dB when the proposed dynamicelement matching scheme is used. The INAs draw about 10uA of quiescent current froma 1.5 dual power supply source. The input referred noise of the INAs is less than3uV/ š»š‘§.

ivACKNOWLEDGEMENTSFirst, I would like to thank my committee chair, Dr. Edgar Sanchez-Sinencio for hisexpert guidance and for encouragement he gave me during the time I have spentpursuing a Masterā€™s degree at Texas A&M University. My thanks also goes to mycommittee members and all the professors in the Analog & Mixed Signal Center ofTexas A&M University for all the technical support I have received during my stay aspart of the group. I also extend my gratitude to Texas Instruments Incorporated forinitiating the AAURP (African Analog University Relations Program) initiative throughwhich I was introduced to Analog Integrated Circuits. My thanks also go to TexasInstruments for sponsoring my Masterā€™s education here at Texas A&M University. Tomy fellow colleagues and office mates of the Analog and Mixed Signal Center (AMSC),thank you for all the help and understanding shown to me at all times. Finally, I want toacknowledge my parents, siblings and all family members who have in numerous wayssupported and encouraged me throughout my pursuit of Electrical Engineering as aprofession. Thank you all.

vTABLE OF CONTENTSPageABSTRACT .iiiACKNOWLEDGEMENTS . ivTABLE OF CONTENTS . vLIST OF FIGURES . viiLIST OF TABLES . ix1. INTRODUCTION . 1A. Amplifiers, Operational Amplifiers and Instrumentation Amplifiers . 2B. Applications of Instrumentation Amplifiers . 4C. Classes of Instrumentation Amplifiers . 122. CMRR OF INSTRUMENTATION AMPLIFIERS . 16A. Definition of CMRR . 17B. CMRR Case Study. 18C. Capacitive vs. Resistive Mismatch of RC Feedback Amplifiers . 23D. Previously Published Works on Biopotential Amplifiers . 283. PROPOSED INSTRUMENTATION AMPLIFIERS . 33A. Dynamic Element Matching . 33B. Concept of Proposed INAs . 34C. Dynamically Matched RC Feedback Fully Differential INA . 40D. Dynamically Matched RC Feedback FBFS INA . 41E. System Level Design . 43F. Transistor Level Design . 504. LAYOUT STRATEGY AND SIMULATION RESULTS . 63A. Layout . 63B. Results ā€“ Dynamically Matched Fully Differential INA . 67C. Results ā€“ Dynamically Matched FBFS INA . 735. SUMMARY AND CONCLUSION . 79

viPageREFERENCES . 80VITA . 82

viiLIST OF FIGURESFIGUREPage1-1Block Diagram of an Amplifier and Operational Amplifier . 21-2Applications of Instrumentation Amplifiers . 41-3Amplitude and Frequency Characteristics of Some Bipotentials . 61-4Electrical Model of Skin-Electrode Interface . 71-5Electrostatic Interference to Human Body . 101-6Three Opamp Instrumentation Amplifier . 121-7Concept of Current Balancing INA . 141-8More Accurate Representation of Current Balancing INA . 152-1Block Diagram of Single Ended and Fully Differential Amplifiers . 162-2Opamp with General Impedance Feedback Configuration . 182-3Common Mode Gain vs. Percentage Mismatch in Y1 . . .212-4Common Mode Gain vs. Percentage Mismatch in Y4 . . .212-5RC Feedback Amplifier Used for EKG Signal Acquisition . 222-6Common Mode Gain vs. Frequency of an RC Feedback Amplifier . 252-7Common Mode Gain vs. Frequency for Varying Capacitor Mismatch . 262-8Biopotential Amplifier with MOS-Bipolar Pseudo Resistor Element. .282-9Two Stage INA with Fully Differential Outputs . 302-10Concept of ACCIA . 312-11Implementation of ACCIA . . . . .323-1Simple Voltage Divider Using Resistors . 333-2Fully Differential Version of INA . 353-3Fully Balanced Fully Symmetric Version of INA . 353-4Swapping of Capacitors to Reverse Polarity of Mismatch . 373-5Emulating the Effect of Swapping Capacitors Using ON/ OFF Switches. 383-6Fully Differential Version of Dynamically Matched INA . 40

viiiFIGUREPage3-7Dynamically Matched FBFS INA . 413-8Test for FBFS Amplifier . 423-9Inverting Opamp Configuration . . .463-10INA for Noise Analysis . . . .483-11Transistor Level Schematic Diagram of Fully Differential Opamp . 503-12Differential Frequency Response of Designed Opamp . 533-13Effect of Transistor M3 Sizing on Input Referred Noise . 553-14Common Mode Feedback Circuit for Opamp . 573-15Transistor Level Schematic Diagram of Single Ended Opamp . 583-16Two-phase Non-overlapping Clock Generator . 593-17Outputs of Non-overlapping Clock Generator . 603-18Snapshot of Non-overlapping Region of Clocks . 603-19Incremental Resistance of Pseudo-resistor Element . 614-1Top Level Layout of Fully Differential INA . 634-2Top Level Layout of Dynamically Matched Fully Balanced FBFS INA . 644-3Top Level Floor Plan of Proposed INAs in Die. 654-4Complete Layout of INAs in Silicon Die . 664-5Transient Simulation of Fully Differential INA . 674-6Differential Mode Frequency Response of Dynamically Matched FullyDifferential INA . 684-7Common Mode Frequency Response . 694-8CMRR of Dynamically Matched Fully Differential INA . 704-9Output and Input Referred Noise of Dynamically MatchedFully Differential INA . 714-10Transient Simulation of Dynamically Matched FBFS INA. 734-11Magnitude and Phase of Dynamically Matched FBFS INA . 744-12Common Mode Gain of Dynamically Matched FBFS INA . 754-13CMRR of Dynamically Matched FBFS INA . 764-14Output and Input Referred Noise of Dynamically Matched FBFS INA. 77

ixLIST OF TABLESTABLEPage1-1Properties of INAs vs. Opamps . 31-2Electrical Characteristics of Commonly Used Electrodes . 81-3Typical Skin Impedance Parameters . 81-4General Requirements of an EKG Amplifier . 112-1Typical Requirements of an EKG Signal Amplifier . 232-2Typical Resistor and Capacitor Values for an EKG Amplifier . 242-3Common Mode Gain of RC Feedback Biopotential Amplifier at 50Hz . 273-1Amplitude and Frequency Characteristics of EKG . 433-2Target Specifications of EKG Signal Amplifier . 443-3Final Component Values for Proposed INAs . 453-4Final Opamp Target Specifications . . .493-5Transistor Sizes for Fully Differential Opamp . 563-6Transistor Sizes for CMFB Circuit . 573-7Transistor Sizes for Single Ended Opamps in FBFS INA . 583-8Aspect Ratios of Static CMOS Gates . 593-9Aspect Ratios of Pseudo Resistor Element . 623-10Sizing of Switches. 624-1Summary of Results - Dynamically Matched Fully Differential INA . 724-2Summary of Results - Dynamically Matched FBFS INA . 785-1Comparison of Results with Other Published Works . 79

11. INTRODUCTIONThe importance of bio-potential signals to physicians for diagnosing medical conditionsand also general in-patient/ out patient monitoring cannot be overestimated.Electrocardiogram (EKG) signals - that is the bio-potential signal that results frominternal electrochemical processes within the heart ā€“ can be used to monitor a patientsā€™health condition.Electroencephalogram (EEG) and EMG (Electromyogram)respectively are electrical signals resulting from the human brain activity and fromcontraction/ relaxation of body muscles.Traditionally, these signals are acquired using electrodes and amplified usinginstrumentation amplifiers. Acquisition of these signals is done differentially while anycommon mode component of the bio-potential is rejected. This is very essential becausethe required bio-potentials are typically weak signals with low voltage levels where asthe likely common mode signals that are coupled with the bio-potentials are much largerin amplitude. For instance, in EKG acquisition, signal amplitudes are typically in microvolt range with maximum values about 0.5mV. A 60Hz interference signal from thesupply mains is typically coupled to the differential electrodes and thus appears as acommon signal which is much larger in voltage compared to the desired EKG signal.This signal is referred to as a common mode signal and has to be rejected where as thedifferential EKG signals is acquired.The ability of an instrumentation amplifier to amplify required differential signals whilerejecting unwanted common mode signals is quantified by its Common Mode RejectionRatio (CMRR). Instrumentation amplifier properties vary depending on its topology andapplication.This thesis follows the style of the IEEE Journal of Solid-State Circuits.

2The most common instrumentation amplifier is the 3 - Opamp instrumentation amplifier.This topology though is not suitable for portable bio-potential signal monitoring since itdemands high power consumption and has very poor CMRR. The poor CMRR of the 3Opamp IA is due to the use of passive components in its feedback network. The CMRRdepends on the mismatch of these passive components and degrades very quickly withslight percentage mismatch in these components.Various Instrumentation amplifiers have been proposed for purposes of bio-potentialsignal acquisition targeting low power, low noise and high CMRR specifications. SingleOpamp topologies such as [1] have the advantage of lower power consumption howeverthe problem of poor CMRR is not addressed. Current feedback topologies as in [2] and[3] have better CMRR however the inaccuracy of the gain of such topologies makesdesign of these amplifiers a little complex.Figure 1-1 Block Diagram of an Amplifier and Operational AmplifierA. Amplifiers, Operational Amplifiers and Instrumentation AmplifiersThe operational amplifier (opamp) is one of the most common circuits used in analogelectronic circuit design. Its uses are very wide and opamps are found in sorts ofapplications from power management systems to RF circuits and data converters. As an

3ideal black box, the opamps magnifies the voltage difference between its inputs byseveral orders. In more specific terms, the ideal opamp has infinite gain, with infiniteinput impedance and zero output impedance. These properties are desirable in allapplications where opamps are used.An amplifier is somewhat of a loose term implying any system or block that produces anoutput quantity that is a scaled version of its input. The input could be a single endedsignal or the difference of two signals. More commonly, the output of an amplifier is ascaled current or voltage. Current and voltage amplifiers can be built using opamps innegative feedback configurations or using entirely different circuit topologies. Figure 1-1shows the block diagrams of amplifiers and operational amplifiers.Table 1-1 Properties of INAs vs. OpampsPropertiesOpampINAVery LargeFiniteGain AccuracyHighVery HighCMRRHighVery High NoiseLowVery Low GainInstrumentation Amplifiers (INAs) are special amplifiers designed where long termaccuracy and stability of the amplifier is desired. They are difference amplifiers in thatthey have two inputs, the difference of which is amplified to produce the desired output.Most instrumentation amplifiers will have at least one opamp and some negativefeedback network to produce the desired fixed gain. However, it should be noted thatthere are a few open loop INA topologies as well. INAs typically have very good

4common mode rejection ratio (CMRR) and high input impedances. Table 1-1 shows thegeneral properties of opamps versus instrumentation amplifiers.B. Applications of Instrumentation AmplifiersThe characteristics of instrumentation amplifiers mentioned in the previous section makethem very suitable for Measurement and Test applications. Besides that they are used ina host of sensor applications such as temperature and pressure sensing. INAs are alsoused in biomedical fields. A typical example of such use is in the front end ofbiopotential acquisition systems. Figure 1-2 shows four major application areas ofinstrumentation amplifiers.Figure 1-2 Applications of Instrumentation AmplifiersFor this research effort, we focus on using instrumentation amplifiers for acquiringbiopotential signals. Such instrumentation amplifiers are also known as biopotentialsignal amplifiers.

51. Biopotential Signal AcquisitionBiopotentials are very important to physicians in modern medical practice. These areelectrical signals generated as a result of electro-chemical processes that occur within thehuman body. The kind of body cells involved in these electro-chemical processesdetermine what biopotential signal is generated and its possible use to physicians. Themost common biopotentials are electroencephalogram (EEG), electrocardiogram (EKG)and electromyogram (EMG). Electroencephalogram (EEG) is generated as a result ofneuron activity within the brain, EMG due to electrical activity of skeletal muscles andEKG as a result of electrical impulses that are generated due to the pumping activity ofthe heart.The above mentioned biopotentials are generated due to combination of action potentialsfrom several cells associated with the tissue/ organ [4]. Each action potential is a cycleof potential changes across the cell membrane. During a cells inactive state, its exhibits apotential referred to as a resting potential. In this state, the membrane of the cell is morepermeable to K than Na . As such, K has higher concentration within the cell in theirinactive state. This diffusion gradient across the cell membrane causes K ions to slowlymove across the membrane to the exterior, making the interior more negative withrespect to the exterior of the cell membrane, and thus building an electric f

2-5 RC Feedback Amplifier Used for EKG Signal Acquisition . 22 2-6 Common Mode Gain vs. Frequency of an RC Feedback Amplifier . 25 2-7 Common Mode Gain vs. Frequency . have the advantage of lower power consumption however the problem of poor CMRR is not addressed. Current feedback topologies as in [2] and

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