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  • FACULDADE DE ENGENHARIA DA UNIVERSIDADE DO PORTO

    An Instrumentation Amplifier for Myoelectric Signals

    Henrique Rodrigues de Castro Mendes Martins

    PREPARAÇÃO DA DISSERTAÇÃO

    PREPARAÇÃO DA DISSERTAÇÃO

    Advisor: PhD:Vitor Grade Tavares

    February 19, 2013

  • c© Henrique Rodrigues de Castro Mendes Martins, 2013

  • An Instrumentation Amplifier for Myoelectric Signals

    Henrique Rodrigues de Castro Mendes Martins

    PREPARAÇÃO DA DISSERTAÇÃO

    February 19, 2013

  • Resumo

    Nas décadas passadas, o setor da eletrónica evoluiu exponencialmente. No nosso dia a dia a eletrónica está presente em tudo, nos nossos carros, e até em algumas das nossas roupas. A verdade é que os desenvolvimentos nesse setor aumentaram a qualidade de vida do Homem. No caso de algumas doenças, apenas com o auxílio de sistemas eletrónicos é possível realizar diagnósticos e tratamentos apropriados. Problemas do foro muscular e de movimento são uma vasta área na qual a eletrónica teve grande influência na ajuda a lidar e a tratar essas doenças. A eletrónica moderna permite uma melhor visão do que se passa ao nivel do músculo. Com instrumentação de grande precisão é possivel obter o sinal gerado pelas fibras das membranas musculares, o sinal miográfico. Os sinais miográficos são signais muito específicos; eles são formados por variações fisiológicas nas fibras das membranas musculares. A medição e o processamento destes sinais é de grande importância, dado que eles permitem olhar diretamente para o músculo. Isto é uma análise importante que precisa de ser feita para :

    • Ajudar na tomada de decisão antes/após da cirurgia;

    • Permitir a medição do desempenho muscular;

    • Ajudar no processo de reabilitação;

    Estes são apenas alguns exemplos daquilo que é possível atingir investindo na investigação e desenvolvimento de sistemas aplicados a esta área em específico. Com isso em mente, a ne- cessidade de um sistema que meça e processe tais sinais surge. O objetivo desta tese é então de desenvolver uma parte desse sistema, um amplificador de instrumentação de baixo ruído e de baixa potência para ser integrado em chip, usando a tecnologia CMOS de 0.35 µm. Este amplificador deve seguir um conjunto de regras, dado que este tipo de sinais requerem caraterísticas muito es- pecíficas no amplificador e no processamento de sinal, nomeadamente um CMRR(Common-mode rejection ratio) elevado, baixo ruído e baixo consumo de potência, dado que o chip a ser produzido será usado em aplicações de elevada autonomia. É então esperado atingir no fim desta tese um am- plificador genérico para sinais de baixa tensão e frequência:caraterísticas estas que estão presente na maior parte dos sinais biológicos.

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  • Abstract

    In the past decades, the electronics sector has evolved exponentially. In our everyday there is electronics everywhere, in our cars, in our houses and even in some of our clothes. Truth is, the developments in that sector have increased the quality of life of the average Man. In the case of some diseases, only with the aid of electronic systems proper diagnosis and treatments can be made. Muscular and movement related problems are a wide area in which electronics have a great influence in dealing with such diseases. Modern electronics allows us to take a better look at what is going on at the muscle level. With great precision instrumentation we can obtain the signal generated by muscle fiber membranes, the Myographic signal. Myographic signals are a very specific type of signals; they are formed by physiological variations in the state of muscle fibre membranes [1]. The measuring and processing of these signals is of great importance, since they allow looking directly into the muscle [1]. This is an important analysis that needs to be done to:

    • Help in decision making both before/after surgery;

    • Allow measurement of muscular performance;

    • Aid in the rehabilitation process;

    These are just a few examples of what we can achieve investing in the research and develop- ment of electronic systems applied to this specific area. With that in mind, the need for a system that can measure and process such signals arises. The goal of this thesis is to develop a part of that system, an instrumentation amplifier to be integrated on chip, using CMOS 0.35 µm technology. This amplifier must follow a set of rules, since these signals demand very specific characteristics on an amplifier and in the signal processing, such as a good CMRR (Common-mode Rejection Ratio), low noise and low power consumption, as this chip is to be used on high autonomy appli- cations. Thus, it is hoped to achieve in the end of this thesis a generic amplifier for low voltage and low frequency signals: characteristics that are present in most of the biological signals.

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  • Contents

    1 Introduction 1 1.1 Document structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    2 Theoretical Background 3 2.1 Myoelectric signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    2.1.1 What are they? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.2 Obtaining and measuring . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    2.2 Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.1 Thermal Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2.2 Flicker Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.3 Noise in Differential Pairs . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2.3 Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3.1 Basic transistor configurations . . . . . . . . . . . . . . . . . . . . . . . 6 2.3.2 Single Stage Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3.3 Differential amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3.4 Differential pair with load resistors . . . . . . . . . . . . . . . . . . . . 11 2.3.5 Common-mode Response . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.6 CMOS Differential Pair . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.7 Stability and Frequency Compensation . . . . . . . . . . . . . . . . . . 17 2.3.8 Common-mode Feedback . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.9 Class Type of Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    3 Bybliographic Review 25 3.1 A CMOS Fully Balanced Differential Difference Amplifier and Its Applications [2] 25 3.2 A New Architecture for Rail-to-Rail Input Constant-gm CMOS Operational Transcon-

    ductance Amplifiers [3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3 A 1 Volt CMOS Pseudo Differential Amplifier [4] . . . . . . . . . . . . . . . . 29 3.4 Low-Voltage Rail-to-Rail CMOS Differential Difference Amplifier [5] . . . . . 31 3.5 A CMOS Chopper-Stabilized Differential Difference Amplifier for Biomedical

    Integrated Circuits [6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.6 System design of a low noise, low offset instrumentation amplifier with chopper

    stabilization [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    4 Problem Presentation 35 4.1 Preliminary topology for an instrumentation amplifier . . . . . . . . . . . . . . . 35

    5 Methodology and Work Plan 37

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  • List of Figures

    2.1 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Differential pair modelled with noise sources . . . . . . . . . . . . . . . . . . . 5 2.3 Common-source topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4 Common-source with source resistor topology . . . . . . . . . . . . . . . . . . . 7 2.5 Common-source with capacitor topology . . . . . . . . . . . . . . . . . . . . . . 7 2.6 Source-Follower topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.7 Common-gate topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.8 Nmos amplifier with an enhancement load . . . . . . . . . . . . . . . . . . . . . 9 2.9 Nmos amplifier with a Depletion load . . . . . . . . . . . . . . . . . . . . . . . 10 2.10 Nmos amplifier with a Pmos load . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.11 Basic Differential pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.12 Basic Differential pair with common mode input . . . . . . . . . . . . . . . . . 12 2.13 Basic Differential pair with CMOS active load . . . . . . . . . . . . . . . . . . . 13 2.14 Differential pair with cascoding . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.15 Differential pair with current source load . . . . . . . . . . . . . . . . . . . . . . 14 2.16 Differential pair equivalent half circuit . . . . . . . . . . . . . . . . . . . . . . . 15 2.17 Differential pair with current source load . . . . . . . . . . . . . . . . . . . . . . 16 2.18 Differential pair with mirror pole representation . . . . . . . . . . . . . . . . . . 17 2.19 Two-stage operational amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.20 Differential pair with inputs shorted to outputs . . . . . . . . . . . . . . . . . . . 19 2.21 Common-mode feedback with resistive sensing . . . . . . . . . . . . . . . . . . 20 2.22 Common-mode feedback with source followers . .

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