Faculdade de Engenharia da Universidade do Porto Integrated System for a High Resolution MEMS Accelerometer Diogo Filipe de Sousa Teixeira e Melo Project carried-out under the framework of the Mestrado Integrado em Engenharia Electrotécnica e de Computadores Major Telecomunicações Supervisor: Prof. Dr. Vítor Grade Tavares Supervisor: Prof. Dr. Luís Rocha February, 2010 Abstract This report discusses a MEMS Accelerometer Integrated System: the MEMS Capacitive Accelerometer, a fully differential amplifier and the Analogue-to Digital Converter. The main goal of the project is to build an Analogue to Digital Converter for a MEMS inertial sensor. This project is being developed in the year 2009/10 in the Faculty of Engineering of the University Of Porto, as the final Dissertation for the Integrated Master in Electrical and Computers Engineering. 2 Table of contents Abstract ............................................................................................. 2 Table of contents.................................................................................. 3 List of Figures ...................................................................................... 4 List of Tables ....................................................................................... 5 Introduction ........................................................................................ 6 Micromachined Inertial Sensors ................................................................ 7 Introduction .................................................................................................... 7 Micromachined Accelerometer.............................................................................. 8 Capacitive Accelerometer .................................................................................. 10 Electrical Model ........................................................................................... 11 Fully Differential Charge Amplifier for Mems Accelerometer [5] ......................13 Introduction ................................................................................................... 13 Fully Differential Amplifier ................................................................................. 14 Charge Amplifier ............................................................................................. 17 Conclusion..................................................................................................... 19 Analog-Digital Converter ........................................................................20 Introduction ................................................................................................... 20 Sigma-Delta Analog-Digital Converter (ΣΔ ADC) ...................................................... 21 Structure of the ΣΔ ADC ............................................................................... 22 ΣΔ ADC Circuits already developed for MEMS Accelerometer.................................... 25 Future Work .......................................................................................27 References.........................................................................................28 3 List of Figures Figure 1 - Block Diagram of the System ..................................................................6 Figure 2 - Mechanical model of an Accelerator [2].....................................................9 Figure 3 - (a) Schematic of a differential Capacitive Accelerometer. (b) Lumped elements model of the accelerometer [3] .................................................................. 10 Figure 4 - Electric model of the MEMS Capacitive Accelerometer ................................. 11 Figure 5 - Differences between Fully Differential Amplifier and a Standard Operational Amplifier.............................................................................................. 14 Figure 6 - Fully Differential Charge Amplifier [5] .................................................... 15 Figure 7 - Frequency response........................................................................... 16 Figure 8 - Rejection Rates................................................................................ 17 Figure 9 - Charge Amplifier schematic and MEMS Accelerometer electric equivalent circuit . 18 Figure 10 - Charge amplifier Results: A)Capacity Variation dC. B)Output of the Differential Amplifier (vp-vn). C) Demodulated Output (outp-outn) ...................................... 19 Figure 11 - Bandwidth Resolution Tradeoffs [6] ...................................................... 21 Figure 12 - First order Sigma Delta ADC................................................................ 23 Figure 13 - Undersampled Signal Spectrum [8] ....................................................... 23 Figure 14 - Oversampled Signal Spectrum [8] ......................................................... 24 4 List of Tables Table 1 - Use of Micromachined Accelerometers [1]................................................... 8 Table 2 - Specification of a few Inertial Accelerometer Systems[2] ................................ 9 Table 3 - Frequency Response Results ..................................................................16 Table 4 - Values of the Rejection Rates ................................................................17 Table 5 - Comparison between various ADC techniques. N stands for resolution and for clock frequency [7] .............................................................................22 5 Introduction Nowadays, we have more and more present in our lives inertial sensors: pressure, optical and others. They are in most of our normal professional tools or in our gadgets. MEMS (Microelectromechanical Systems) technologies are present in most of these sensors. One of the most used, still with an enormous development margin, are the MEMS accelerometers. Accelerometers are used in automotive applications, such as airbags and stability systems; in biomedical systems, like activity monitoring; in our consumer electronics like mobile phones and camcorders or even gamepads. The intense use of these products demands a development of the technology and a huge decrease of sensor costs, as they are being integrated in our devices, either to improve performance or to enable features that they were not yet capable of doing. The Integrated system that we are developing consists of 3 major parts. The sensor itself, a fully differential amplifier that amplifies the sensors readings, and an A/D Converter, composed by a Sigma Delta Modulator and a Digital Decimator to convert the analogue signal to Digital, so that it can be processed digitally. The goal of this project consists on the development of the Analogue to Digital converter that will convert the signal from the sensor, already amplified, and make it ready to be used by a digital machine. In the next figure is presented a block diagram of the system that is being developed at the time. Figure 1 - Block Diagram of the System In this work we will discuss all the integrated system, the sensor, the amplifier and the ADC. It will be explained why the use of a Sigma-Delta modulator, instead of other types of modulations, as well as its structure. 6 Micromachined Inertial Sensors Introduction Micromachined inertial sensors are a very important group of silicon-based sensors. They can measure linear accelerations (Accelerometers) or angular accelerations (Gyroscope). The constant development of the technology is improving the resolution of the systems, and reducing the fabrication costs, enabling a wider use of this technology. This reduction of costs is leading to an integration of this technology in many electronic types of equipment, to improve its performance or features. The Micromachined inertial sensors technology is not exclusive of powerful companies that develop state of the art and costly electronic for use in military or aerospace applications. This technology is mostly used in automotive applications, where they are integrated in safety systems like airbags, stability systems or suspension. The constant feature size reduction, associated with a less or more effective cost, opens a wide range of applications. Biomedical use, consumer application, industrial application, navigation systems, seismography and earthquake prevention are a few areas that the sensors are now being used. In this project we will discuss in particular the micromachined accelerometers. In table 1 we have some of the applications where micromachined accelerometers are used. 7 Table 1 - Use of Micromachined Accelerometers [1] Measurement Application Front and side airbag crash sensing Electrically controlled car suspension Safety belt pretensioning Acceleration Vehicle and traction control systems Inertial measurement, object positioning, and navigation Human activity for pacemaker control Engine management Condition-based maintenance of engines and machinery Vibration Security devices Shock and impact monitoring Monitoring of seismic activity Inclinometers and tilt sensing Vehicle stability and roll Headlight leveling Angles of inclination Computer peripherals (e.g., joystick, head mounted displays) Handwriting recognition (e.g., SmartQuill from British Telecom plc) Bridges, ramps, and construction Micromachined Accelerometer All accelerometers have a basic mechanical equivalent model. This model is based on a proof mass that is suspended by a spring and a Damper that are attached to a fixed frame. The next figure shows that