Low-power front-ends for capacitive three-axis accelerometers

This thesis consists of six publications and an overview of the research topic. The overview concentrates on background information of the capacitive accelerometers and front-ends. The publications focus on two low-power front-ends that were implemented for capacitive three-axis accelerometers and their operation as a part of an interface. The switched-capacitor front-ends that were implemented are based on the charge-balancing structures, namely a self-balancing bridge and a ΔΣ front-end, which convert the capacitive acceleration information to analog and digital signals, respectively. Both structures operate mechanically in open-loop configuration and are capable of reducing the effects of the electrostatic forces and displacement-to-capacitance conversion. According to the performance comparison presented in this thesis, both interfaces, which were implemented around the front-ends, exhibit competitive performance when compared to the commercial products of the day

A Real-Time Thermal Monitoring System Intended for Embedded Sensors Interfaces

RÉSUMÉ: This paper proposes a real-time thermal monitoring method using embedded integrated sensor interfaces dedicated to industrial integrated system applications. Industrial sensor interfaces are complex systems that involve analog and mixed signals, where several parameters can influence their performance. These include the presence of heat sources near sensitive integrated circuits, and various heat transfer phenomena need to be considered. This creates a need for real-time thermal monitoring and management. Indeed, the control of transient temperature gradients or temperature differential variations as well as the prediction of possible induced thermal shocks and stress at early design phases of advanced integrated circuits and systems are essential. This paper addresses the growing requirements of microelectronics applications in several areas that experience fast variations in high-power density and thermal gradient differences caused by the implementation of different systems on the same chip, such as the new-generation 5G circuits. To mitigate adverse thermal effects, a real-time prediction algorithm is proposed and validated using the MCUXpresso tool applied to a Freescale embedded sensor board to monitor and predict its temperature profile in real time by programming the embedded sensor into the FRDM-KL26Z board. Based on discrete temperature measurements, the embedded system is used to predict, in advance, overheating situations in the embedded integrated circuit (IC). These results confirm the peak detection capability of the proposed algorithm that satisfactorily predicts thermal peaks in the FRDM-KL26Z board as modeled with a finite element thermal analysis tool (the Numerical Integrated elements for System Analysis (NISA) tool), to gauge the level of local thermomechanical stresses that may be induced. In this paper, the FPGA implementation and comparison measurements are also presented. This work provides a solution to the thermal stresses and local system overheating that have been a major concern for integrated sensor interface designers when designing integrated circuits in various high-performance technologies or harsh environment

Integrated reference circuits for low-power capacitive sensor interfaces

This thesis consists of nine publications and an overview of the research topic, which also summarizes the work. The research described in this thesis concentrates on the design of low-power sensor interfaces for capacitive 3-axis micro-accelerometers. The primary goal throughout the thesis is to optimize power dissipation. Because the author made the main contribution to the design of the reference and power management circuits required, the overview part is dominated by the following research topics: current, voltage, and temperature references, frequency references, and voltage regulators. After an introduction to capacitive micro-accelerometers, the work describes the typical integrated readout electronics of a capacitive sensor on the functional level. The readout electronics can be divided into four different functional parts, namely the sensor readout itself, signal post-processing, references, and power management. Before the focus is shifted to the references and further to power management, different ways to realize the sensor readout are briefly discussed. Both current and voltage references are required in most analog and mixed-signal systems. A bandgap voltage reference, which inherently uses at least one current reference, is practical for the generation of an accurate reference voltage. Very similar circuit techniques can be exploited when implementing a temperature reference, the need for which in the sensor readout may be justified by the temperature compensation, for example. The work introduces non-linear frequency references, namely ring and relaxation oscillators, which are very suitable for the generation of the relatively low-frequency clock signals typically needed in the sensor interfaces. Such oscillators suffer from poor jitter and phase noise performance, the quantities of which also deserve discussion in this thesis. Finally, the regulation of the supply voltage using linear regulators is considered. In addition to extending the battery life by providing a low quiescent current, the regulator must be able to supply very low load currents and operate without off-chip capacitors

Sub-Femto-Farad Resolution Electronic Interfaces for Integrated Capacitive Sensors: A Review

Capacitance detection is a universal transduction mechanism used in a wide variety of sensors and applications. It requires an electronic front-end converting the capacitance variation into another more convenient physical variable, ultimately determining the performance of the whole sensor. In this paper we present a comprehensive review of the different signal conditioning front-end topologies targeted in particular at sub-femtofarad resolution. Main design equations and analysis of the limits due to noise are reported in order to provide the designer with guidelines for choosing the most suitable topology according to the main design specifications, namely energy consumption, area occupation, measuring time and resolution. A data-driven comparison of the different solutions in literature is also carried out revealing that resolution, measuring time, area occupation and energy/conversion lower than 100 aF, 1 ms 0.1 mm2, and 100 pJ/conv. can be obtained by capacitance to digital topologies, which therefore allow to get the best compromise among all design specifications

System design of a low-power three-axis underdamped MEMS accelerometer with simultaneous electrostatic damping control

Recently, consumer electronics industry has known a spectacular growth that would have not been possible without pushing the integration barrier further and further. Micro Electro Mechanical Systems (MEMS) inertial sensors (e.g. accelerometers, gyroscopes) provide high performance, low power, low die cost solutions and are, nowadays, embedded in most consumer applications. In addition, the sensors fusion has become a new trend and combo sensors are gaining growing popularity since the co-integration of a three-axis MEMS accelerometer and a three-axis MEMS gyroscope provides complete navigation information. The resulting device is an Inertial measurement unit (IMU) able to sense multiple Degrees of Freedom (DoF). Nevertheless, the performances of the accelerometers and the gyroscopes are conditioned by the MEMS cavity pressure: the accelerometer is usually a damped system functioning under an atmospheric pressure while the gyroscope is a highly resonant system. Thus, to conceive a combo sensor, aunique low cavity pressure is required. The integration of both transducers within the same low pressure cavity necessitates a method to control and reduce the ringing phenomena by increasing the damping factor of the MEMS accelerometer. Consequently, the aim of the thesis is the design of an analog front-end interface able to sense and control an underdamped three-axis MEMSaccelerometer. This work proposes a novel closed-loop accelerometer interface achieving low power consumption The design challenge consists in finding a trade-off between the sampling frequency, the settling time and the circuit complexity since the sensor excitation plates are multiplexed between the measurement and the damping phases. In this context, a patenteddamping sequence (simultaneous damping) has been conceived to improve the damping efficiency over the state of the art approach performances (successive damping). To investigate the feasibility of the novel electrostatic damping control architecture, several mathematical models have been developed and the settling time method is used to assess the damping efficiency. Moreover, a new method that uses the multirate signal processing theory and allows the system stability study has been developed. This very method is used to conclude on the loop stability for a certain sampling frequency and loop gain value. Next, a 0.18μm CMOS implementation of the entire accelerometer signal chain is designed and validated

System and circuit design for a capacitive MEMS gyroscope

In this thesis, issues related to the design and implementation of a micro-electro-mechanicalangular velocity sensor are studied. The work focuses on a system basedon a vibratory microgyroscope which operates in the low-pass mode with a moderateresonance gain and with an open-loop configuration of the secondary (sense) resonator.Both the primary (drive) and the secondary resonators are assumed to have a high qualityfactor. Furthermore, the gyroscope employs electrostatic excitation and capacitivedetection. The thesis is divided into three parts. The first part provides the background informationnecessary for the other two parts. The basic properties of a vibratory microgyroscope,together with the most fundamental non-idealities, are described, a shortintroduction to various manufacturing technologies is given, and a brief review of publishedmicrogyroscopes and of commercial microgyroscopes is provided. The second part concentrates on selected aspects of the system-level design of amicro-electro-mechanical angular velocity sensor. In this part, a detailed analysis isprovided of issues related to different non-idealities in the synchronous demodulation,the dynamics of the primary resonator excitation, the compensation of the mechanicalquadrature signal, and the zero-rate output. The use of ΣΔ modulation to improveaccuracy in both primary resonator excitation and the compensation of the mechanicalquadrature signal is studied. The third part concentrates on the design and implementation of the integratedelectronics required by the angular velocity sensor. The focus is primarily on the designof the sensor readout circuitry, comprising: a continuous-time front-end performingthe capacitance-to-voltage (C/V) conversion, filtering, and signal level normalization;a bandpass ΣΔ analog-to-digital converter, and the required digital signal processing(DSP). The other fundamental circuit blocks, which are a phase-locked loop requiredfor clock generation, a high-voltage digital-to-analog converter for the compensationof the mechanical quadrature signal, the necessary charge pumps for the generationof high voltages, an analog phase shifter, and the digital-to-analog converter used togenerate the primary resonator excitation signals, together with other DSP blocks, areintroduced on a more general level. Additionally, alternative ways to perform the C/Vconversion, such as continuous-time front ends either with or without the upconversionof the capacitive signal, various switched-capacitor front ends, and electromechanicalΣΔ modulation, are studied. In the experimental work done for the thesis, a prototype of a micro-electro-mechanicalangular velocity sensor is implemented and characterized. The analog partsof the system are implemented with a 0.7-µm high-voltage CMOS (ComplimentaryMetal-Oxide-Semiconductor) technology. The DSP part is realized with a field-programmablegate array (FPGA) chip. The ±100°/s gyroscope achieves 0.042°/s/√H̅z̅spot noise and a signal-to-noise ratio of 51.6 dB over the 40 Hz bandwidth, with a100°/s input signal. The implemented system demonstrates the use of ΣΔ modulation in both the primaryresonator excitation and the quadrature compensation. Additionally, it demonstratesphase error compensation performed using DSP. With phase error compensation,the effect of several phase delays in the analog circuitry can be eliminated, andthe additional noise caused by clock jitter can be considerably reduced

Integrated interface electronics for capacitive MEMS inertial sensors

This thesis is composed of 13 publications and an overview of the research topic, which also summarizes the work. The research presented in this thesis concentrates on integrated circuits for the realization of interface electronics for capacitive MEMS (micro-electro-mechanical system) inertial sensors, i.e. accelerometers and gyroscopes. The research focuses on circuit techniques for capacitive detection and actuation and on high-voltage and clock generation within the sensor interface. Characteristics of capacitive accelerometers and gyroscopes and the electronic circuits for accessing the capacitive information in open- and closed-loop configurations are introduced in the thesis. One part of the experimental work, an accelerometer, is realized as a continuous-time closed-loop sensor, and is capable of achieving sub-micro-g resolution. The interface electronics is implemented in a 0.7-µm high-voltage technology. It consists of a force feedback loop, clock generation circuits, and a digitizer. Another part of the experimental work, an analog 2-axis gyroscope, is optimized not only for noise, but predominantly for low power consumption and a small chip area. The implementation includes a pseudo-continuous-time sense readout, analog continuous-time drive loop, phase-locked loop (PLL) for clock generation, and high-voltage circuits for electrostatic excitation and high-voltage detection. The interface is implemented in a 0.35-µm high-voltage technology within an active area of 2.5 mm². The gyroscope achieves a spot noise of 0.015 °/s/√H̅z̅ for the x-axis and 0.041 °/s/√H̅z̅ for the y-axis. Coherent demodulation and discrete-time signal processing are often an important part of the sensors and also typical examples that require clock signals. Thus, clock generation within the sensor interfaces is also reviewed. The related experimental work includes two integrated charge pump PLLs, which are optimized for compact realization but also considered with regard to their noise performance. Finally, this thesis discusses fully integrated high-voltage generation, which allows a higher electrostatic force and signal current in capacitive sensors. Open- and closed-loop Dickson charge pumps and high-voltage amplifiers have been realized fully on-chip, with the focus being on optimizing the chip area and on generating precise spurious free high-voltage signals up to 27 V

High Aspect-ratio Biomimetic Hair-like Microstructure Arrays for MEMS Multi-Transducer Platform

Many emerging applications of sensing microsystems in health care, environment, security and transportation systems require improved sensitivity and selectivity, redundancy, robustness, increased dynamic range, as well as small size, low power and low cost. Providing all of these features in a system consisting of one sensor is not practical or possible. Micro electro mechanical microsystems (MEMS) that combine a large sensor array with signal processing circuits could provide these features. To build such multi-transducer microsystems we get inspiration from “hair”, a structure frequently used in nature. Hair is a simple yet elegant structure that offers many attractive features such as large length to cross-sectional area ratio, large exposed surface area, ability to include different sensing materials, and ability to interact with surrounding media in sophisticated ways. In this thesis, we have developed a microfabrication technology to build 3D biomimetic hair structures for MEMS multi-transducer platform. Direct integration with CMOS will enable signal processing of dense arrays of 100s or 1000s of MEMS transducers within a small chip area. We have developed a new device structure that mimics biological hair. It includes a vertical spring, a proof-mass atop the spring, and high aspect-ratio narrow electrostatic gaps to adjacent electrodes for sensing and actuation. Based on this structure, we have developed three generations of 3D high aspect-ratio, small-footprint, low-noise accelerometers. Arrays of both high-sensitivity capacitive and threshold accelerometers are designed and tested, and they demonstrate extended full-scale detection range and frequency bandwidth. The first-generation capacitive hair accelerometer arrays are based on Silicon-on-Glass (SOG) process utilizing 500 µm thick silicon, achieving a highest sensor density of ~100 sensors/mm2 connected in parallel. Minimum capacitive gap is 5 μm with device height of 400 μm and spring length of 300 μm. A custom-designed Bosch deep-reactive-etching (DRIE) process is developed to etch ultra-deep (> 500 µm) ultra-high aspect-ratio (UHAR) features (AR > 40) with straight sidewalls and reduced undercut across a wide range of feature sizes. A two-gap dry-release process is developed for the second-generation capacitive hair accelerometers. Due to the large device height at full wafer thickness of 1 mm and UHAR capacitive transduction gaps at 2 µm that extend > 200 µm, the accelerometer achieves sub-µg resolution (< 1µg/√Hz) and high sensitivity (1pF/g/mm2), having an area smaller than any previous precision accelerometers with similar performance. Each sensor chip consists of devices with various design parameter to cover a wide range. Bonding with metal interlayers at < 400 °C allows direct integration of these devices on top of CMOS circuits. The third-generation digital threshold hair accelerometer takes advantage of large aspect-ratio of the hair structure and UHAR DRIE structures to provide low noise (< 600 ng/√Hz per mm2 footprint proof-mass due to small contact area) and low power threshold acceleration detection. 16-element (4-bit) and 32-element (5-bit) arrays of threshold devices (total chip area being < 1 cm2) with evenly-spaced threshold gap dimensions from 1 µm to 4 µm as well as with hair spring cross-sectional area from 102 µm to 302 µm are designed to suit specific g-ranges from < 100 mg to 50 g. This hair sensor and sensor array technology is suited for forming MEMS transducer arrays with circuits, including high performance IMUs as well as miniaturized detectors and actuators that require high temporal and spatial resolution, analogous to high-density CMOS imagers.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143975/1/yemin_1.pd

SMARAD - Centre of Excellence in Smart Radios and Wireless Research - Activity Report 2008 - 2010

Centre of Excellence in Smart Radios and Wireless Research (SMARAD), originally established with the name Smart and Novel Radios Research Unit, is aiming at world-class research and education in Future radio and antenna systems, Cognitive radio, Millimetre wave and THz techniques, Sensors, and Materials and energy, using its expertise in RF, microwave and millimetre wave engineering, in integrated circuit design for multi-standard radios as well as in wireless communications. SMARAD has the Centre of Excellence in Research status from the Academy of Finland since 2002 (2002-2007 and 2008-2013). Currently SMARAD consists of five research groups from three departments, namely the Department of Radio Science and Engineering, Department of Micro and Nanosciences, and Department of Signal Processing and Acoustics, all within the Aalto University School of Electrical Engineering. The total number of employees within the research unit is about 100 including 8 professors, about 30 senior scientists and about 40 graduate students and several undergraduate students working on their Master thesis. The relevance of SMARAD to the Finnish society is very high considering the high national income from exports of telecommunications and electronics products. The unit conducts basic research but at the same time maintains close co-operation with industry. Novel ideas are applied in design of new communication circuits and platforms, transmission techniques and antenna structures. SMARAD has a well-established network of co-operating partners in industry, research institutes and academia worldwide. It coordinates a few EU projects. The funding sources of SMARAD are diverse including the Academy of Finland, EU, ESA, Tekes, and Finnish and foreign telecommunications and semiconductor industry. As a byproduct of this research SMARAD provides highest-level education and supervision to graduate students in the areas of radio engineering, circuit design and communications through Aalto University and Finnish graduate schools such as Graduate School in Electronics, Telecommunications and Automation (GETA). During years 2008 – 2010, 21 doctor degrees were awarded to the students of SMARAD. In the same period, the SMARAD researchers published 141 refereed journal articles and 333 conference papers

Interface Circuits for Microsensor Integrated Systems

ca. 200 words; this text will present the book in all promotional forms (e.g. flyers). Please describe the book in straightforward and consumer-friendly terms. [Recent advances in sensing technologies, especially those for Microsensor Integrated Systems, have led to several new commercial applications. Among these, low voltage and low power circuit architectures have gained growing attention, being suitable for portable long battery life devices. The aim is to improve the performances of actual interface circuits and systems, both in terms of voltage mode and current mode, in order to overcome the potential problems due to technology scaling and different technology integrations. Related problems, especially those concerning parasitics, lead to a severe interface design attention, especially concerning the analog front-end and novel and smart architecture must be explored and tested, both at simulation and prototype level. Moreover, the growing demand for autonomous systems gets even harder the interface design due to the need of energy-aware cost-effective circuit interfaces integrating, where possible, energy harvesting solutions. The objective of this Special Issue is to explore the potential solutions to overcome actual limitations in sensor interface circuits and systems, especially those for low voltage and low power Microsensor Integrated Systems. The present Special Issue aims to present and highlight the advances and the latest novel and emergent results on this topic, showing best practices, implementations and applications. The Guest Editors invite to submit original research contributions dealing with sensor interfacing related to this specific topic. Additionally, application oriented and review papers are encouraged.