A low noise, sub-1ppm/oC piecewise second-order curvature compensated bandgap reference for high resolution ADC

학위논문 (석사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 김수환.본 논문에서는 고해상도 analog to digital converter를 위한 저 잡음, 고 정밀 bandgap voltage reference를 제안한다. reference 회로의 성능 중 가장 중요한 것들은 바로 낮은 온도 계수(temperature coefficient)와 저주파 대역의 전기적 잡음이다. 제안된 Bandgap reference 회로는 위 두가지 요소를 개선 하였다. 먼저 낮은 온도 계수를 성취하기 위해서는 BJT Emitter-Base전압의 비선형적 온도의존성을 보상해주어야 하고, bandgap core을 이루는 Error amplifier의 DC offset을 제거해야 하며, 마지막으로 process variation에의한 추가적인 온도 의존성을 상쇄시켜야 한다. 제안된 bandgap reference는 여러가지 회로 기술들을 활용해 위 요소들을 보상하였다. BJT Emitter-Base전압의 비선형적 온도 의존성을 온도에 대해 2차 의존성을 갖는 compensation 전류를 생성하고 bandgap core에 흘려주어 제거하였다. Compensation 전류는 크게 current subtraction 동작과 current squaring 동작을 통해 생성되는데, 위 동작은 모두 process variation에 둔감하다. 두 번 째로 process variation에 의한 온도 특성의 변화를 보상해 주기 위해 trimming resistor를 사용하였다. 마지막으로 error amplifier에 chopping을 적용하여 Error amplifier DC offset을 약화시켰다. Bandgap reference의 저 주파수 전기적 잡음의 근원은 대부분 Error amplifier이므로 chopping 동작을 통해 저주파대역의 전기적 잡음 또한 제거된다. Chopping 동작을 통해 생겨난 리플 과, 고주파 대역으로 변조된 저주파 대역의 전기적 잡음은 RC filter를 통해 제거하였다. 제안된 bandgap reference는 스탠다드 0.13um CMOS 공정의 3.3V 전원 소자로 설계하였으며 레이아웃 사이즈는 0.0534mm2이다. Post layout simulation 결과 제안된 bandgap reference의 -40°C부터 125°C 사이의 온도 계수는 약 0.64ppm/°C이다. 0.1Hz부터 10Hz사이의 integrated noise는 약 2.7uVrms이다. 제안된 bandgap reference는 상온에서 약 44uA의 전류를 소모한다.In this thesis a low noise and high precision bandgap reference is presented. One of the most important characteristics of reference circuit for analog to digital converter with high resolution is low temperature drift and low noise. The proposed bandgap reference improves these two characteristics. To achieve low temperature coefficient(TC), non-linear temperature dependence of emitter-base voltage of bipolar transistor should be compensated. Also, degradation of TC due to dc offset of the error amplifier and process variation is another concern. The proposed bandgap reference compensates these factors by utilizing various circuit technique. Because non-linear temperature dependence of bipolar transistor has a concave shape with temperature, second order curvature compensation current is generated by using current subtraction circuit and current squaring circuit and injected into bandgap core. The current subtraction and squaring operation is tolerant to process variation. To achieve low temperature coefficient regardless of process variation, PTAT trimming is utilized to compensate added linear temperature dependence. At last, to remove dc offset of the error amplifier, chopping technique is applied to the error amplifier. Ripple and up-modulated low frequency caused by chopping operation is removed through RC-filter. The proposed bandgap reference is designed in 0.13um standard CMOS process. Layout size of the bandgap reference is 0.0534mm2. Post layout simulation shows that TC of the bandgap reference from -40°C to 125 °C is 0.64ppm/°C. In addition, integrated noise from 0.1Hz to 10Hz is about 2.7uVrms. The proposed bandgap reference consumes 44uA at room temperature제 1 장 서론 1 제 1 절 연구의 배경 1 제 2 절 기본적인 bandgap reference의 동작 원리. 4 1. bipolar 트랜지스터의 온도 특성 4 2. 기본적인 bandgap voltage reference의 동작 원리 7 3. 기본적인 bandgap current reference의 동작 원리 9 제 2 장 기본적인 bandgap reference의 성능적 한계 12 제 1 절 비선형적 온도 의존성 12 1. error amplifier dc offset 14 2. emitter-base 전압의 비선형적 온도 의존성 16 3. bipolar 트랜지스터 전류 이득에 의한 비선형적 온도 의존성 17 4. bipolar 트랜지스터의 베이스 저항에 의한 비선형적 온도 의존성 19 제 2 절 Bandgap reference의 전기적 잡음. 20 제 3 장 제안하는 저 잡음 고 정밀 bandgap voltage reference 22 제 1절 제안된 bandgap reference의 전체 구조 22 1. PTAT전류 생성 회로 23 2. reference 전류 생성 회로 24 3. bandgap core 25 제 2절 Curvature compensation technique 25 제 3절 Noise reduction technique 30 제 4절 Resistor trimming 32 제 5절 주요 성분 파라 미터 테이블 33 제 4 장 Layout 및 모의 실험 결과 34 제 1 절 Layout 34 제 2 절 모의 실험 결과 35 제 5 장 결론 40 제 6 장 부록 current squaring 회로의 동작 원리. 41 참고문헌 43 Abstract 43Maste

Low temperature coefficient bandgap voltage reference generator

The maximum achievable performance of almost all mixed-signal and radio frequency systems is dependent on the accuracy of voltage references. The bandgap voltage of silicon at zero Kelvin, VGO is a physical constant with unit Volts. It is independent of process, supply voltage and temperature variations. This work proposes a strategy for extracting VGO and expressing it at the output of a voltage reference circuit. The concept is implemented in UMC 65nm process and the simulation results indicate that the circuit design can achieve very low temperature coefficients (\u3c1ppm/°C). The proposed concept is validated using measurements and the associated constraints are carefully investigated. The measured output voltage reference of the two tested units record a temperature coefficient of 3.4ppm/°C and 4.57ppm/°C across the industrial temperature range (-40°C to 85°C)

Robust Design With Increasing Device Variability In Sub-Micron Cmos And Beyond: A Bottom-Up Framework

My Ph.D. research develops a tiered systematic framework for designing process-independent and variability-tolerant integrated circuits. This bottom-up approach starts from designing self-compensated circuits as accurate building blocks, and moves up to sub-systems with negative feedback loop and full system-level calibration. a. Design methodology for self-compensated circuits My collaborators and I proposed a novel design methodology that offers designers intuitive insights to create new topologies that are self-compensated and intrinsically process-independent without external reference. It is the first systematic approaches to create "correct-by-design" low variation circuits, and can scale beyond sub-micron CMOS nodes and extend to emerging non-silicon nano-devices. We demonstrated this methodology with an addition-based current source in both 180nm and 90nm CMOS that has 2.5x improved process variation and 6.7x improved temperature sensitivity, and a GHz ring oscillator (RO) in 90nm CMOS with 65% reduction in frequency variation and 85ppm/oC temperature sensitivity. Compared to previous designs, our RO exhibits the lowest temperature sensitivity and process variation, while consuming the least amount of power in the GHz range. Another self-compensated low noise amplifiers (LNA) we designed also exhibits 3.5x improvement in both process and temperature variation and enhanced supply voltage regulation. As part of the efforts to improve the accuracy of the building blocks, I also demonstrated experimentally that due to "diversification effect", the upper bound of circuit accuracy can be better than the minimum tolerance of on-chip devices (MOSFET, R, C, and L), which allows circuit designers to achieve better accuracy with less chip area and power consumption. b. Negative feedback loop based sub-system I explored the feasibility of using high-accuracy DC blocks as low-variation "rulers-on-chip" to regulate high-speed high-variation blocks (e.g. GHz oscillators). In this way, the trade-off between speed (which can be translated to power) and variation can be effectively de-coupled. I demonstrated this proposed structure in an integrated GHz ring oscillators that achieve 2.6% frequency accuracy and 5x improved temperature sensitivity in 90nm CMOS. c. Power-efficient system-level calibration To enable full system-level calibration and further reduce power consumption in active feedback loops, I implemented a successive-approximation-based calibration scheme in a tunable GHz VCO for low power impulse radio in 65nm CMOS. Events such as power-up and temperature drifts are monitored by the circuits and used to trigger the need-based frequency calibration. With my proposed scheme and circuitry, the calibration can be performed under 135pJ and the oscillator can operate between 0.8 and 2GHz at merely 40[MICRO SIGN]W, which is ideal for extremely power-and-cost constraint applications such as implantable biomedical device and wireless sensor networks

Design and verification approaches for reliability and functional safety of analog integrated circuits

New breakthroughs in semiconductor design have enabled a rapid integration of semiconductor chips into systems that affect all aspects of the society. Examples of emerging systems include spacecraft, Internet of Things (IoT), intelligent automotive, and bio-implantable devices. Many of these systems are mission-critical or safety-critical, meaning that failure or malfunction may lead to severe economical losses, environmental damages or risks to human lives. In addition to performances improvement, the reliability and functional safety of the underlying integrated circuit (IC) have attracted more and more attention and have posed grand challenges for semiconductor industries. This dissertation introduces an approach for high performance voltage reference design and investigates two subjects that improve the reliability and functional safety of analog circuits. The first part of this dissertation studies design strategies of a low temperature-coefficient voltage reference generator, which is a fundamental building block and determines the maximum achievable performance of almost all analog/mixed-signal systems. The proposed method is targeted at extracting a physical quantity of the silicon bandgap, and has the potential of designing a voltage reference that has qualitatively better temperature dependence. An implementation of the proposed approach in GlobalFoundries 130nm process shows that the design can achieve temperature coefficients as low as 0.7ppm/°C over a temperature range of -40°C to 125°C over all process corners. The second part of this dissertation focuses on multi-states verification of analog circuits. The multiple DC equilibrium points or multi-states problem traces back to IC design. It is a well-known problem in many basic self-stabilized analog circuits because of the existence of positive feedback loops (PFLs). This work proposes systematic and automatic approaches for locating all PFLs to identify circuits vulnerable to undesired equilibrium states and methods for automatically identifying break-points to break all PFLs in the vulnerable circuits. The proposed methods make it possible to efficiently identify a circuit’s vulnerability to undesired operating points by considering circuit topology only, without the need for finding all possible solutions to a set of simultaneous nonlinear equations which is an open problem with no solution. Moreover, the automatic break-points identification enables easy use of homotopy analysis to guarantee absence of undesired states. The third part of this dissertation focuses on fault coverage simulation of analog circuits. This work describe two methods, one is to reduce the fault coverage estimation time and the other is to improve the fault coverage for analog circuits. The first method incorporates graph theory and sensitivity analysis and leads to dramatic reduction in fault coverage simulation time by 10’s of times for a moderately sized analog circuit. The second method discusses a systematic test-points selection technique to improve the analog fault coverage with simple DC tests and a concurrent sampling technique for monitoring these points. This work could be applied to manufacturing testing or for real-time fault detection

Portable spectroscopy system for ultra-sensitive, real-time measurement of breath ethane

This thesis describes the development, characterisation and application of a portable spectroscopy system for ultra-sensitive, real-time detection of breath ethane. In healthcare, breath ethane is a widely accepted marker of free radical-induced cell damage and may be used to indicate changes in oxidative stress. The aim was to deliver a compact instrument capable of long-term, on-site use in a clinical environment, while also retaining the high performance previously achieved by lab-based systems at the University of Glasgow. The newly developed instrument has a sensitivity of 70 parts per trillion with a 1 Hz sampling rate. The system incorporates a cryogenicallycooled lead-salt laser and uses a second derivative wavelength modulation detection scheme. A thermally-managed closed-loop refrigeration system has eliminated the need for liquid coolants. The instrument has been field-tested to ensure target performance is sustained in a range of environments, both indoor and outdoor. It has since been used in a number of pilot clinical studies, both off-site and on-site, in which breath ethane was monitored as a marker of oxidative stress. The three main clinical areas investigated were dialysis, radiotherapy and intensive care. In the intensive care study, the instrument was modified to enable automatic breath sampling of inspired and expired gases of ventilated patients. This technique proved highly successful and the instrument then remained at the Southern General hospital, where it continued to be used as part of a wider study into breath ethane in intensive care patients. The use of the new spectroscopy system has enabled ultra-sensitive, rapid analysis of a large number of breath samples. The use of the new instrument, in particular for continual breath monitoring, has enabled the detection of short-lived fluctuations in breath ethane, yielding some interesting findings in a number of pilot clinical studies. Our results suggest that breath ethane may be used as an indicator of dynamic changes in oxidative stress. Further studies will be required to determine if such monitoring is of clinical benefit. Chapter 1 gives a general introduction to spectroscopy and some background to our project. A number of spectroscopic techniques and laser sources are discussed, along with a review of previous work in ethane detection. In chapter 2 some background theory of molecular spectroscopy is given, with a more detailed discussion of the wavelength modulation technique. Chapter 3 describes in detail the development of the portable spectroscopy system. The achieved performance and factors contributing to this performance are discussed in chapter 4. The field test of the instrument is reported on in chapter 5. In chapter 6 the application of the technology to breath analysis and the current challenges in this field are discussed. Example breath ethane measurements for healthy controls are provided. The clinical pilot studies conducted using the new system in areas of dialysis, intensive care and radiotherapy are discussed in chapters 7, 8, and 9 respectively. Chapter 10 contains the thesis summary and conclusions, with suggestions for future work

Improving the Efficiency, Stability, and Flexibility of Perovskite Solar Cells

Solar cells are a renewable, clean alternative to fossil fuels. Silicon solar cells are used in about 90% of commercial modules. Despite having high efficiency and stability, silicon solar cells are costly and rigid. To produce an efficient, stable, cost-effective, and flexible module, different strategies have been explored. One strategy is photon upconversion, where multiple sub-bandgap photons can be converted into a single photon of higher energy, allowing it to be absorbed by the semiconductor of a solar cell. Another strategy is the cost-effective fabrication of efficient perovskite solar cells, composed of CH3NH3I and PbI2. Unlike silicon, the perovskite can be fabricated on top of flexible plastic substrates coated with transparent conductive metal oxide electrodes; however, the limit of its flexibility needs to be explored and improved for real-world applications. More importantly, the major barrier to the commercialization of perovskite solar cells is their instability when exposed to either high humidity or O2 and sunlight. Therefore, improving stability, while not sacrificing efficiency, is extremely important. In this thesis, different strategies are developed to fabricate such a multi-faceted solar cell. Plasmonics are used to enhance the quantum yield of the photon upconversion process. As another strategy, the inflexible metal oxide electrodes of perovskite devices are replaced by flexible polymer-based ones. Then, the intrinsic flexibility of the CH3NH3PbI3 layer is assessed by comparing the results of bending tests performed on a perovskite device and on a highly-flexible organic solar cell. Furthermore, the results of compositional engineering are shown, where partially replacing the CH3NH3+ cation with formamidinium is shown to improve the stability of the perovskite film. Following this, the possibility of completely eliminating CH3NH3+ from state-of-the-art devices is shown, resulting in improved stability and similar efficiencies

Langasite bulk acoustic wave resonant sensor for high temperature applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references (p. 175-188).(cont.) The self consistent defect model established the defect chemistry of langasite, enabling important parameters describing reduction (Er = 5.70± -0.06eV and 6.57±-0.24eV for acceptor and donor doped langasite respectively) and oxidation (Eo = 2.18±0.08eV), intrinsic electron-hole generation (Eg [approx. equals] 4.0-4.4eV) and defect ionization (ED-ion = 52±0.06eV for Nb ionization), to be extracted. The predictive defect model was used to calculate the dependence of the partial ionic and electronic conductivities and mass change as functions of temperature, dopant level and pO₂. Given that the magnitudes of conductivity and mass change directly affect the resolution and sensitivity limits of langasite resonators, their predictions allowed for the definition of acceptable operating limits and/or the design of properties for optimum resolution and sensitivity. Two high temperature applications of resonant sensors were studied. Praseodymiumcerium oxide was selected for oxygen partial pressure monitoring and is representative of films which change mass upon absorption or desorption of gaseous species. Barium carbonate film was selected for NO₂ sensing and is representative of films which change mass upon reaction with the gas phase to form a new product phase. Both sensors showed sensitivity to their respective target chemicals and demonstrated the feasibility of high temperature sensor applications. The performance of each sensor was discussed and suggestions for improving sensor performance were presented.The high temperature transport properties of langasite, La₃Ga₅SiO₁₄, were investigated with special attention focused on their potential impact on the utilization of langasite as a mass sensitive resonant platform for high temperature sensor applications. The electrical properties of acceptor and donor doped langasite were examined at temperatures ranging from 700 to 1000 ⁰C, and pO₂ of 1 to 10-25atm. Acceptor doped langasite was shown to exhibit mixed ionic-electronic conductivity behavior, with predominant ionic conduction due to mobile oxygen vacancies at high pO₂, and n-type electronic conduction due to electrons at low pO₂. Increasing acceptor level resulted in the appearance of p-type hole conduction at high pO₂ and increased ionic conductivity, while the n-type electron conduction was depressed. Donor doped langasite was shown to be electronic at all temperatures and pO₂. The electron mobility of langasite was found to be activated (polaron hopping) with an activation energy of 0.15(±0.01)eV, whereas the holes were assumed to be quasi free carriers. The activation energy for oxygen vacancy migration was estimated to be 0.91(±0.01)eV under dilute solution conditions and 1.27(±0.02)eV for 1% Sr level under concentrated solution conditions. Both values of activation energy of ionic conductivity-temperature product are consistent with activation energy of oxygen self-diffusivity in the respective materials. The electrical properties were related to the underlying defect and transport processes using defect modeling.by Huankiat Seh.Ph.D

RF MEMS reference oscillators platform for wireless communications

A complete platform for RF MEMS reference oscillator is built to replace bulky quartz from mobile devices, thus reducing size and cost. The design targets LTE transceivers. A low phase noise 76.8 MHz reference oscillator is designed using material temperature compensated AlN-on-silicon resonator. The thesis proposes a system combining piezoelectric resonator with low loading CMOS cross coupled series resonance oscillator to reach state-of-the-art LTE phase noise specifications. The designed resonator is a two port fundamental width extensional mode resonator. The resonator characterized by high unloaded quality factor in vacuum is designed with low temperature coefficient of frequency (TCF) using as compensation material which enhances the TCF from - 3000 ppm to 105 ppm across temperature ranges of -40˚C to 85˚C. By using a series resonant CMOS oscillator, phase noise of -123 dBc/Hz at 1 kHz, and -162 dBc/Hz at 1MHz offset is achieved. The oscillator’s integrated RMS jitter is 106 fs (10 kHz–20 MHz), consuming 850 μA, with startup time is 250μs, achieving a Figure-of-merit (FOM) of 216 dB. Electronic frequency compensation is presented to further enhance the frequency stability of the oscillator. Initial frequency offset of 8000 ppm and temperature drift errors are combined and further addressed electronically. A simple digital compensation circuitry generates a compensation word as an input to 21 bit MASH 1 -1-1 sigma delta modulator incorporated in RF LTE fractional N-PLL for frequency compensation. Temperature is sensed using low power BJT band-gap front end circuitry with 12 bit temperature to digital converter characterized by a resolution of 0.075˚C. The smart temperature sensor consumes only 4.6 μA. 700 MHz band LTE signal proved to have the stringent phase noise and frequency resolution specifications among all LTE bands. For this band, the achieved jitter value is 1.29 ps and the output frequency stability is 0.5 ppm over temperature ranges from -40˚C to 85˚C. The system is built on 32nm CMOS technology using 1.8V IO device

Pathlength calibration of integrating sphere based gas cells

Integrating sphere based multipass cells, unlike typical multipass cells, have an optically rough reflective surface, which produces multiple diffuse reflections of varying lengths. This has significant advantages, including negating scattering effects in turbid samples, removing periodicity of waves (often the cause of etalon fringes), and simple cell alignment. However, the achievable pathlength is heavily dependent on the sphere wall reflectivity. This presents a challenge for ongoing in-situ measurements as potential sphere wall contamination will cause a reduction in mean reflectivity and thus a deviation from the calibrated pathlength. With this in mind, two techniques for pathlength calibration of an integrating sphere were investigated. In both techniques contamination was simulated by creating low reflectivity tabs e.g. ≈5x7mm, that could be introduced into the sphere (and removed) in a repeatable manner. For the first technique, a four beam configuration, adapted from a turbidity method used in the water industry, was created using a 5cm diameter sphere with an effective pathlength of 1m. Detection of methane gas was carried out at 1650nm. A mathematical model was derived that corrected for pathlength change due to sphere wall contamination in situ, thus enabling gas measurements to continue to be made. For example, for a concentration of 1500ppm of methane where 1.2% of the sphere wall was contaminated with a low reflectivity material, the absorption measurement error was reduced from 41% to 2% when the model was used. However some scenarios introduced errors into the correction, including contamination of the cell windows which introduced errors of, for example, up to 70% if the particulate contamination size was on the order of millimetres. The second technique used high frequency intensity modulation with phase detection to achieve pathlength calibration. Two types of modulation were tested i.e. sinusoidal modulation and pulsed modulation. The technique was implemented using an integrated circuit board which allowed for generation of modulation signals up to 150MHz with synchronous signal processing. Pathlength calibration was achieved by comparison of iii the phase shift for a known length with the measured phase shift for the integrating sphere with unknown pathlength over a range of frequencies. The results for both modulation schemes showed that, over the range of frequencies detected, 3-48MHz, the resultant phase shift varied as an arctangent function for an integrating sphere. This differed from traditional single passes where frequency and phase have a linear relationship