Anahtarlı kapasite (SC) devrelerinin spice programı ile analizi
Analysis of switched capacitor (SC) circutis using spice program
- Tez No: 39128
- Danışmanlar: PROF.DR. ALİ NUR GÖNÜLEREN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Anahtarlı kapasite devreleri, İletişim sistemleri, Switched capacity circuits, Communication systems
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Elektronik ve haberleşme sistemlerinde, anahtarlı kapasite (SC) devrelerinin önemli bir yeri vardır. Bu çalışmanın ilk bölümlerinde SC devreleri genel olarak ele alınmış ve temel özellikleri belirtilmiştir. Bu devrelerde anahtar olarak kullanılan MOS tranzistorlar devrenin yapısına göre tek ya da çift saat fazları ile kontrol edilmektedirler. Bu tranzistorların doğru birer anahtar olarak çalışabilmesi saat işaretlerinin periyotları, devrenin giriş işareti ve devrede kullanılan kapasite değerleri ile doğrudan ilişkilidir. Anahtarlı kapasite devreleri örneklenmiş veri prensiplerine dayandığı için z-domeninde tasarlanması ve analizi uygun olmaktadır. Bu çalışmada anahtarlı kapasite devreleri z-domeni transfer fonksiyonlarının bulunması konusuna da değinilmiştir. Çalışmanın esas konusu ise anahtarlı kapasite devrelerinin bilgisayar destekli tasarımının araştırılmasıdır. Bu amaçla elektronik devrelerin analizinde sıklıkla kullanılan SPICE pogramı ele alınmıştır. Anahtarlı kapasite devrelerinde kullanılan temel elemanlar, MOS anahtarlar, kapasiteler ve işlemsel kuvvetlendiricilerdir. Bu devrelerin bilgi sayar destekli analizlerinin yapılabilmesi için öncelikle temel elemanların kullanılan programa iyi bir şekilde tanıtılması gerekmektedir. Bu amaçla SPICE programı için notasyonlar verildikten sonra, Anahtarlı kapasite devrelerinde kullanılan elemanlar için modeller oluşturulmaya çalışılmıştır. MOS tranzistorlar için uygun büyüklük ölçüleri ve parametreler seçilmiştir. Ayrıca ikinci dereceden bir CMOS işlemsel kuvvetlendirici için tasarım değerleri kullanılan anahtarlı kapasite devreleri gözönüne alınarak belirlenmiştir. Çalışmanın son bölümünde de uygun devre elemanları kullanılarak seçilen anahtarlı kapasite devrelerin analizleri SPICE programı ile gerçekleştirilmiş ve analizin sonuçları grafikler halinde sunulmuştur. IV
Özet (Çeviri)
SUMMARY ANALYSIS OF SWITCHED CAPACITOR (SC) CIRCUITS USING SPICE PROGRAM Switched capacitor (SC) circuits play a very important role in elect ronics and communications systems. In last twenty years the MOS integrated- circuit technology had found wide usage in industry because of superior logic density, as compared to that achievable with bipolar technology. Particularly since the 1980's the improvements in switched capacitor circuits have resulted in the wide use of these circuits in various fields. Today, millions of MOS transistors can be placed on a single chip. The denvelopments show that the packing density will continue to increase and the cost for complex digital/analog MOS integrated circuits will continue to decrease. Switched capacitor circuits are most widely used in filter implementations. Also these circuits are frequ ently used in coding, modulating, scrambling, analog/digital and digital/ana log converting circuits. By the use of MOS technology, many filters can be mounted on a very small silicon surface using only one integrated circuit. Today, using wide scale integration, the circuit intensity in an integrated circuit is increased while the cost is decreased. MOS integrated circuits have ability to store charge on a node for several milliseconds and to sens this stored charge continuously and nondestructively. This propety in MOS integrated circuits comes about natu rally and cheaply. With this advantage, MOS integrated circuits were first used in dynamic random access memories and dynamic logic. Using MOS integrated circuits in this area, provided some important advantages to realize a sampled- data transversal filters. More recently, it was found more attractive to implement MOS analog samled-data circuits as active switched capacitor circuits. In addition, switched capacitor circuits take full advantage of the inherent precision achi eved by MOS processing. In this study, it is shown that the transfer function coefficients are completely determined by a single, precise crystal-controlled cloc frequency and ratioed capacitors. Capacitors ratios can be held to about 0.3%, with appropriate circuit techniques, capacitances as small as 5 pf can be used. Furthermore, MOS capacitors are nearly ideal, with very low dissipation factors and good temperature stability. Because of economical importance, precision switched capacitor circuits can be fabricated using memory-like -V-NMOS and CMOS processing. Thus, analog and digital circuitry can be placed on the same chip. In this study we describe and demostrate techniques for the analysis and design of active switched capacitor circuits. Because of their sampled-data character, switched capacitor circuits are most conveniently analyzed and design, in the z-transform domein, like digital circuits. However, switched capacitor circuits are analog circiits. So, analog concepts of impedance and loading are retained. Like active-RC circuits, there are many SC topologies that can be used to realize a given z-domein transfer function. There are several sampled-data waveforms which can be modeled as special cases of the waveform. One can immediately invoke the z-transform to mathematically describe these waveforms. Since switched capacitor circuits can be characterized in terms of charge-transfer operation, discreate-time voltages and discreate-time charge variations or transfer are used as port variables. For single capacitor switched capacitor blocks z-transformed nodal charge equations lead directly to simple z-domein equivalent circuits. The z-transformed voltage transfer functions plays an equivalents important role in specifying and designing active SC filters. The two port can be represented by an equivalent four-port. In general, a 2x2 transfer matrix is required to fully characterize the input-output relations for this four-port network. The initial step in the synthesis of an SC network is to obtain an appropriate z-domein transfer function. Since filters are typically specified by frequency-domein requirements, it is convenient to have a mathematical expression that allows us to transform rational s-domein transfer functions to rational z- domain transfer functions. In lieu of computing a z-domain transfer function and synthesizing the SC network in the z-domain, analog active RC circuits can be transformed into z-domein circuits using the frequency transformation. This type of synthesis enables us to transform low-sensitivity active-RC filter design into active-SC realizations of compatible quality. At this point, we saw the analysis and systhesis of swiched capacitor filters. For analysis and general understanding, we have relied to great extent on their similarity to digital filters. For topological realizations and practical MOS implementation, we have sought to exploit their unique properties. An analysis procedure using z-domain equivalent circuits has been given. The sampled-data behavior of SC networks permits the use of the z-transform, but basically their analog nature allows them to be characterized using basic linear circuit theory. Many examples can be given which demostrate the versatility of SC networks to realize filters of all types..VIBecause of its load storing properties, MOS technology has begun to be used in processing analog signals. In these circuits, the load transfer between the capacities is done by a switching operation. The switching ope ration is realized by pulses produced outside the circuit driving the MOS transistors [2]. Sampled-data circuits requires synchronization between the clocks that control the switched capacitor circuits and those that control the external sampling operations. This is accomplished by passing synchroniza tion pulses between the switched capacitor circuits and external samplers. The pulse source (<j>), which is aplied the gate terminal of MOS transistor, causes it to work as a switch. When the pulse source has the status ON, the MOS transistor is in the forwarding state and as a switch it is closed. When (<j>) is in the OFF state, the MOS transistor is in the cutting state and the switch is open. In switched capacitor circuits, structures consisting of MOS transistors and capacities working according to this principle are used. The operation of an ideal switched capacitor circuits, comprised of ideal capacitors, ideal switches, and CMOS operational amplifiers when excited by sampled-data voltage inputs. In switched capacitor circuits, switches are controlled by a two-phase, nonoverlapping clock of frequency fc=2T. Note that <j>e is used to denote the even clock phase, which instantaneously closes the e switch on the even 2nT times. Similarly, $° denotes the odd clock phase, which instantaneously closes the o switch on the odd (2n+l)T times. The switches are assumed to have a 50% duty cycle with equal on and off time periods. In practice, the clock rate is tipically chosen no higher than is required to achieve the desired degree of anti-aliasing protection with a second order continuous filter of sufficiently high cutoff frequency to render its main passband variation acceptably small. Although to take the even and odd times of the clock signals unequal simplifies the analysys, it is by no means necessary. Thus, concider the more general case where the clock period is still t but the desired input signal is sampled and held only for the interval xe(xe<x). The odd phase of the clock period is refered to as x (xo=x - xe). The input during this phase is assumed to be undesiradbe. Apart from MOS transistors and capacities, operational amplifiers are also used in switched capacitor circuits. Operational amplifiers play a very important role in analog circuit design. The operationa amplifiers, which has become one of the most versatile and important building blocks in analog circuit design, amplifiers that have sufficiently high forward gain so that when negative feedback is aplied, the closed-loop transfer function is practically independent of the gain of the operational amplifier. This principle has been exploited to develop many useful analog circuits and systems, ideally, an operational amplifier has infinite differential-voltage gain, infinite input resistance, and zero output resistance.In reality, an operational amplifier only approaches these VII-values. In switched capacitor circuits usually CMOS operational amplifiers that are selected or designed according to the circuit structure are used. Using these operational amplifiers circuit conditions are well defined. The purpose of this thesis is to introduce switched capacitor circuits in detail and to find realization possibilities for computer aided simulation of such circuits. Today as electronic circuits advance, predesigning of the circu its and defining the working conditions of the circuits are as important as the physical implementation. That is why the computer aided design became popular. In computer aided design, defining the circuit elements and circuit conditions properly is very important. In the examples of this work, it is used a two-stage operational amplifier. For this two-stage operational amplifier, assumed that specificati ons for the following parameters are given; gain at dc, gain bandwidth, input common-mode range, load capacitance, slew rate, output voltage swing and power dissipation. At the end of the design procedure the value of the devices and sizes of the MOS transistors are defined. SPICE is a widely used program in simulating electronic circuits. The last section of this study is about analysis of switched capacitor circuits by using SPICE program. Because of the enormous complexity of modern integrated circuits, computer-aided circuit analysis is essential and can provide information about circuit performance that is almost impasible to obtain with laboratory prototype measurements. SPICE is a very popular software for analyzing electrical and electronic circuits. In this program, circuit elements are defined by various models and parameters. The electronic circuits can be analyzed in detail. In this work the general properties of switched capacitor circuits are introduced first and the elements and the constraints of these circuits are analyzed. Then, data sampling waveforms and transfer function equations are examined. This information is used in some circuits. In particular, the working conditions of MOS transistors as an accurate switch and the implementation of the (<j>) signals have been analyzed. In later chapters a general overview has been given of the SPICE program and modeling for SPICE has been introduced. For this purpose the SPICE models both for the elements of SC circuits and the ones that are used in our examples have been implemented. The necessary conditions and para meters for these elements are also taken into consideration where appropriate. Also, apart from switched capacitor circuits elements explanations about modeling of certain basic blocks that are used in switched capacitor circuits are given. ?VIIIUsing the information explained in previous chapters, the final chapter gives examples of the simulation of switched capacitor circuits using the SPICE program. For this purpose, sample and hold circuitsand integra tor circuits without loss are examined. Graphical results are presented using appropriate SPICE models. With this examples, we can say that, if we use suitable device models for SPICE, analizing of active switched capacitor circuits is posible. IX
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