资 源 简 介
There is an ever increasing trend towards putting entire systemson a single chip. This means that analog circuits will have tocoexist on the same substrate along with massive digital systems.Since technologies are optimized with these digital systems in mind,designers will have to make do with standard CMOS processes in theyears to come. We address analog filter design from this perspective.Filters form important blocks in applications ranging from computerdisc-drive chips to radio transceivers.In this book, we develop the theory and techniques necessaryfor the implementation of high frequency (hundreds of megahertz)programmable continuous time filters in standard CMOS processes.Since high density poly-poly capacitors are not available in thesetechnologies, alternative capacitor structures have to be found. Metalmetalcapacitors have low specific capacitance. An alternative is touse the (inherently nonlinear) capacitance formed by MOSFET gates.In Chapter 2, we focus on the use of MOS capacitors as integratingelements. A physics-based model which predicts distortion accuratelyis presented for a two-terminal MOS structure in accumulation.Distortion in these capacitors as a function of signal swing and biasvoltage is computed.Chapter 3 reviews continuous-time filter architectures in the lightof bias-dependent integrating capacitors. We also discuss the meritsand demerits of various CMOS transconductance elements. Theproblems encountered in designing high frequency programmablefilters are discussed in detail.Chapter 4 considers time-scaling in electrical networks and introducesa technique called constant-capacitance scaling. We show thatwide-range programmable filters implemented using this techniqueare optimal with respect to noise and dynamic range.Chapter 5 documents the detailed design and layout of a 60 –350 MHz Butterworth filter implemented in a 0.25 μm digital CMOSprocess. A simple circuit arrangement is proposed to keep thefilter bandwidth constant with respect to temperature and processvariations. Simulation results for the filter test chip are shown.For filters in the frequency range of interest to us, special care hasto be taken to measure the filter response accurately. In Chapter 6,we present measurement techniques and the implementation resultsof the prototype filter chip. This experimental data demonstrates theeffectiveness of the techniques proposed in Chapter 4.In Chapter 7, we discuss the application of scaling techniques toother filter architectures.For very high-quality filters, the simple tuning technique proposedin Chapter 5 may not guarantee sufficient precision. In Chapter 8, amore accurate and complex method based on the behavior of a filtercomparatoroscillator is proposed. This scheme converts a filter intoan oscillator. The amplitude and frequency of oscillation are measuresof the center frequency and quality factor of a biquadratic filtersection. Analytical relations for the behavior of the filter–comparatorsystem are developed and verified through a breadboard prototype.This book is based on a doctoral thesis (submitted by one of theauthors [S.P.] to Columbia University). It describes the results ofa research project carried out to determine if it is at all possibleto realize robust, production quality VHF filters in standard deepsubmicron CMOS technologies. The filter design discussed in thisbook was implemented at Texas Instruments (New Jersey).ACKNOWLEDGMENTS: We are grateful to the staff of TexasInstruments at Warren for their support - T.R. Viswanathan and K.Nagaraj have been sources of great encouragement all along. Wethank the students of the Columbia Integrated Systems Laboratory- especially M. Tarsia, G. Palaskas, N. Krishnapura and A. Dec foruseful discussions. Finally, one of the authors [S.P.] would like tothank Professors Anthony Reddy and John Khoury - their superbclasses on filter theory and circuit design have been profoundlyinfluential over the course of creating this book.
