EE 338L CMOS Analog Integrated Circuit Design Lecture 1, Introduction and Background With the advances of VLSI (very large scale integration) technology, digital signal processing is proliferating and penetrating into more and more applications. Many applications which have been traditionally implemented in analog domain have been moved to digital, such as digital audio and wireless cellular phones. Actually, we will know later, even though digital signal processing techniques have been introduced in these applications, analog circuits are still required. Digital signal processing has a number of advantages compared with analog signal processing, such as : digital signal processing is insensitive to process variations, supply voltage change, temperature variation, interference, and aging; : digital signal processing algorithms can be changed fairly easily by changing the coefficients or the software codes; : some signal processing algorithms have extra degree of freedom if implemented in digital, such as linear phase filter. As many applications have moved to digital domain, analog circuits seem obsolete. But actually analog circuit designers are in strong demand today. Why?!!! Because even though many signal processing functions have been implemented in digital, some functions can not be replaced by digital signal processing, such as analog-to-digital and digital-to-analog conversion, anti-alias and reconstruction filtering, and so on. These functions are to be implemented in analog domain, independent of technology improvement. 1.1 Why is Analog Signal Processing Required? The physical world is in analog (at macroscopic level), such as, such as microphones, a temperature sensors in automobile engines, and the photo cells in digital cameras, are all in analog form; The speakers in mobile phones and computer monitors need analog input signal. The in analog. in space and at the antenna are also 1
We need to digitize the input signal, and to reproduce the analog signal after digital signal processing. (before the ADC) and _ amplification, filtering, equalization. (after the DAC) are needed, such as Physical World (in Analog) Analog Pre- Processing A/D Conversion Digital Signal Processing D/A Conversion Analog Post- Processing amplification, filtering, and etc amplification, filtering, and etc Fig. 1.1, ADCs, DACs, and pre-/post- processing analog circuits are required to interface the DSP core and the physical world 1.2 Integrated Circuit Technologies 1.2.1 History of Integrated Circuits Integrated circuits, i.e., devices with multiple electronic devices on the same substrate, were invented in late 1950s, by Jack St. Clair Kilby at Texas Instruments, Inc. In 1970s, Gordon Moore, one of the founders of Intel, predicted that the number of transistors per chip doubles every one and a half years. The minimum channel length of MOS transistors dropped from 25 m in 1960s to 90 nm in the year 2002, with the benefit of much higher complexity, smaller volume, and higher speed. Without integrated circuit technologies, computers might still be as huge as ENIAC, and mobile phones would be as big as suitcases! 2
1.2.2 Mixed-Signal Integrated Circuits 1) Integrated circuits of ten or twenty years ago A signal processing system required multiple analog integrated circuit (IC) chips (such as amplifiers, filters, and A/D and D/A converters), digital IC chips (such as memory, digital signal processors, and interfacing logic), and plenty of passive discrete components. The analog and digital IC chips were traditionally designed and fabricated in different technologies. Usually, analog circuits use bipolar technologies, while digital circuits are in MOS technologies. A system, which consisted of a large number of integrated and discrete components, was power-hungry, huge, and expensive! 2) Integrated circuits of today Most of the integrated chips have both analog and digital circuits. This is called mixed-signal integration. Penetrating into every corner of our everyday life, from supercomputers, space probes and medical diagnostic equipment, to printers, DVD players, cellular phones, and children s toys. Digital circuit design is mostly automated from logic synthesis to placement and routing, while analog circuit design remains as an almost allhandcrafted art. Feature size Amp ADC 0.6m 0.35m 0.18m RAM DSP Filter DAC System on a chip integration SoC With the continued scaling down of semiconductor technology, more and more devices could be integrated onto a single chip. Amp ADC DSP Filter DAC RAM Fig. 1.2, Mixed-signal system-on-a-chip integration 3
1.2.3 CMOS, Bipolar, and BiCMOS technologies CMOS and Bipolar in silicon are the two mainstream semiconductor technologies. BiCMOS is the combination of the above two, which has both CMOS and bipolar transistors. 1.3 Why CMOS Mixed-Signal Integration CMOS technologies have the advantages of of both high-density digital circuits (such as DSP and memory) and analog circuits (including amplifiers, filters, and A/D & D/A converters) for low cost. Ideal properties of MOS switches for high accuracy sample-data circuits, such as switched-capacitor filters and A/D & D/A converters. New CMOS technologies with (such as 0.25m and 0.18m) can operate at increasingly high speed (5GHz), comparable to some bipolar technologies. Bipolar silicon technologies Bipolar transistors can operate at higher frequencies than CMOS with relatively smaller power consumption (due to large gm/ic ratio). Suitable for pure analog integration with relatively high operating speed (such as RF circuits) or relatively high power (such as ADSL line drivers) applications. Digital circuits in bipolar are power hungry, prohibiting very large scale integration. BiCMOS technologies have most advantages of both CMOS and bipolar technologies but at the expense of higher manufacturing cost due to required extra processing steps. The performance of bipolar transistors in BiCMOS are usually inferior to that of pure bipolar technologies. Thus CMOS technologies become mainstream technologies for mixed-signal integration due to the advantages of low cost and high integration density. 1.4 Example Applications of Analog and Mixed-Signal Integrated Circuits 1.4.1 Wireless communications Such as cellular phones and wireless local area networks. Fig. 1.3 shows the block diagram of a 5-GHz CMOS transceiver for Wireless LAN application [zargari02]. 4
1.4.2 Wireline communications Such as cable modems and xdsl. Fig. 1.4 illustrates the block diagram of an ADSL transceiver. Fig. 1.3, A 5-GHz CMOS transceiver for Wireless LAN application Local Memory Data Framer, FEC, Data Interface DMT, Mod./ Demod. DSP DAC ADC Filter Amp VGA Filter Analog Hybrid, Line Driver Phone wires Host Interface Fig. 1.4, the block diagram of an ADSL transceiver 1.4.3, Digital communications through copper wires or optical fibers Including local area network using copper wires and internet backbone through optical fibers. 5
Transmitter Laser diode Photo-diode Receiver 1.4.4 Circuits for Sensors Fig. 1.5, Optical communication system [razavi01] Including mechanical, electrical, chemical, and optical sensors. New processing technologies may manufacture both the sensor and analog/digital processing circuitry onto a single chip. For example, a CMOS image sensor [yoon02] for automobile applications is shown in Fig. 1.6. (a) Block diagram (b) Microphotograph Fig. 1.6, A CMOS image sensor [yoon02] for automobile applications 6
1.4.5 Disk drive write/write electronics Different analog building blocks are needed in disk drive circuits, such as VGAs (variable gain amplifiers), filters, VCOs (voltage controlled oscillators), PLLs (phase locked loops), ADCs (analog-to-digital converters), and DACs. Fig. 1.7, Block diagram of Disk drive write/write electronics 1.4.6 Microprocessors and memories The design of microprocessors and memories requires a lot of analog expertise [razavi02]. High speed clocks and digital signals are treated as analog signals in checking the timing of clock and data. The high-speed sense amplifiers in memories are actually analog circuits! 1.5 Analog and Digital Signals Analog signals may be i) signals, or ii) signals (also called sampled-data signals). Analog signals are in amplitude, but may or may not be continuous in time. Digital signals are and signals. 7
Fig. 1.8, Analog and digital signals, (a) signal (continuous in both time and amplitude), (b) signal (discrete in both time and amplitude), (c) signal (discrete in time, continuous in amplitude), or sampled-data signal 1.6 Notation, Symbols, and Terminology 1.6.1 Notation Defination Quantity Subscript Example Total instantaneous value of the signal lowercase UPPERCASE q A DC value of the signal UPPERCASE UPPERCASE Q A ac value of the signal lowercase lowercase q a Complex variable, phasor, or RMS value of the signal UPPERCASE lowercase Q a 8
1.6.2 MOS transistor symbols Fig. 1.9, Notation for signals Fig. 1.10, MOS transistor symbols If the bulk terminals are not connected to power supplies (NMOS negative supply, PMOS positive supply), they need to be drawn explicitly, such as M1 and M2 in Fig. 1.10. 9
Vdd=1.65 M6 M5 Vin+ M1 M2 Vin- Vout I B M3 M4 CL Vss=-1.65 Fig. 1.11, An example CMOS amplifier 1.6.3 Amplifier and current/voltage sources Fig. 1.12, Amplifier and current/voltage sources (VCCS is most frequently used) 10
1.7 Levels of Abstraction M5 Vin+ M1 M2 Vin- Vout M3 M4 Device Circuit V IN 2 C S V O V C O S V REF 1 Architecture System 1.8 Analysis and Design Fig. 1.13, Levels of abstraction [razavi01] Fig. 1.14, Analysis and design processes 11
Fig. 1.15, Design process for analog integrated circuits 1.9, Summary and conclusion There is a strong need for excellent analog and mixed-signal designers. To prepare a career in the field of mixed-signal integrated circuits, this course is the first step. 12
References [allen02] P. Allen and D. Holberg, CMOS Analog Circuit Design, 2nd Ed., Chapter 1, Oxford University Press, 2002. [razavi01] B. Razavi, Design of Analog CMOS Integrated Circuits, Chapter 1, McGraw Hill, 2001. [zargari02] M. Zargari et al., A 5-GHz CMOS transceiver for IEEE 802.11a wireless LAN systems, IEEE J. Solid-State Circuits, vol. 37, no. 12, pp. 1688-1694, Dec. 2002. [yoon02] K. Yoon, C. Kim, B. Lee, and D. Lee, Single-chip CMOS image sensor for mobile applications, IEEE J. Solid-State Circuits, vol. 37, no. 12, pp. 1839-1845, Dec. 2002. 13