Analog design in the 21st century: challenges, tools, and IC advances
Steve Taranovich, Contributing Technical Editor - January 19, 2012
We are now more than a decade into the 21st century, and on an ever-accelerating fast track to technological innovation in electronics. The transistor and progression into the IC, or microchip, lit the fuse leading to the explosion of innovations in electronics that is now taking place. Since the widespread introduction of the microchip in the early 1970s, more medical, mathematical, and scientific breakthroughs have occurred than during any other time, and big breakthroughs are happening more frequently. This surge is due in large part to the performance and computing power that the ever-shrinking but increasingly dense silicon wafer has brought.
Let’s take a look into the analog sector’s crystal ball and see what’s coming to the rescue of analog designers to help them in the scope of variables they must manage, the extreme sensitivities to circuit constraints, and the quality of results necessary to make analog design as much of an art form as it is a science (Reference 1).
Beginning at the system level, you’ll find that the silicon components embody much of the system (Reference 2). A good example of this trend is Intel’s Huron River mobile platform, which includes the Sandy Bridge microprocessor and Cougar Point PCH (platform-controller hub). Signaling speeds of today’s platforms can reach 1600 Mbps for single-ended DDR3 and 5 Gbps for differential PCIe Generation 2. The typical PCB material in systems has limitations that require advanced analog-design complexity to overcome limits. Analog designers often use interface chips, such as Texas Instruments’ DS25BR204 LVDS (low-voltage-differential-signaling) repeater, which has built-in pre-emphasis on the transmitter side and equalization on the receiver side of a signal trace running on a PCB across a motherboard. The need to conserve power and silicon-die area is also challenging to analog-circuit designers.
Another factor affecting analog circuitry is in the mixed-signal domain, which you can observe primarily in the timing-convergence domain. As signal speeds increase, clock and timing budgets continue to shrink. This situation also fuels the risk of circuit bugs (Reference 2).
Despite its critical nature, especially in high-speed circuitry, PCB layout is often one of the last steps in the design process. As in real estate, location is everything. Critical decisions include where to place an IC on a board, the location and thickness of copper traces, where to locate components, and what other nearby circuits might affect performance. Many designers rely on IC suppliers’ evaluation boards and data sheets to properly integrate a chip into a copper-clad, fiberglass PCB. Also check IC suppliers’ white papers on the subject.
The TI/National Semiconductor merger will enable more powerful analog designer tools. National’s Webench Designer in seconds creates and presents all of the possible power, lighting, or sensing circuits that meet a design’s requirements. This feature enables a user to make value-based comparisons at a system and supply-chain level before committing to a design. According to Phil Gibson, manager of Webench design at Texas Instruments, the goal is to make tools more visual, easier to use, and more intuitive. Webench brings analog designers from concept to selecting and ordering components for the breadboard. Stay tuned for more tools, which leverage the “cloud,” from the combined expertise of both of these companies. TI and National will integrate the breadth of their combined portfolio into existing and new tools (Figure 1).
Precision sensing systems employing customized analog designs take weeks—or even months—to design. From IP (intellectual property) to prototype and test to writing system algorithms, designers have been unable to simplify the development and production cycle for critical development systems. The TI and National sensor analog-front-end system and ICs offer an integrated hardware- and software-development platform that allows circuit designers to quickly create a design online, configure it for their requirements, and rapidly prototype it.
A seemingly simple calculation of input-signal common-mode range with TI’s INA-CMV-Calc (instrumentation-amplifier common-mode-voltage calculator) can save a designer valuable design time. You can avert many common instrumentation-amplifier design problems with this simple spreadsheet calculator.
Analog Devices has enhanced its Web site to support design engineers, with increasing emphasis on technical content. This support includes the Engineer Zone online technical-support community. The company also offers RF tools, such as ADIsimRF, an easy-to-use RF-signal-chain calculator for cascaded gain, noise figure, third-order intercept point, 1-dB gain compression, and total power consumption. Users can switch the calculator between transmitting and receiving modes, which present output-referred and input-referred calculations, respectively. The company’s Circuits from the Lab lists proven reference designs so that designers can be confident in their performance.
On the amplifier side, Intersil offers the iSim active-lowpass-filter tool, which Michael Steffes, senior applications manager at the company, developed. Steffes recently updated the product to include a noninverting-design tool and design algorithms for inverting, both of which will be available shortly. These tools might seem simple. However, the inverting input capacitance in the models and in the device interacts with the resistance values, creating frequency-response issues, and this tool can help address these issues.
According to Steffes, companies can automate a range of ubiquitous application circuits for op amps into online tools. Although vendor and third-party tools apply textbook approaches to the external elements to achieve a design, emerging tools combine the features of a parametric search engine to home in on suitable devices and then execute the design algorithms, including device-specific features. Whereas earlier tools might do a design by looking at just the gain-bandwidth product for voltage-feedback-amplifier-based circuit blocks, newer tools might also consider slew rate, parasitic input capacitance, noise and flicker-noise corners, output-loading issues, and how these issues interact with the amplifier’s frequency response.
These “expert system” approaches quickly and effectively narrow the range of device selections by considering both basic effects, such as supply-voltage range, and second-order effects. The new tools also enhance the design algorithms to reduce the required design margin in a device. This feature leads to lower-power and lower-cost devices by considering and compensating for the device’s nonidealities in the initial design pass. For example, older tools might be looking for a 100-times greater bandwidth margin in a second-order Sallen-Key filter design, whereas new tools might reduce this margin into the 10-times region, greatly reducing the required design margin in the active device and leading to lower-cost and lower-power designs.
Starting from a few common circuit building blocks, these tools quickly deliver proposed devices; execute designs for the external elements; and, in some cases, port those designs into more general circuit simulators, in which the design can continue using a larger library of board-level components. The online tools thus incorporate accurate and more complete subcircuit models. They model key parasitics and device nonidealities, and designers then use these models in the design algorithms to anticipate and manage second-order issues.
The subsequent designer interface targets the desired outcome for a range of common applications circuits, along with system constraints, such as supply voltage. The tools combine this outcome with a parametric search and a weighting algorithm to eliminate unsuitable devices and then rank the remaining devices by closeness of fit. This weighting closely couples with the intended design algorithms for each of the common topologies. After selection of a device and target design, the tool then executes external element algorithms that are considering parasitic effects, leading to a proposed circuit block. After a circuit block is designed using these interactive tools, some tools can then port that block into a more general simulation platform in which design can continue, combining several of these blocks and inserting other required components.
These analog-circuit-design tools enhance engineers’ capabilities and lead to a streamlined and rapid transition of a design idea to a realizable piece of circuit hardware that they can then tweak into a final, working design.
As CMOS processes move into the nanoscale—that is, less-than-100-nm—range, it becomes increasingly difficult to maintain the energy efficiency for medium- to high-accuracy analog circuits (Reference 3). Accuracies may decrease as the technology scales down. Analog-chip designers can improve energy efficiency using different approaches, from selecting the optimum CMOS technology to clever system-level design. Although these approaches can improve efficiencies, doing so is becoming increasingly challenging. Efficiency is essential to analog-circuit designers who are trying to pack 10 lbs of functions into a 5-lb bag. Proper transistor biasing and new clever circuit-level techniques can maintain or even improve energy efficiency in future analog CMOS circuits.
Data-converter performance has been steadily increasing over the years. Advancements in scaling and design techniques exploit the high density and speed of modern process technology. More intelligence is being put into data converters. Managing more system functions, converters will simplify programming software, shrink, and become less complex and more cost-effective with digital enhancement. Adaptive power management and sensor integration and enhancement are here, with more to come, eliminating the analog-circuit designer’s messy task of signal conditioning the sensor and keeping the sensor, conditioning, and data-conversion chips as close as possible. Enter TI’s TMP006 infrared thermopile sensor in an ultrasmall chip-scale package with integrated signal conditioning, an ADC, and a digital-signal output to a serial data bus (Figure 2).
Until recently, ADC converters employing a pipeline architecture were the most appropriate for processing wideband signals with the required resolution. Recent advances in oversampling-data-converter technology have enabled new alternatives for the definition of baseband analog front ends for wireless broadband communication systems. The technology for ADCs using oversampling sigma-delta architectures has evolved to the point that the devices can effectively convert signals with bandwidths of 10 MHz or more. This achievement is significant because it allows for the use of this ADC architecture in the receiver channel of broadband communication interfaces, such as LTE (long-term evolution), WiMax, or 802.11abg. They do not suit use for 802.11n, for which the required signal bandwidth is 20 MHz (Reference 4).
Analog-circuit designers now have more choices in data converters, especially with the ability to integrate analog front ends in video, medical, and power-line-communication applications. These converters have lower-power, smaller-footprint, and lower-cost sigma-delta and pipeline architectures than ever before, allowing for implementation in new markets and applications that were previously off limits to such units. TI’s ADS1298, targeting use in the medical arena, is a primary example of such a device.
With last year’s advances in data converters and analog front ends, wideband oversampling sigma-delta ADCs will find use in handset systems that support both narrowband and broadband communications with signal bandwidths as high as 10 MHz. They will also find use in applications integrating the ADC together with the RF transceiver, such as fully integrated RF/broadband chips. Pipeline ADC converters, on the other hand, will find use in applications supporting broadband only, such as PC dongles and Pico cells, and applications, such as 802.11n, in which the baseband signal’s bandwidth is wider than 10 MHz. They will also find use in applications in which the RFIC is an external device.
The wireless-communication market will be a key driver of data-conversion performance, power efficiency, and calculated integration. Emerging 4G cellular networks and the demand to keep pace with the exploding popularity of wireless video transmission will keep circuit designers busy. Meanwhile, data-converter manufacturers, such as Analog Devices, will develop products with faster sampling rates and more usable bandwidths at higher immediate frequencies, bringing the antenna closer to the data converter, thereby eliminating costly and complex microwave and RF components. This approach greatly simplifies the analog-circuit designer’s task of developing new infrastructure to keep pace with consumer needs.
Vendors are developing amplifiers to complement the ever-increasing high speed and high bandwidth of ADCs. Intersil, for example, offers the ISL55210 SiGe amplifier with large dynamic range for high-end data acquisition. Another exotic new amplifier might be quiet enough for quantum computing (Reference 3). Quantum computers have the potential to solve seemingly intractable problems in no time flat. A big stumbling block on the path to practical quantum computing, however, is figuring out how to observe the tiny quantum signals that drive computation. In an advance that may ease that observation, a group at Aalto University in Finland has created a new kind of microwave amplifier. It uses a mechanical resonator—essentially, a nanometer-scale tuning fork.
MEMS chips are ubiquitous. They tell Wii controllers that you’re swinging your virtual tennis racket and iPhones that you’re giving them a shake. Now, they’re popping up in unexpected places, including ski goggles and surfboards (Reference 5). STMicroelectronics’ MEMS group has entered the market with the three-axis L3G4200D digital gyroscope. Highly integrated products such as this one are easing the analog-circuit designer’s burden of interfacing sensors on through to the serial bus.
ASICs and ASSPs
Companies including Cactus Semiconductor have expertise in power-management and analog circuits that find value in products for the medical- and portable-system market. An analog-circuit designer’s dream come true is the level of integration that such innovators make possible (Figure 3).
Touchstone Semiconductor, a 2010 start-up, is initially focusing on marketing parts as drop-in replacements for those from other manufacturers, such as Maxim Integrated Products and Linear Technology. The company will eventually bring to market proprietary parts, which are already in development. Low-power and low-offset current-sense amplifiers are among the new developments, according to Brett Fox, chief executive officer at Touchstone. Targeting existing high-margin markets will enable the company to build revenue and credibility with customers, Fox says. Using TSMC (Taiwan Semiconductor Manufacturing Co) as a foundry allows Touchstone more flexibility and exceeds performance typically available in other semiconductor fabs with state-of-the-art processes. Touchstone now offers 15 analog ICs as part of its Maxim alternative-source product family. Engineers can use Touchstone’s alternative-source parts with Maxim’s products to ensure a constant supply, so companies can build products and meet shipping deadlines. Analog-circuit designers look to such innovative companies as Touchstone and Maxim for specialized, high-integration ASICs and ASSPs to ease their design task. The availability of multiple sources can be helpful when selecting an analog part for a design.
SOC technology requires smooth interfacing between peripherals and processors. Multichannel ADCs with high-speed and serial interfaces bring more challenges to analog designers for effectively interfacing these devices with the processor (Reference 6). For example, Analog Devices’ AD9219 offers low cost, low power, a small footprint, and simple implementation for a VLSI design to pass the data from the ADC to the DSP. You can use asynchronous FIFO buffers with Gray-code synchronization to handle the synchronization issues that occur with two clock domains. This DSP-based ADC interface can be programmed as not only an ideal component to offload processing requirements but also as an analog-input signal preprocessor for the host.
Digitally Enhanced Analog
Implementing digital control in the design of power controllers allows analog-circuit designers to easily monitor multiple operational parameters of the power converter. These parameters include input and output voltage, output current, and temperature, which are only a few of the basic parameters of many critical parameters that you can measure. It is more difficult for analog controllers than their digital counterparts to achieve the diverse functions and adaptive capability that power converters require. Digital controllers more easily achieve these tasks because they can easily digitize data and make it readily available for an external device to read (Reference 3).
TI is the leader in digital power, providing flexible and configurable digital-power products for ac/dc and dc/dc designs. The company provides power designers with a broad portfolio of processors, controllers, and drivers, as well as modular approaches to any digital-power-system design challenge. Whether you are designing for isolated or nonisolated products from ac/dc to dc/dc POL (point-of-load) systems, TI’s flexible, customizable, and intuitive digital-power portfolio enables a variety of designs.
Analog designers, take heart! More help is on the way as new designs demand smaller footprints, longer battery life, or energy-harvesting power and increased performance in all electronic devices, even in areas you never expected, such as newspapers. Within 10 years, you will see the e-Sheet, a virtually indestructible e-device that will be as thin and as rollable as a rubber placemat (Reference 7). The full-color, interactive device requires little power to operate because it charges using sunlight and ambient room light. However, it will be tough and will use only wireless connection ports, so you can leave it out overnight in the rain. You’ll be able to wash or drop it without damaging the thin, highly flexible casing.
For more on this topic, see the sidebar “Analog design in the 21st century: semiconductor processes and design.”
You can reach Contributing Technical Editor Steve Taranovich at firstname.lastname@example.org.
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