Electronic Design

Analog/Mixed-Signal: Overview

Diverse Trends Shake Up Product Portfolios

Last year was supposed to ring in the "Recovery" after the worst year in semiconductor history, yet everyone is still holding their breath for the economy to revive. Most analysts have reported that semiconductor sales grew a meager 1% in 2002, but expectations are for the market to finally pick up steam in 2003. Nevertheless, the analog and mixed-signal segments haven't struggled as much as the rest of the industry. According to iSuppli (www.isuppli.com), analog IC sales are expected to hit $27.752 billion in 2002 and should grow to $30.776 billion in 2003.

Although the nature of analog and mixed-signal devices is quite different from the digital world, the trends between the two remain parallel—faster and higher performance. The main drivers for the analog and mixed-signal sectors are higher precision and linearity, as well as lower distortion. At the same time, the market is looking for products with lower operating power and smaller packages.

The drivers identified by Intel's Gordon Moore for ever-higher levels of integration, even though not directly applicable to the analog space, are still important in many areas of analog and mixed-signal components. The metrics for higher-performing analog and mixed-signal ICs, however, differ significantly from the digital measurements. A faster part is not necessarily a better analog or mixed-signal part.

In general, higher performance relates directly to the number of active devices in the circuit. Incorporating digital functions in analog circuits allows the circuits to achieve performance that approaches the ideal, enabling analog functions to be programmable in-circuit. But in the nondigital world, the need for more transistors must be balanced against the time to simulate a few hundred or thousand devices in Spice.

Analog designs always have very tight coupling between the circuit topology—the way that the internal components are interconnected—and the layout, or the physical implementation of that design. Because of a dependence on Spice-like tools for detailed device-level high-accuracy simulations, analog designs are never very large by most other measures. A mixed-signal design might have as many as 10,000 transistors, and most analog functions have less than a couple of hundred active devices in them. Because analog designs tend toward larger geometry devices for better matching, an analog process may be at least one process generation, if not more, behind the latest digital processes.

The challenge for IC manufacturers is to develop designs that maximize the abilities of the few analog IC designers out there. The analog IC design community always depends on engineers with a good understanding of semiconductor device physics and good mathematics abilities to design next-generation analog ICs. The tools and methodologies for analog design still rely on the knowledge, experience, and intuition of the designers, compared to the fairly high levels of automation for digital designs. Even though ICs tend to go to higher levels of integration over time, the parts in the analog and mixed-signal space only partially buck this trend.

One way that vendors address the need for more functionality is to put together many formerly discrete components into whole analog subsystems. These analog subsystems include the complete analog-signal-processing chain—amplifiers, filters, and converters—along with the interfaces and control signals for the digital handoff. In many cases, the subsystems include some level of programmability to allow for changes in circuit parameters to better optimize the subsystem in operation.

Because the digital processes continue to decrease the minimum device dimensions, the supply has to scale accordingly. This reduction in supply voltage creates two divergent trends. One is to design new analog functions that can operate at lower supplies. Consequences of the lower supplies are reduced dynamic range and increased sensitivity to noise. Analog precision is traded off for a simpler design with a single common power supply optimized for the low-voltage digital functions. Although the lower supply voltages help the digital function's power consumption, it hurts the power consumption of the analog sections.

The counter trend is to move all high-performance analog functions to a separate section of the design and operate on a separate analog supply. This supply diversity eases the design and layout by permitting almost complete analog and digital segregation. Concurrently, the separation doesn't cost too large a penalty in board space for extra regulators and supply filters.

One unique trend at the system level is the decrease in expertise and knowledge in the user engineering groups. The systems people don't have time to learn all of the details of the analog subsections, so they're deferring to the analog IC companies for expertise. The lack of analog knowledge in the user base is forcing these companies to make their parts more highly integrated and, hopefully, easier to use. By putting more functional units on a single chip, such as real-time clock, data converter, multiplexers, and the rest of the analog signal chain, they help users by eliminating a lot of the analog interfacing and design work.

TAGS: Intel
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