Electronic Design

Analog/Mixed-Signal> Overview

Analog/Mixed-Signal: Application Needs Inspire Groundbreaking Innovations

Developers of analog and mixed-signal ICs have many options at hand to enhance the value of their products. They can soup up key ac and dc performance specifications. They can shrink device size or add more functionality to a given-sized device. Or, they can find ways to provide the same functionality or performance with lower power consumption. Very often, IC manufacturers will chase lower cost.

Rarely, if ever, can they pursue all of these goals simultaneously when developing a single chip. But they are able to tackle one or more of these objectives at a time. In some cases, chip makers can develop families of components that allow customers to make tradeoffs in performance, packaging, and price that best suit their application.

The application is the common thread among all development paths for new components. Sometimes, there's no single application but a set of applications that shapes product development. This is common in the data-converter area.

Consider the high-speed category of analog-to-digital converters (ADCs). One ongoing trend involves the creation of faster pipeline converters that can sample higher and higher intermediate frequencies for communications designs. This trend is being driven by system requirements for more integrated, less costly pc-board designs based on a single stage of frequency conversion, rather than two stages.

There are other, similar examples in data conversion in which boosting the conversion speed, accuracy, or number of channels per device can significantly enhance system design. But these main trends also lead to other subtrends. For example, as conversion rates began rising into the hundreds of megasamples per second, noise-related problems started creeping into designs with older CMOS interfaces. This led to the adoption of low-voltage differential signaling (LVDS) as a means of getting the data out of high-speed ADCs without compromising their dynamic performance.

However, a technology that takes root by solving a problem in one area often spurs other interesting developments. In the case of LVDS, the use of parallel LVDS among high-speed ADCs was followed by the introduction of serial LVDS.

The latter technology has made it possible to build speedy data converters in tiny, low-pin-count packages. It also has enabled multiple channels of high-speed analog-to-digital conversion to be combined in some of those same packages. As a result, not only does LVDS bring about high-speed operation, it also helps to further shrink system designs.

Aside from the increased use of LVDS among data converters, interface developments have helped spur on a host of analog and mixed-signal innovations. For example, industrial applications are driving the development of more digital-to-analog converters (DACs) with bipolar outputs, despite the general trend toward low-voltage, single-supply CMOS designs. In the op-amp area, more components now feature rail-to-rail inputs and outputs.

Meanwhile, functional integration lies at the heart of many analog and mixed-signal developments. Within the data-converter spectrum, this can mean the integration of precision voltage references or amplifiers on-chip. Or, it may involve the combination of ADCs with DACs or other elements of the signal chain.

In some cases, vendors now incorporate a measure of programmability into their data converters. That trend has also touched the amplifier area, where programmability offers a means of nulling out offset and gain errors in the system. This development relates to the increasing focus on application-specific requirements as well, because programmable amplifiers are often tailored to perform signal conditioning for particular types of sensors.

Many of these trends are enabled by the migration to more advanced CMOS processes based on finer lithography. However, in the analog world, moving from one submicron process to the next has become difficult. As the supply voltage falls below 3.3 V, it becomes harder to maintain good signal-to-noise performance with each step down in voltage. That's why mixed-signal parts like ADCs are typically a couple of process generations behind leading-edge digital ICs.

Because migrating designs from one CMOS process to another is no cakewalk, chip developers look for alternatives ranging from innovations in circuit architecture to the co-packaging of multiple die.

System-in-a-package techniques such as die stacking are now moving beyond their initial applications with memory and logic, ultimately promising new possibilities for data converters and other mixed-signal designs. At the same time, vendors aiming to get to higher speeds are breaking away from parasitic-laden bond wires and leadframes and looking to BGA-style packages with shorter interconnects.

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