Modern electronic equipment for military and aerospace applications presents systems designers with a fundamental contradiction: the real world we observe with electronic and electro- optical sensors is analog, yet the way we process sensor information is digital. There has to be a fast, efficient, and accurate way to translate analog sensor information into digital data for efficient processing, and to translate digital data back to analog for wireless transmission and - let's face it - for human understanding. Enter the modern analog- to- digital (AD) and digital-to-analog (D- A) converters. These devices, designed and integrated by a handful of industry specialists, represent the crucial glue that links the analog world to the digital. Without them, much of today's high- performance digital processors would be useless in RF and electrooptical applications like radar, softwaredefined radio, electronic warfare, missile guidance, high-end test and measurement equipment, and counter improvised explosive device (IED) systems.

Instead, systems designers would have to rely on complex and expensive analog data processors, and most likely would render information not as quickly and accurately as do today's systems that use modern A-D and D-A converters. With the plethora of widely available, high-performance, and relatively inexpensive digital processing available today, such as multiprocessing embedded computers and field-programmable gate arrays (FPGAs), the value of the A-D converter cannot be understated.

"Typically the A-D converter (ADC) is selected first, and then the [digital] signal processing path behind the ADC; it's one of the long poles in the tent," explains Phil Lopresti, director of the high-speed data converter products at A-D converter manufacturer Intersil Corp. in Austin, Texas. The A-D and D-A converter "really defines the overall system capability and system limits," adds Mike Althar, vice president and general manager of the Intersil Special Products segment in Palm Bay, FIa.

Radar is one of the driving applications for the advancement of analog-to-digital converter technology, as radar systems designers constantly need faster, higherresolution parts to keep up with modern threats.

As designers use the A-D and D-A con- verters as bridges between analog and digi- tal data, they also must balance the amount of processing necessary for each realm. "It is always a compromise in the processing you do in the analog part of the world, and processing you do once the data becomes digital," says Andrew Reddig, president and chief technology officer at TEK Microsystems Inc., a high-performance signal processing specialist in Chelmsford, Mass. "It's easy to do lots of manipulations once you get a signal into the digital realm," Reddig explains. "Analog processing is complicated and very expensive."

One goal of systems integrators is to place the A-D converter as close as possible to the sensor gathering the analog signal - typically an antenna or electro-optical sensor. In this way, they can simplify their architectures and do the maximum amount of processing digitally, which is fast and inexpensive.

Performance in balance

A-D and D-A converters essentially have four crucial benchmarks that enable manufacturers and users to characterize their performance for different kinds of applications:

1) speed, or bandwidth, as measured in MHz or megas amples per second (the larger the number of megasamples per second, the faster the device);

2) resolution, or accuracy, as measured in bits (the larger the number of bits, the higher the resolution of the device);

3) noise and distortion rejection, as measured in decibels, or dB (the higher the decibel level, the better the device rejects noise and distortion); and

4) size, weight, and power consumption.

The speed of the A-D and D-A converter describes how many signal samples per second the device can process. Typically this is measured in MHz, or millions samples per second. Some devices can take only hundreds of samples per second, and some of today's fastest devices can take 3 billion samples per second (gigas amples).

The resolution of A-D and D-A converters refers to the detail and depth of each sample taken, and is measured in bits. The higher the number of bits, the more detailed is each sample taken. High resolution is particularly important in applications like imaging radar that must discern small objects close by large objects, or in signals intelligence that must be able to characterize even the faintest radio signals in the presence of many other signals and electronic noise.

Noise and distortion rejection is measured in two ways. The first is spurious noise dynamic range (SNDR), and the second is signal-to- noise ratio (SNR), both of which are measured in decibels, or dB. The higher the dB level of these two measurements, the better the A- D or D-A is at detecting and characterizing weak signals that may be important. Strong noise and distortion rejection is particularly important for applications like signals intelligence, radio communications, or sophisticated radar jammers.

Some A-D and D-A makers will list a device's "useful bits" rating for resolution, which considers not only the part's resolution, but also its noise and distortion rejection ability. An A-D or D-A may have a high bit resolution rating, but poor noise rejection, which effectively would reduce its useful resolution bits rating.