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issue: March 2004 APPLIANCE Magazine

Electronics Report
Quad Analog-to-Digital Converter

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Analog Devices Inc. offers what it claims is the industry's first quad analog-to-digital converters (ADCs) for medical and communications appliances.

Analog Devices Inc., a Norwood, MA, U.S.-based developer of semiconductors for signal processing applications and data converters, has integrated four ADCs into one chip in an effort to solve the space problems of particular applications such as medical imaging systems, hand-held test equipment, and wireless infrastructure, says Tom Tice, product line manager for Analog Devices' high-speed converters.


Some converter systems may have the analog portion in only a corner of the board, with the rest being digital. In others, there may be a separate board for the analog portions of the system. With Analog Devices' quad ADCs (pictured), "it would be entirely possible to route these signals through a back plane to a separate board," says Tom Tice, product line manager for Analog Devices' high-speed converters.
For example, the medical ultrasound market uses digital beam forming, and these systems require a large number of receiver channels, Mr. Tice explains. A typical system may have 128-receiver channels, which would require 128 ADCs. However, by using a serial low-voltage differential-signaling (LVDS) data output, four ADCS can be integrated onto one chip. These LVDS outputs reportedly reduce pin count, package size, number of board traces, and substrate noise. The interconnectivity of the chips may also be a concern when using converters. With the LVDS, all output signals are combined or converted into a serial format and reportedly come out at a rate of 12 times the sampling rate of the converter.

"The demand for increased system resolution and speed is driving the number of ADCs used in these systems higher, so manufacturers are striving to pack more converters into smaller spaces," says Kevin Kattmann, product line director for High-Speed Converters, Analog Devices Inc. "The serial LVDS digital outputs allow the new quad ADCs to radically simplify board layout, enabling more data conversion paths to be routed on a board."

Mr. Tice says the first members of the new AD92x9 quad ADC family are the 12-bit 50-/65-MSPS (mega samples per second) D 9229, and the 8-bit 65-MSPS AD9828. Mr. Tice says both feature the highest signal-to-noise (SNR) ration and spurious-free dynamic range (SFDR) of any converters within their resolution grades currently on the market. Instead of having several outputs coming out parallel on the converters, there is a serial channel that is 750 megabits per second - with each one having a differential output. "Not only does going to a serial output allow us to have fewer lines, but it allows us the luxury of having two outputs per channel," Mr. Tice explains. "Having a LVDS output or differential output greatly reduces the noise that comes off the board and also the noise that comes off the substrate in the device. By having this very high-speed output, we can have a very high-speed serial output with very little noise with coupling from the high-speed data that is switching."

One of the problems with high-speed ADCs is that output lines can't go very far or their signal will be degraded because they are so fast, which may result in data loss, Mr. Tice points out. Although there have been continued arguments as to whether the ADC should be placed on the analog or the digital side, engineers and designers have been forced to put them on the digital section.

"In a lot of cases, you are sacrificing some of the performance of the ADC because you have it on the board with all the digital components that are creating a lot of noise - and perhaps corrupting the input," Mr. Tice notes. However, with Analog's new ADCs, Mr. Tice says that the serial LVDS stream is a low-noise interface and is designed to go for significantly longer distances. "This now allows the system designers to put the ADCs wherever they would like," Mr. Tice says. Not only could this lead to better performance for the systems, he adds, "but it is going to open up a lot of new possibilities for system partitioning."


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