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Analog Devices ADCs push architectural limits

Posted: 02 Jul 2004 ?? ?Print Version ?Bookmark and Share

Keywords:analog devices? adi? ad7641? ad7760? adc?

Analog Devices Inc. (ADI) will announce two products that represent significant advances in analog-to-digital converter (ADC) technology.

The AD7641 successive-approximation converter and the AD7760 sigma-delta converter highlight the outer borders of the possible in production mixed-signal technology as well as the remarkable paths chip designers are taking to reach that edge.

Each part sets a new standard of performance for ADI, and each shows the fingerprints of a very different, very intensive engineering effort.

The 7641 pushes all key specs at once, with 18-bit resolution, integral nonlinearity of plus/minus 2 least significant bits and 2Msps performance. The device is general purpose, said Mike Britchfield, product line director for ADI, but would be ideal for a variety of instrumentation and medical applications in which high-resolution conversions are done on a multiplexed stream of analog signals.

Another factor in reducing cost and design hassles for 7641 users will be the number of external parts the chip can eliminate. By moving from 16- to 18-bit resolution, marketing manager Leo McHugh said, the dynamic range of the converter is increased enough to eliminate programmable-gain amplifiers on the front-ends of many designs.

Standard transistor designs also are used, but with thoughtful circuit design. Often a traditional device, such as an analog switching transistor, will be surrounded by considerable digital logic to improve its behavior. That approach is vital to a device that must make 70 million precise comparisons per second, the finest of them in the tens of microvolts.

The 7760 is another example of applying leading-edge thinking to a mature process. This 2.5-mega-update/s oversampling sigma-delta converter has 20-bit resolution and 100dB SNR at full speed (120dB at 50 kilo-updates/s). It brings the precision and low in-band noise of sigma-delta techniques to a new range of signal frequencies.

The design is relatively conventional looking, with the usual sigma-delta modulator, reconstruction circuitry, programmable decimator and output filter. Virtually everything that goes on in the chip is under digital control, including the output FIR filter, which is somewhat user-programmable.

The 7760 also integrates some critical components to ease things for users. One is the input differential amplifier. "When we originally conceived the 7760, we didn't have a differential amp on the part," said Bob Brewer, manager of the Newbury, England, design center. "But as we got into the design of the sigma-delta modulator, we saw two things. First, we were getting a lot of benefit out of the fast, thin-oxide transistors in the quarter-micron process. They have a very high transconductance, which in turn means a very low thermal noise floor. But the thin oxides also mean very low signal swings.

In the CMOS multipath amp, low- and high-signal bands are routed along different paths. "At these speeds we are charging up a pretty large capacitor with a few PPM accuracy in about 25ns," Brewer said. "That takes a good gain-bandwidth product." Other amps in the switched-capacitor design go back to an early technique in precision amplifier design: chopper stabilization to control flicker noise.

The two devices are indicative of a trend in analog design to explore deep-submicron transistors. Willingness to do innovative design work in order to use existing digital-process transistor and passive-device models is also clear in those efforts.

- Ron Wilson

EE Times

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