Two A/D converters push architectural limits
San Mateo, Calif. - Analog Devices Inc. will announce two products this week that represent significant advances in analog-to-digital converter 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 the key specs at once, with 18-bit resolution, integral nonlinearity of plus/minus 2 least significant bits and 2-Msample/second performance. The device is general-purpose, said Mike Britchfield, product line director for ADI (Wilmington, Mass.), 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. The ability of the 7641 to crank out 2 million conversions per second on multiplexed, completely independent values can substantially reduce the number of converters required in a design, he said. This is particularly important in the CT-scan market, where low-end scanners are trying to reduce parts count and high-end scanners are adding huge numbers of data samples per scan.
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-bit 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.
Raising the dynamic range is not just a matter of adding bits and conversion cycles, of course, but requires strict attention to noise. The 7641 claims a 93-dB signal-to-noise ratio (SNR) and -100-dB total harmonic distortion. It might be a surprise, then, that the device uses a stock Taiwan Semiconductor Manufacturing Co. 0.25-micron logic process with only added poly-poly capacitors.
Even standard transistor designs 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.
Design manager Mike Coln used the on-chip voltage reference to illustrate the circuit work that has gone into the 7641. The reference achieves drift of less than 3 parts per million/ degrees C-better than many dedicated voltage references.
The designers implemented "a fairly conventional Brokaw bandgap cell," Coln said, "with the substrate pnp transistors that come as a part of the process." But then they added circuitry "to permit us to calibrate the part at final test. We calibrate not only absolute voltage output, but also the slope and the curvature of the drift curve. Those values are recorded digitally in the chip and are sufficient to keep the reference in spec over the life of the device."
Moving the voltage reference on-chip wasn't enough, Coln said. "A switched-capacitor successive-approximation circuit tends to kick a lot of noise back to the voltage reference. To get things to work well we had to design our own buffer for the reference. That in itself was a challenge."
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 100-dB SNR at full speed (120 dB at 50 k-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-tens of microvolts. But the thin oxides also mean very low signal swings.
"After looking at the situation, we worried we were creating the A/D converter that no one could drive," Brewer said. "So we decided to include the input differential amplifier on the device. That way we can do the level conversion and gain scaling functions, and the user doesn't have to worry about it. We just present a nice resistive load with 5-volt capability to the inputs." This was possible in part because the 7760 used a core of 0.25-micron CMOS and an added module of 0.5-micron CMOS transistors to give added voltage swing. Quarter-micron, thin-oxide transistors were used at critical nodes for their low thermal noise; the larger devices were used where signal swing was important.
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 25 nanoseconds," 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. And the unprecedented blending of analog and digital functions, often creating circuits in which the analog portions could not function without the supporting digital circuits, is a clear harbinger of the future.
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