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Product Focus: A/D Converters

Facing customer pressure to reduce board space requirements and improve performance, makers of analog-to-digital converters are integrating signal-conditioning functionality into their products in hopes of furthering both objectives. Analog Devices, Linear Technology, Maxim Integrated Products, National Semiconductor and Texas Instruments are among those offering A/Ds that tout signal quality, hand customers the ability to control signal parameters or both. Typically these chips incorporate functions that would otherwise require one or several additional components.

For example, the MAX1463, from Maxim Integrated Products (Sunnyvale, Calif.), is a 16-bit delta-sigma A/D converter that integrates a programmable-gain amplifier (PGA) and a digital signal processor. The low-power, dual-channel device functions as a temperature- or pressure-sensing signal processor in process control and other industrial applications.




“Put the 1463 in a standalone system, and it feeds digital data back to the main host processor. The data is immediately ready for use,” said Tom Sparkman, Maxim's product-marketing manager for temperature sensors and signal-conditioning products. “Its size allows it to fit where multiple components would have been impossible, and it can perform complex functions that you wouldn't expect a sensor to do.”

Further up the resolution scale, the MSC1211 from Texas Instruments Inc. (Dallas), a 24-bit data acquisition system-on-chip introduced in April, integrates a 24-bit delta-sigma A/D with an enhanced 8051 processor core, flash memory and various on-chip peripherals. The enhanced core executes up to three times faster than the standard 8051 core and does so with lower power, said Skip Osgood, TI's market development manager for precision data converters and data acquisition products.

Osgood said the A/D in the MSC1211 offers a faster sampling rate than competing devices, with programmable gains of up to 128. At 75 nanovolts, it also offers lower noise performance than other mixed-signal designs, he said.

“In the old days you could have analog inputs up to plus/minus 10 V, but the trends toward lower-cost, lower-power devices and lower-voltage processes have put greater constraints on the noise floor,” Osgood noted. “When you lose dynamic range, you need lower-noise front ends.”

To reduce noise problems caused by switching transients, especially at sampling rates of 100 Msamples and higher, A/D makers are migrating to low-voltage differential-signaling (LVDS) outputs, with fixed dc load currents that minimize the likelihood of current spikes.

National Semiconductor Corp. (Santa Clara, Calif.) has combined a 10-bit, 40-Msample/second A/D with an LVDS serializer. “A/Ds operating at high speeds generate a lot of noise that results in lower dynamic performance and higher EMI electromagnetic interference,” said Suresh Ram, marketing manager for National's data conversion systems group. “LVDS outputs have been used in parallel mode to reduce noise by lowering the voltage swing, but serialized LVDS outputs improve dynamic performance and reduce EMI further. They also offer a much lower pin count and a significant reduction in the number of traces needed on a board.”

Ram cited as one example an ultrasound application with 128 A/D converter channels, where the integrated LVDS output structure with an embedded clock eliminated 1,024 traces.

“The technology saves board space, requires less time in manufacturing and improves reliability,” said Ram. National says it has working silicon and expects to introduce products combining an A/D and an LVDS serializer this year.

Lowering noise as well as trimming external component requirements, the 16-bit, 195-ksample/s delta-sigma AD7722 from Analog Devices Inc. (Norwood, Mass.) integrates two finite- impulse-response digital filters and thus limits external anti-alias filtering requirements to first-order, in most cases.

Group delay for the filter is 215.5 microseconds; settling time for a step input is 431 microseconds. An analog modulator samples analog input continuously, eliminating the need for external sample-and-hold circuitry. A 1-bit D/A in the modulator is said to boost linearity and dc accuracy, while endpoint accuracy is governed by on-chip calibration that also minimizes zero-scale and full-scale errors, the company said.

Analog Devices' 14-bit, 80-Msample/s AD9245, introduced in February, integrates a clock duty-cycle stabilizer that allows the device to maintain performance over a range of clock pulse widths. The converter supports input frequencies up to 100 MHz, which helps eliminate a downconversion stage and reduces the need for mixers and filters.

Another strategy for improving signal quality is to increase sampling rates. This tactic requires more system data-processing capability but also reduces performance demands on input anti-alias filters, increasing their effectiveness and lowering their cost.

Analog Devices' 14-bit AD6645, with sampling rates of 105 and 80 Msamples/s, is said to enable higher signal quality levels in wireless basestations and other applications that require high dynamic range.

The part's conversion rate is fast enough to enable intermediate-frequency sampling, which eliminates a downconversion stage and its associated components, thus enhancing a basestation's reliability.

It also helps designers to reduce costs by using less expensive analog filters, the company said. The AD6645 and a companion product, the AD6624 decimating digital receiver, allow designers to place undesirable harmonics outside a frequency band of interest.

Taking a different tack, Linear Technology Corp. (Milpitas, Calif.) doesn't integrate programmable-gain amplifiers and filters that, “by definition, place compromises on the part by assuming that what's been integrated is what the customer wanted most,” said product-marketing manager Erik Soule.

He said customers who want an external PGA can sometimes use a less-expensive A/D.

Linear Technology claims precise accuracy at output rates from 6.9 Hz to 3.5 kHz for its 24-bit delta-sigma LTC2440. Leveraging a patented, low-noise variable-speed, variable-resolution architecture, the converter features simultaneous 50/60-Hz rejection, 25 million counts at a plus/minus 2.5-V input range and 500,000 counts at a plus/minus 50-mV range.

“The way we designed the 2440 was harder than it would have been to integrate a programmable-gain amplifier,” said design engineer Mike Mayes, “but it offers more advantages. We have a very high oversampling ratio that gives us a higher rejection rate along with low RMS noise and full-scale accuracy.”

Company Contacts

Analog Devices Inc.
(800) ANALOGD (262-5643)
http://www.analog.com

Linear Technology Corp.
(408) 432-1900
http://www.linear.com

Maxim Integrated Products
(800) 998-8800
http://www.maxim-ic.com/

National Semiconductor Corp.
(800) 272-9959
http://www.nsc.com/

Texas Instruments Inc.
(800) 477-8924, ext. 4500
http://www.ti.com/

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