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Focus on D/A converters: D/As ride LVDS to gigasample realm

As signal-processing requirements increase in speed and complexity, system designers require data converters that can synthesize high-quality signals at much higher frequencies than ever before. Fujitsu Microelectronics America Inc. and Analog Devices Inc. are meeting these needs using low-voltage differential-signaling (LVDS) inputs to boost sampling rates in digital-to-analog converters.

LVDS results in improved performance in D/A converters with output frequencies above 100 MHz. The conventional digital interface for D/A converters has been limited to 250 to 300 Msamples/second and is also single-ended-with voltage swings of the full power supply. The 300-Msample/s data rate limits D/A converter output frequency synthesis to 150 MHz.

The LVDS interface, on the other hand, allows data rates up to 840 MHz, or 840 Msamples/s. LVDS is also a differential signal with lower voltage swings. Thus, it will be much less likely to couple into the critical analog-circuit sections, creating additional unwanted noise, said Thomas Guy, product line manager for D/A converters at Analog Devices (Norwood, Mass.). “So when applications require signal synthesis of the D/A converter output above 150 MHz, LVDS enables this without having to multiplex multiple ports that can create unwanted tones at the multiplex rate,” he said.

Fujitsu (Sunnyvale, Calif.) was the first company to begin mass-producing a 14-bit gigasample D/A converter. The company increased the performance of its 14-bit CMOS D/A converter from 800 Msamples/s to 1 Gsample/s (MB86064) using LVDS inputs. The move was timed to align itself with the rising LVDS I/O rates of FPGAs, said Paul Maddox, technical marketing manager for Fujitsu Microelectronics Europe.


“The increase to 1 Gsample/s is key to addressing a variety of new market segments that have highlighted a need for speed following Fujitsu's 800-Msample/s D/A converter launch last year,” Maddox said.

The raw bandwidth achievable using an LVDS interface is what has pushed Analog Devices into this market, said David Carr, marketing manager for high-speed converters. ADI's AD9736 (see diagram below), 14-bit transmit D/A converter (TxDAC) clocks at an impressive 1.2-Gsample/s sample rate. Indeed, Carr maintained that the converter sets a new data rate benchmark, breaking the 1,000-Msample/s speed barrier. Production quantities are slated for February. Meanwhile, ADI can support preproduction quantities (hundreds of units) until then.


ADI could not have achieved this data-sampling rate without LVDS, Carr said. The only other way to get there would be to multiplex numerous inputs or to use some other interface. “In the past, we'd have to multiplex four, 300-Msample/s buses together inside the D/A converter to get to 1.2 Gsample/s,” Carr said.

Using LVDS, a 14-bit bus would have 28 lines, while multiplexing four buses would result in twice as many pins (28 x 2). “FPGAs and ASICs get more expensive when you have more pins,” Carr added. Prior to utilizing LVDS, ADI's fastest D/A converters provided a sampling rate of 400 Msamples/s.

Both National Semiconductor Corp. (Santa Clara, Calif.) and Intersil Corp. (Milpitas, Calif.) confirmed they have plans to produce D/A converters with LVDS inputs.

“It is our intention to integrate LVDS technology into our high-speed D/A converters within the next year, assuming there is continued customer interest,” said Leonardo Azevedo, National Semiconductor's product-marketing manager for high-speed A/D converters.

ADI's AD9736 provides a fast LVDS input interface using a double-data-rate mode, which enables high conversion rates over a wide bandwidth. This allows it to receive data at a high speed, while maintaining low distortion and noise, simplifying the transmit signal chain and enabling high-quality synthesis of wideband signals at intermediate frequencies up to the Nyquist rate (one half of the D/A converter sampling rate).

A combination of applications is driving demand for these products, including military/aerospace, different types of electronic intelligence and warfare, and telemetry in the aerospace area. These types of applications require very wide-bandwidth signals, Carr said. ADI is also seeing interest in cellular basestations. Although D/A converters were used in these applications previously, they are being used in a different way now, Carr said.
The main attraction for cellular-basestation makers is not so much the speed of the D/A converters. But with the higher sample rate, there is a lot more flexibility in the frequency placement of the output signal, he said. A very low-frequency signal, such as voice, is encoded digitally, then turned into an analog signal by the D/A converter. Usually a bunch of RF components is needed to transmit at a higher frequency.

Higher-speed D/A converters like the AD9736 allow the first conversion process to happen at a much higher frequency so they don't have to upconvert as far. Consequently, the bill-of-materials cost is lower since there are fewer components, but the design is also simplified because frequency planning is easier, Carr said. RF designers need to consider how different clock frequencies and signal frequencies can mix and intermodulate in an undesired fashion, Carr said. “Fewer upconversion stages means less to worry about, and this makes the design more manufacturable and robust,” he added.

Another market in which ADI is seeing interest is cable-related services, such as cable TV, video-on-demand and high-speed Internet access. Currently, expensive RF techniques are being used to transmit signals and upconvert them. “Our D/A converters could displace part of the RF chain,” Carr said. “Right now, we still can't produce the signal, though. The cable plant operates over a large bandwidth, up to 860 MHz, and the D/A converter can't directly synthesize this entire bandwidth. ADI is working toward this goal, though.”

In addition to speed, ADI's AD9736 features low power dissipation, suiting it for applications that require a low-power D/A converter to process high-frequency, wide synthesis bandwidth signals. These applications include high-bandwidth test-and-measurement equipment, radar, avionics and wideband communications, such as point-to-point wireless, local multipoint distribution systems and power amplifier linearization.

The AD9736 operates from 1.8- and 3.3-volt supplies, consuming 380 milliwatts at 1.2 Gsamples/s, with the interpolation filter bypassed, and 550 mW with the interpolation filter enabled. Conversion in the AD9736 is initiated on the rising edge of each input clock at the full D/A converter sample rate. Sampling only on the rising clock edge eliminates potential performance problems related to clock duty cycle sensitivity, the company said.
D/A converters that sample on both rising and falling clock edges can exhibit noise feedthrough of the half-rate clock if a nearly perfect 50 percent duty cycle is not maintained. Even small variations in duty cycle can create significant half-rate spurs and images that degrade spurious-free dynamic-range (SFDR) performance over the Nyquist bandwidth.

“The AD9736 clocking architecture renders it largely insensitive to clock duty cycle variations,” Carr said. “That makes our part easier to use, since other, competing parts may require tight control.”

SFDR for the AD9736 is 63 dBc at a 300-MHz output frequency and 53 dBc at 600 MHz, sampling at 1.2 Gsamples/s. Offered in a 160-pin ball grid array package, the AD9736 is priced at $34.95 in 1,000-piece quantities.

For more technical information about the AD9736, please click here: www.analog.com/AD9736.

Fujitsu's D/A converters feature a gain in quoted adjacent-channel leakage ratio performance, according to Maddox. Completion of characterization prior to production approval showed the improvement, he said.

“The figure, when generating four Universal Mobile Telecommunications System carriers at 276 MHz, has moved to 74 dBc in the 1-Gsample/s version, from 85 dBc in the 800-Msample/s product,” Maddox said, using a D/A conversion rate of 737 Msamples/s.

Pricing for Fujitsu's MB86064 started out at $120 in 1,000-piece quantities when the products were initially announced in March. Since then, however, prices have dropped to $79 each, Maddox said.

For additional technical information on Fujitsu’s MB86064 please click here:

Texas Instruments Inc. (Dallas) was the first company on the scene with a megasample-version D/A converter with LVDS. Production volumes began in May 2002. The 14-bit, 400-Msample/s DAC5675 converter is aimed at multicarrier wireless base transceiver stations. It enables transmission of 3G multicarrier wideband CDMA and supports additional major wireless standards, such as cdma2000, IS-95, GSM, GPRS, Edge and IS-136, as well as other digital applications based on quadrature-amplitude and quadrature phase-shift-keying modulation techniques. Packaged in a 48-lead HTQFP with PowerPad, the converter is priced at $25 in 1,000-piece quantities.

TI sees LVDS as a good interface for high-speed A/D converters in the gigahertz range, said Ed Fullman, product-marketing manager for TI's high-speed data converter products.

Company Contacts:

Analog Devices Inc.
www.analogdevices.com

Call: 1-800-Analog-D

Fujitsu Microelectronics America, Inc.
www.fujitsu.com/us/

Call: (800) 866-8608

Intersil Corp.
www.intersil.com

Call: (408) 935-4300

National Semiconductor Corp.
www.national.com

Call: (408) 721-5000

Texas Instruments, Inc.
www.ti.com

Call: (972) 644-5580

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