EBN
(10/16/2000)
Technology Focus: A/D and D/A
Converters
By John H. Mayer
The
data-converter-IC market, like the analog-IC market in general, is famous
for its non-flashy stability compared with its digital counterparts. But
this year, it has been acting out of character.
Fueled by the growth in communications-infrastructure and emerging
portable-consumer applications, data-converter ICs are expected to grow
at a better than 40% annual clip. That's a remarkable achievement for a
market that for much of the past decade has averaged a compound annual
growth rate of about 6%, according to market research firm IC Insights
Inc., Scottsdale, Ariz.
“They may not be as glamorous as microprocessors or DRAMs, but demand is very strong,” said Jim Feldhan, president of Semico Research Corp., Phoenix. “Manufacturers have told us they're booked up and shipping as much as they can and would ship more if they can get their hands on more wafers.”
Through the first eight months of this year, shipments of data-conversion circuits reached $1.1 billion-roughly equivalent to the entire 12 months of 1999, which took in $1.2 billion, Feldhan said. “It looks like we're going to see pretty good growth this year,” he predicted.
Innovations
for wireless
While the wireless-infrastructure segment represents just one of many high-growth markets for data converters, it has been the driver for some of the industry's most innovative recent products. Many have been developed to support base- station designers' ongoing migration from single to multicarrier radio designs.
Unlike a traditional radio design, where each channel has its own receive and transmit chain with its own amplifier, filters, and mixers, the multicarrier approach promises to eliminate that parallelism by bringing those same channels into a single high-performance analog-to-digital converter (ADC). By replacing those multiple receive chains with a single high-performance radio where each RF carrier is processed in the digital domain, designers hope to scale down base stations' footprints, power consumption, and costs.
The problem for data-converter developers has been to design a device capable of meeting the stringent requirements of such a system for bandwidth, signal integrity, and dynamic range. While a 14-bit ADC will support the dynamic-range requirements of next-generation cellular architectures, combining high resolution and a high sampling rate in the same device has been no simple task.
To reach that goal, leading converter manufacturers have been pushing the performance limits of their product lines. Within the past year, Analog Devices Inc. announced the AD6644, a high-performance, 14-bit ADC that combines low distortion with extremely low noise.
Similarly, National Semiconductor Corp., Santa Clara, Calif., has brought to market a 14-bit ADC designed to meet the dynamic-range requirements of GSM1800, PCS, W-CDMA, and other high-performance wireless formats.
“SFDR [spurious free dynamic range] really needs to be outstanding in these apps,” said Jon Hall, strategic marketing manager for high-speed converters at Analog Devices, Norwood, Mass. “That's the key spec that third-generation W-CDMA architectures must meet to handle wideband signals.”
Most ADC manufacturers are shooting for SFDR of 100 dB, although the specific requirements vary with each wireless air interface. For signal-to-noise ratio (SNR), the industry target is around 75 dB or lower. National's CLC5958, for instance, features a SFDR of 90 dB and a SNR of 70 dB at 20 MHz.
More recently, vendors have been pushing the performance barriers of their direct IF-sampling devices. In August, Analog Devices introduced the AD9226, a 12-bit ADC that is targeted at IF-sampled wireless base- station receivers. “Basically, we built on its predecessor's specs by pushing the limits of our CMOS technology and improving the IF-sampling capability,” Hall said.
Converter manufacturers are facing a two-pronged challenge similar to other markets: Customers are demanding higher performance but with lower power consumption.
“What we're seeing is a lot of these cellular-infrastructure guys want to go to a picocell design where they're able to put a lot of these base stations out there so they have better coverage,” said Robert Eddy, marketing director for the data-converter systems product line within the analog division at National Semiconductor. “But they need to run these base stations off batteries that are solar recharged, so they'd like CMOS versions at about 12 bit or 14 bit and running at 66 MHz.”
“They basically want the same kind of functionality they currently get in a lower-power version,” Analog Devices' Hall said. Both Analog and National are working on new low-voltage versions of their devices.
IC manufacturers are also working hard to collapse the transmit chain of the base station into as few components as possible, and migrate as much functionality as they can to the digital domain. This would take advantage of direct processor control, on-the-fly programmability of data rates, and precision frequency hopping.
One goal is to “future-proof” designs by physically separating the RF interface block from the digital functions. That, in turn, will help designers deal with changing interface standards or system enhancements with simple re-spins of their digital ASIC and by keeping the same RF interface for different system designs.
Representative of this trend is a new device announced by Analog Devices in September. The AD9857, a 14-bit quadrature digital upconverter, combines a high-speed direct digital synthesizer, a 14-bit DAC (digital-to-analog converter), digital filter, and clock multiplier circuitry in a single 80-lead LQFP. The AD9857 accepts complex data in a 14-bit-wide data input and splits the data into two channels. It then modulates the output of the NCO in quadrature, recombines it, and upconverts to an agile frequency.
Earlier this year, Fujitsu Microelectronics Inc., San Jose, extended its DAC product line for cellular applications. The MB86060 is a 16-bit DAC with integrated interpolating filters to provide the wideband, high-IF synthesis required in cellular base stations. To maximize analog output, the filters supply data while driving the internal DAC core at up to four times the input data rate.
Data-converter manufacturers are also setting their sights on broadband-communications applications such as local multipoint distribution services (LMDS), a microwave-based high-speed broadband-access technology for data, voice, and video transmission. Just this month, Analog Devices announced a new 10-bit ADC that company executives claim can boost broadband data capability by as much as 200%. The AD9410 features a 500-MHz input bandwidth and SNR of 54 dB at 99 MHz. At the same time, Analog Devices has added the AD9412, a low-voltage 10-bit device for applications such as multichannel multipoint distribution services (MMDS) or home powerline networks.
For 8-bit communications and networking systems, National Semiconductor has developed a new ADC that features no signal-to-noise and distortion degradation through a clock frequency range of 20 to 100 million samples per second (MSPS). Based on a switched-capacitor bandgap design, the ADC08100's power consumption is a linear function of its clock rate. “The power actually scales with the ADC frequency, so if you run it at a lower speed, the power is lower,” Eddy said. “There's no other ADC on the market that'll do that.” It sells for $5.25 in 1,000-piece quantities.
Powering portables
In high-growth portable consumer markets, small system footprint and low power dissipation remain the driving forces behind data-converter design. “Form factor and issues such as the height of a package are very important,” Eddy said.
Last month, National Semiconductor announced a pair of ADCs that feature a unique package design. The devices come in a 24-pin leadless leadframe package that measures a mere 4 ¥ 5 mm and stands only 0.8 mm high. With this proprietary package design, the converters occupy 2.5 times less board area than comparable devices in a traditional 24-pin TSSOP, National executives said.
“It's unique not only because it's small, but because it features very small wires from the pads to the die, so inductance is very low,” Eddy said. The package was designed to fit the limited clearance in ever-shrinking portable camcorders, CD players, mini-disks, and other handheld devices. The package offers a pin pitch of just 0.5 mm.
The ADCs also meet the power requirements of portable markets. The ADC08351 typically consumes just 40 mW from a 3-V supply. A power-down mode drops that to 7 mW. The device also offers strong dynamic performance with a SNR of 45 dB. It sells for $2.25 in 1,000-piece volumes.
National's second device, the 8-bit ADC1175-50, is a 5-V part targeted at wired applications in printers, copiers, and other video and imaging equipment. National is currently selling this device at $2.50 in 1,000s.
Low-power data converters are also playing an important role in the portable-instrumentation market. Linear- and mixed-signal-IC supplier TelCom Semiconductor Inc., Mountain View, Calif., recently lowered power dissipation in two new products by removing some of the power-management interface circuitry it used in earlier products. The TC3400 and TC3402 ADCs are two new members of TelCom's 16-bit Delta-Sigma converter family. The two converters operate from an unusually wide input range-1.8 to 5.5 V-and support serial communications with a system microcontroller via a two-wire MicroPort interface.
“By eliminating some of the power-management interface circuitry, we can offer these two devices with lower quiescent power dissipation in a smaller, reduced-cost package,” said Don Ashley, manager of advanced products marketing. Both devices automatically enter sleep mode when not in use.
Each device supports speeds adjustable from 8 conversions per second at 16-bit resolution to 12 conversions per second at 10-bit resolution. The TC3400 features a single differential input while the TC3402 supplies four differential inputs with a built-in precision analog multiplexer. Pricing starts at $2.58 and $2.85, respectively.
Low power isn't a design criterion only in portable applications. Many OEMs building communications systems, automated test equipment, and video displays are looking for ADCs with low power but not necessarily at the expense of sample rates. An ADC with a high sample rate can help a display designer support higher-resolution products yet be under similar constraints when it comes to power.
The SPT7721 from Signal Processing Technologies Inc., Colorado Springs, Colo., attempts to offer the best of both worlds by combining 250 MSPS in an 8-bit ADC that dissipates only 310 mW. Priced at $15 in 1,000-piece quantities, the device sells for a fraction of the price of competitive parts, according to Robert Roper, SPT's director of worldwide sales and marketing.
For those low-power applications in high-speed communications, imaging, and instrumentation requiring wide bandwidth, Maxim Integrated Products Inc., Sunnyvale, Calif., has developed a family of 3-V, 10-bit monolithic ADCs. At the high-performance end of the family, the 80-MSPS MAX1448 delivers SFDR of 70 dB at 40-MHz input frequency while consuming only 118 mW of power. Key to the MAX1448's performance is an innovative differential pipelined architecture, according to Maxim executives. The device family supports both differential and single-ended input configurations.
New DVD-based audio applications are also driving innovations in data-converter designs. Late last month, Cirrus Logic Inc., Austin, Texas, announced the CS4392, a new DAC offering 114-dB dynamic range. Integrated selectable digital filters support design flexibility by enabling designers to customize system performance to maximize the listening performance of the user for any given application.
Targeting motion control
High-performance, high-resolution DACs are also seeing growing demand in the single-supply-type systems typically found in industrial process-control and fiber-optic-communications applications.
Designed to support low-power operation and provide a voltage output that will support fast settling in single and low-voltage dual-supply applications, the DAC7631 and DAC7641 from Burr-Brown, Tucson, Ariz., support the conversion of a digital output from a microprocessor into the analog output required by robotics, motor-control, and equipment-motion controllers. Both devices accept 16-bit data input and feature double-buffered DAC-input logic structure to support synchronous updates. For industrial-control applications, the DAC7641 adds a data readback mode to the internal input registers. Packaged in an SSOP-20, the DAC7631 is priced from $5.50 in 1,000-piece quantities. The DAC7641 sells from $5.85 in similar quantities and is packaged in a TQFP-32.
ADCs play a crucial role in industrial applications by converting analog inputs of temperature or pressure, for instance, into a digital signal that can be used by a microproecesor or a DSP to enable control at a high level of precision. Many data-converter vendors offer higher-resolution upgrades to existing 12-bit products to better performance.
A case in point is the ADS8341 brought to market by Texas Instruments Inc., Dallas, after it acquired Burr-Brown earlier this year. The four-channel ADC features 16-bit resolution and 2.7- to 5-V operation. The part also saves power by running at 8 mW at 100 kHz. Packaged in a 16-pin SSOP, pricing for the ADS8341 starts at $7 in voluime quantities.
For remote-sensing applications, Linear Technology Corp., Milpitas, Calif., has developed a compact, 24-bit delta-sigma ADC that is less than half the size of an SO-8. Capable of fitting inside the shell of a sensor or transducer, the LTC2411 features a proprietary no-latency architecture that allows its digital filter to settle in a single cycle. That eliminates the need to throw away data after the input signal step, as required by many other high-resolution ADCs. The compact device packs the ADC, differential inputs, and reference into a tiny MSOP. Pricing begins at $7.70 in large volumes.
The market for high-resolution 16-bit ADCs is growing by leaps and bounds, particularly as a wide range of imaging technologies mature. The SPT8100 from Signal Processing Technologies, for example, combines a fast monolithic 16-bit ADC with a programmable gain amplifier. “It's ideal for high-end scanners and digital copiers as well as for infrared imaging and high-end CCD imaging,” SPT's Roper said. “With 16 bits of resolution, three SPT8100s can increase a scanner or digital copier's resolution or color depth to 48 bits, providing sharper, clearer images and enhanced color to meet the demands of the high-end graphics industry.”
Copyright © 2000 CMP Media, Inc
