EE Times
April 16, 2001
A/D converters embrace CMOS for
high performance
By John H. Day
Leading
makers of high-end analog-to-digital converters have all but abandoned
Bi-CMOS process technology, which combines bipolar and CMOS elements, in
favor of all-CMOS, in a push to boost the performance of their most sophisticated
devices.
Analog Devices, Cirrus Logic, National Semiconductor, Signal Processing Technologies and Texas Instruments are among the companies that tout CMOS as the path to higher performance at lower power. They and other leading converter makers, like Maxim Integrated Products (Sunnyvale, Calif.) and Linear Technology Corp. (Milpitas, Calif.), are benefiting from design innovations that place them on the threshold of major advances. Sipex Corp. (Billerica, Mass.), meanwhile, is among the holdouts continuing to use BiCMOS technology.
"There is an exponential ramp under way in performance and volume, and customers are very excited," said Pat O'Doherty, product-line director for precision converters at Analog Devices Inc. (ADI; Norwood, Mass.). "Barriers are being knocked down not just for more accuracy in the precision space, but also for speed and cost-and we're achieving those gains in low-cost CMOS vs. 'boutique' BiCMOS."
O'Doherty said ADI has achieved a breakthrough in successive approximation
register A/D converters in CMOS. "Since the AD677 in 1992,
the focus has been on lowering cost and accepting lower performance," he
said, referring to the company's 16-bit, 100-ksample/second device. "But
now a new family is coming out that is blowing the doors off previous limitations."
He cited as an example the AD7676, which is sampling now and
due out this summer. The part offers a fivefold improvement in speed and
accuracy over the AD677, O'Doherty said, with integrated nonlinearity (INL)
of plus/minus 1.5 least significant bit and power consumption of 0.12 milliwatt
per ksamples/s, against 4 mW in the earlier device. Markets for the new
converter include medical, industrial, communications, automotive and motor
control. In fact, the AD677 was a two-chip device consisting of a BiCMOS
analog chip and a CMOS digital chip.
In 1997, ADI introduced the AD977, a single BiCMOS device
that offered twice the speed of the AD677, at 200 ksamples/s, but less
accuracy-an INL of plus/minus 2 LSB rather than plus/minus 1.5.
Now, with the CMOS single-chip AD7676, the company is increasing
the accuracy a little over the AD677, to nearer plus/minus 1 LSB, but ramping
speed to 500 ksamples/s. "A lot of effort has been put into CMOS within
the past year and a half," O'Doherty said, "and in the inexorable march
of design technology, we've found ways around problems that previously
caused bottlenecks."
O'Doherty said ADI's CMOS breakthroughs were related to the design of references and comparators.
Bipolar processes were embraced as a way of achieving higher accuracy and resolution, but manufacturers relying on bipolar incurred a penalty in wafer costs. "CMOS is driven by the digital industry and is very efficient," O'Doherty explained. CMOS requires 12 process steps, for example, against more than 20 for BiCMOS. "Also, with submicron CMOS, the circuitry is far more dense," he said. "Bipolar technology hasn't kept up. With CMOS we're getting silicon at a lower cost per unit area, and we've increased what we can put in a unit area."
ADI is using CMOS for its high-speed converters as well as for its precision devices. Jon Hall, marketing manager for the high-speed converter group, said Analog Devices is using 0.35-micron CMOS processes on two of its newer products: the 10-bit AD9214 series, which offers 105-Msample/s performance and power consumption of less than 0.5 W, and the soon-to-be-introduced, 12-bit AD9235, with 65-Msample/s performance and sub-400-mW power consumption.
National Semiconductor Corp. (Santa Clara, Calif.) is also firmly entrenched in the CMOS camp. "We're very focused on CMOS. The industry is headed that way," said Robert Eddy, marketing manager for National's data-conversion systems product line. "With pipeline and folded architectures, it's possible to achieve dynamic performance in CMOS comparable to that of bipolar devices, with significantly reduced power." National has a 12-bit, 40-Msample/s device in the works with an INL of plus/minus 0.7 LSB; signal, noise and distortion of better than 68 dB; and power consumption of 340 mW at 40 MHz. "It's rivaling bipolar products," Eddy said.
Eddy sees the major advantage of bipolar as "faster transistor speeds. But now, CMOS process geometry is smaller and faster-the die-size saving can be as much as 50 percent-and the effective number of bits from a bipolar or a CMOS converter is about the same, with CMOS at one-third to one-half the power." Faster devices tend to consume more power, he noted, but the power/speed ratio is dropping, thanks to innovative designs as well as better processes. National's 8-bit, 100-Msample/s ADC08100 consumes 130 mW, but a new part in development offers 200-Msample/s performance at 210 mW-double the speed with 60 percent more power.
"Ten years ago, a 100-Msample/s converter would dissipate as much as 2 W," Eddy said.
In Eddy's view, further developments in bipolar are unlikely. "I don't think [bipolar] can improve to a performance level exceeding that of CMOS at the speeds that are needed today. Eventually, processes like silicon germanium will provide faster speeds, with power consumption close to that of CMOS. They'll take over CMOS and completely displace bipolar. In the not-too-distant future we'll see 8-bit A/Ds at speeds in excess of 1 Gbit/s and 14- to 16-bit A/Ds at speeds of 100 Msamples/s and beyond." National has elected to focus on imaging and communications applications with a product line that consists largely of 8-, 10- and 12-bit converters. "We're using the A/D as a building block and 'up-integrating' to add value for our selected markets," Eddy said. Previously the company has integrated D/A converters, programmable-gain amplifiers and application-specific digital logic in analog front ends.
National alliances
National is working on a 10-bit, 40-MHz dual A/D, the two matched channels
of which can be used to simultaneously capture the I and Q signals in a
digital receiver or to reduce the component count in a portable ultrasound
system. On the communications front, Eddy said National has forged alliances
with "tier-one" companies for wireless local-loop applications, but he
declined to be more specific.
At Texas Instruments Inc. (Dallas), meanwhile, strategic-marketing manager Ed Pullmann said the company's 12-bit ADS809, ADS2806 and ADS-2807 A/Ds are manufactured on a 0.5-micron CMOS process. "We're concentrating on wideband applications, so it's important to have a wide Nyquist zone," Pullmann said. "We're conscious of power but focused on usability and performance over the Nyquist range, for CDMA applications."
TI product manager Robert Schreiber added that the 40-Msample/s ADS1251 and ADS1252, based on a delta-sigma architecture, offer 19.5 and just under 18 bits, respectively, of effective resolution, coupled with fast dc specifications. "The next closest is a 16-bit SAR [successive approximation register]," he said. "Delta-sigma offers higher resolution through the use of oversampling," said Schreiber. "With dc-accurate parts, the trade-off is between speed and accuracy. Our architecture can push the sampling rate while maintaining resolution. We use a fourth-order modulator and a fifth-order synch filter, compared with more typical second- and third-order modulators and filters respectively. The recent ADS1251 is optimized for customers that want to connect DSPs with converters."
For its part, Signal Processing Technologies Inc. (SPT; Colorado Springs, Colo.) sells converters for communications, video/imaging and instrumentation applications, three markets where designers are looking for more speed, more resolution and less power consumption, said converter-marketing manager Olav Lindquist. While power dissipation can be sacrificed for better noise or linearity performance in some applications, SPT is manufacturing converters on 0.35-, 0.25- and 0.18-micron CMOS processes for applications in portable devices. The company has also noted increased demand for multichannel converters. Ultrasound, for example, can encompass as many as 256 channels, with a converter required for each. Multichannel converters can lower system power, size and cost requirements. SPT's 16-bit, 5-Msample/s, self-calibrating SPT8100 for high-end imaging applications includes a programmable-gain amplifier, and Lindquist noted that three of the converters provide 48-bit color depth for sharper, clearer images and better throughput.
Gsample speeds
For instrumentation applications, the 6-bit SPT7610 and the
8-bit SPT7760 each sample at 1 Gsample/s. Lindquist said both offer
a wide-input bandwidth and low-input capacitance to eliminate the need
for external track-and-hold amplifiers.
The 8-bit, 250-Msample/s SPT7721 combines fast sampling with low (310-mW) power dissipation. The 12-bit, 20-Msample/s SPT7935 requires a single 3.3-V power supply and dissipates 79 mW; and the 10-bit, 40-Msample/s, dual-channel SPT7862 is said to consume about 20 percent less power than two single-channel A/Ds with comparable performance.
Maxim Integrated Products is focusing on battery-powered portable applications with the 8-bit, 100-ksample/s MAX1115-MAX1119 serial interface A/Ds it introduced last November. The parts combine an internal track-and-hold, voltage reference, clock and VDD monitor in an eight-pin SOT23 package. They operate from a single 2.7- to 3.6-V (MAX1115 and MAX1117), 4.5- to 5.5-V (MAX1116 and MAX1119) or 2.7- to 5.5-V (MAX1118) supply, and have an internal or external reference and one or two input channels. Power consumption is 175 microamps for all but the MAX1118, which consumes 135 microamps.
Linear Technology aims its high-end A/Ds at industrial and instrumentation
markets, as well as communications applications, said marketing manager
Todd Nelson. LTC was among the first to market with a 16-bit, 500-ksample/s
A/D, he said-the LTC1608. It offers 16-bit no-missing-codes performance
over the -40 degrees C to 85 degrees C temperature range, with no pipeline
delay. Applications include high-speed, multiplexed data acquisition, industrial
process control and instrumentation.
In February the company introduced the LTC2415, a 24-bit device
that offers a 15-Hz output rate at 60-Hz rejection with 1.1-microvolt rms
output noise, 2-ppm INL performance and differential nonlinearity that's
said to guarantee true 24-bit output with no missing codes. The device
is intended for use in weigh scales, temperature measurements, gas analyzers
and strain gauge transducers. "We're concentrating on high resolution,
where we have a lot of expertise," said Nelson, "and our converters balance
speed with precision."
On the BiCMOS front, Sipex uses mixed-signal and dielectrically isolated BiCMOS process technologies to produce high-performance analog ICs. Its strategy is to provide A/D converters within a portfolio of signal-conditioning and signal-processing functions that includes power regulation, interface, electroluminescent back-lighting and D/A converters. The company's A/D line includes 10- and 12-bit single and dual converters operating from 29 to 38 ksamples/s and drawing 230 to 250 microamps; and 12-bit converters from 40 to 333 ksamples/s with power dissipation ranging from 1 to 18 mA.
Cirrus' contribution
Cirrus Logic Inc. (Austin, Texas) last November introduced the CS5451
and CS-5471, six- and dual-channel A/Ds for industrial energy markets,
where three-phase power is widely used. Each includes an MCU serial interface
and an on-chip voltage reference.
Earlier Cirrus introductions include the Crystal CS53-L32A,
an audio A/D that consumes 9.7mW. It provides a 98-dB dynamic range, -88-dB
total harmonic distortion plus noise, a 12-dB variable-input gain in 1-dB
steps, single-ended inputs, a 2:1 input multiplexer and digital gain and
attenuation. It also integrates a microphone amplifier, yet is still said
to reduce board space requirements by 75 percent over existing solutions.
COMPANY CONTACTS
Analog Devices Inc. - www.analog.com
Cirrus Logic Inc. - www.cirrus.com
(Linear Technology Corp. - www.linear-tech.com
Maxim Integrated Products - www.maxim-ic.com
National Semiconductor Corp. - www.national.com
Signal Processing Technologies Inc. - www.spt.com
Sipex Corp. - www.sipex.com
Texas Instruments Inc.- www.ti.com
All material on this site Copyright © 2001 CMP Media Inc.
