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A/D converters break power barrier

Munich, Germany — Xignal Technologies AG claims to have launched the lowest power, 14-bit and 12-bit, 40-MHz analog-to-digital (A/D) converters.

Based on the company's continuous time delta-sigma technology, the 12-bit (XT11200) and 14-bit (XT11400) devices consume only 70 mW — while operating at 20 to 40 Msamples/second. “This is half the power consumption of the nearest best-in-class alternatives and the first time such performance has been available at the sub-100 mW power level,” said Mark Holdaway, Xignal's marketing director.

The 70-mW power consumption spec includes power consumed by the crystal oscillator and phase-locked loop (PLL), according to Holdaway. “With traditional pipeline converters, this circuitry is external to the A/D converter. Striping out these blocks would give the devices an equivalent power draw of approximately 55 mW, instead of 70 mW,” he said.

Despite offering a power figure of merit (FOM) that is half that of current pipeline A/D converters, there is no trade-off in linearity or electrical performance, Holdaway said. The XT11400 has a SNR of 76 dB and total harmonic distortion (THD) of -82 dB. The XT11200 turns in an SNR of 71 dB and THD of -78 dB.

The A/D converters, intended for medical imaging, ultrasound, radar-based applications, communication systems, image sensing, and test and measurement, are easy to drive, and require no differential input buffer, Holdaway said. The converters can handle 4-V peak to peak input signals while operating from a 1.2-V dc supply.

The XT11 A/D converter family utilizes a fast, third-order continuous time delta-sigma modulator, combined with an on-chip digital filter and tuneable loop filter. These circuit innovations substantially reduce the design effort needed to deploy a high performance data acquisition system, Holdaway said. “In addition, the architecture eliminates the need for external anti-aliasing filters — allowing the A/D converter to sample the entire first Nyquist frequency zone (0 to 20 MHz) with almost no wasted bandwidth,” he said.

The converters utilize a proprietary self-clocking circuit that eliminates the need for an external highly accurate, complex clocking scheme. The on-chip clock is driven from an inexpensive external crystal (ranging from 13.5 to 27 MHz). An on-chip inductive resonator based PLL generates a clean (low jitter) sample clock that is also brought to an external pin and made available for use as an accurate reference clock for other components on the printed circuit board.

Unlike conventional pipeline A/D converters, Xignal's XT11 family requires no sample and hold circuit to function, but rather uses a simple, current driven (resistive) input stage, according to Holdaway.

The XT11400 and XT11200 are pin-compatible and are available in a 6 x 6mm, 40-pin QFN package. Evaluation kits will be available at the time of sampling.

The XT11400 is sampling now with production slated for Q1, 2006. Pricing of the XT11400 in 1,000s is $18.00. The XT11200 will start sampling in December with production quantities planned for Q2, 2006. Pricing for the XT11200 is $9.95 in same quantities. Click here to request the data sheets.

Xignal Technologies AG , + 011-49-89-32-22-720, www.Xignal.com.

The A/D converter market has grown somewhat stale in terms of technology advancements, according to newcomer Xignal Technologies. The company hopes to stir things up a bit with its first A/D converters. In addition to being the lowest power 12- and 14-bit converters in the industry, Xignal also claims to have conquered the complicated signal path beast with a much simpler design.

This is Xignal's second major announcement in eight weeks. In September, the fabless company unveiled its Continuous Time Delta-Sigma architecture that was used to create these converters. Essentially, the architecture provides high resolution at MHz speeds with significantly improved power consumption compared to pipelined A/D converters, Holdaway said.

Click here for more details on Xignal’s Continuous Time Delta-Sigma technology.

The company attributes the A/D converter's technology improvements to the architecture, as well as to its 0.13-micron CMOS process. “I believe that our use of 0.13 micron CMOS is the first time such a sub-micron technology has been used for a commercial high-speed A/D converter. Some of the latest pipeline A/D converter products are now using 0.18 micron,” Holdaway said.

Based on this Continuous Time Delta-Sigma technology, Xignal is rolling out two basic A/D converters that Taiwan Semiconductor Manufacturing Company Ltd. (TSMC) is expected to build.

Although dynamic performance is equivalent to existing converters from the likes of Analog Devices Inc., Linear Technology Corp., Maxim Integrated Products Inc., National Semiconductor Corp., and Texas Instruments Inc. — Xignal's power consumption is two times less than competing devices, Holdaway said.

The converter's also sport the lowest operating supply voltage, according to Holdaway. “These A/D converters work on a 1.2-V single battery cell, which is unique since the majority of the A/D converters out there work on 3.3-V single cell batteries,” he said.

The top line of the energy efficiency comparison graph below shows the way A/D converters are traditionally implemented using the pipeline architecture, while the bottom line represents Xignal's continuous time delta-sigma technology. “We are getting the same level of dynamic performance at a much lower power level,” Holdaway said.

 
 
Energy Efficiency Comparison Chart

One way that Xignal is simplifying the converter design is by eliminating external filter requirements. “All data conversion systems, including the pipeline architecture, require an external anti-aliasing filter. We have a digital filter on-board, which combined with the delta-sigma loop filter and over-sampling, effectively forms the anti-aliasing filter,” Holdaway said.

Although the cost of the anti-aliasing filter depends on the application, it can range from 50 cents per channel up to $3.00 per channel, Holdaway said.

The diagram below shows the first stage of a generic pipeline A/D converter. It's a challenge to design this circuit because it requires a lot of bandwidth to settle rapidly. “When we digitize the signal, we are integrating it continuously, prior to its sampling at 16 times the input frequency, which eliminates the sample-and-hold function as needed by the pipelined A/D converter,” Holdaway explained. As you can see, Xignal's design has eliminated the switches altogether.

 Sample and Hold Amplifier

The bandwidth required to implement this circuit also results in a huge power draw. Xignal completely does away with the sample-and-hold amplifier, but achieves a similar sample rate (20 to 40 MHz) — but at much lower power (70 mW compared to 120 mW for the nearest competitor), Holdaway said. Typically, 12- and 14-bit converters require 250 to 300 mW power consumption at this sample rate, he said.

Furthermore, extra buffer ICs aren't needed to drive inputs, Holdaway said. “The pipeline architecture has a limited input signal swing. We can scale inputs based on the input resistance we have so it’s easier to drive our inputs. We are simplifying the signal path significantly, which is normally very complicated within a pipeline architecture. It’s a far simpler beast,” he said.

Another significant advantage of using this type of architecture is that it continues to scale over time. When Xignal moves down to finer process geometries like 90 nm and 60 nm processes, the company will still be able to use this technology platform to build new pin-compatible converters so a major redesign won't be necessary, Holdaway said.

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