Advertisement

Article

Two fast, two accurate A/D converters help eliminate front-end conditioning

Norwood, MA — Analog Devices, Inc., a global provider in high-performance semiconductors for signal processing applications and the market leader in data converters, today announced two new data converters that deliver industry-best speeds and unparalleled accuracy at resolutions of 18-bits and above. The new analog-to-digital converters (ADCs) simplify circuit design and decrease system cost by eliminating the need for cumbersome front-end signal conditioning. These advancements provide significant benefits in applications requiring high-performance data conversion, such as instrumentation, imaging and vibration analysis in industrial, medical and communications markets.

ADI's new converters are:
The fastest, most accurate 18-bit SAR ADC — At 2 MSPS, the AD7641 is four times faster than the closest competition, while improving accuracy by a factor of three, with a typical nonlinearity of 2 LSB and SNR (signal-to-noise ratio) of 93 dB.

The most accurate, high-speed 20-bit sigma-delta ADC — the AD7760 delivers the industry's highest SNR: 100 dB at 2.5 MSPS.

“Data-conversion technology has evolved to the point that it is now easier and more cost-effective to use higher resolution ADCs in many data acquisition applications including medical imaging and high-speed communications, than the lower resolution converters and front-end PGAs that have often been used,” said Mike Britchfield, product line director for precision converters at Analog Devices. “These new high resolution converters function better in the analog realm, allowing designers to reduce the complexity and cost of the design process, while achieving unparalleled accuracy and performance levels.”

Greater Cost Savings Through Reduced Design Complexity
The front-end signal conditioning traditionally required in high accuracy data acquisition systems is cumbersome and expensive, especially in many-channel systems. The wide dynamic range of the AD7641 and AD7760 allows low-level signals to be converted with minimal front-end analog signal conditioning, which reduces design effort and cost in comparison to solutions using lower-resolution ADCs. In many applications such as vibration analysis and medical imaging, the AD7641 eliminates the need for a discrete, low noise PGA (programmable gain amplifier), while the AD7760 simplifies complex anti aliasing filtering which reduces design work, complexity, cost and time to market. The AD7760 also integrates an on-chip buffer and differential amplifiers, allowing the device to be used with minimal external component selection.

The AD7641 – Fastest, Most Accurate SAR ADC
The fully differential AD7641 successive-approximation register (SAR) ADC joins Analog Devices' PulSAR family of ADCs. Dissipating only 100mW at 2 MSPS, the AD7641 enables significant cost savings because of its low power consumption. In impulse mode, the new ADC's power dissipation decreases with throughput to allow users total flexibility, from fastest throughput to lowest power consumption. Its 2-MSPS sampling rate lets the AD7641 replace several ADCs in multi-channel applications, reducing the number of discrete parts required. The 18-bit AD7641 operates from a single 2.5-V supply and provides an internal conversion clock, an internal reference buffer, error correction circuits and serial and parallel system interface ports.

To drive the ADC in high-precision applications the AD7641 can be coupled with Analog Devices' AD8021, a high-performance amplifier featuring low noise and low distortion.

The AD7760 – Most Accurate, High-Speed Sigma-Delta ADC

The first in Analog Devices' AD776x family of 20-bit sigma delta converters, the AD7760 sigma delta ADC combines wide input bandwidth and high speed with the benefits of sigma-delta conversion and provides breakthrough performance of 100dB SNR at 2.5MSPS, making it ideal for high speed data acquisition. Wide dynamic range and significantly reduced anti aliasing requirements truly simplify the design process, providing cost savings when compared to lower performance devices requiring considerable front end signal processing.

An integrated buffer to drive the reference, differential amplifiers for signal buffering and level shifting, an over-range flag, internal gain- & offset registers, and a low-pass digital FIR filter combine to make the AD7760 a compact highly integrated data acquisition device requiring minimal peripheral component selection. In addition, the AD7760 allows designers to adjust the filter if its default characteristics are not appropriate to the application. By simplifying high performance data acquisition, the AD7760 becomes ideal for applications demanding industry leading SNR without the need to design complex front end signal processing circuitry.

Analog Devices, Inc. is a leading manufacturer of precision high-performance integrated circuits used in analog and digital signal processing applications. ADI is headquartered in Norwood, Massachusetts, and employs approximately 8,700 people worldwide. It has manufacturing facilities in Massachusetts, California, North Carolina, Ireland, and the Philippines. Analog Devices' common stock is listed on the New York Stock Exchange and ADI is included in the S&P 500 Index.

www.analog.com

This announcement is a big deal because ADI has managed to introduce two top-notch converter products that are similar yet different. Similar because both have what may be the highest speed at a very high resolution and different because they use different architectures; similar because they are 18- and 20-bit converters and different because they target dissimilar applications.

The amplifier design on the AD7760 and the comparator and error correction methodology on the 7641 were the significant advancements to these parts. The 7641 and 7760 are focused on high-accuracy for industrial, instrumentation and medical applications. The 7641 is an 18-bit, 2MSPS SAR A/D converter. The 7641 will typically find a home in the main signal chain in a CAT scanner while the 20-bit AD7760, a 2.5MSPS 20-bit sigma-delta A/D converter, would be used as the main A/D converter for signal acquisition in vibration analysis applications.

ADI thinks the AD7641 is a major introduction because it jumps the sampling rate from 800 kSPS to head-turning 2MSPS at 18-bits, and it still maintains the integral non linearity (INL) and differential non linearity (DNL) of the existing slower parts. The big benefit for AD7641 SAR architecture is the ability to multiplex over the channels with just one converter.

In general, the envelope for the SAR converter is being pushed by ADI with better error correction methods and techniques for lower power consumption. The process size for the AD7641 SAR moved from 0.6 micron to 0.25 micron. ADI also said it used some tricks in the comparator design's error correction method enabling it to go faster and still achieve the same accuracy. So this part isn't just about speed because speed is only useful if you can maintain the accuracy. The design capabilities and tricks used by ADI allowed it to achieve the linearity specs at high speeds.

The AD7760 sigma-delta converter process also became smaller with a move from 0.6 to 0.25 micron. One of the key benefits of shrinking the geometry is the reduction in the size of the capacitors. In converters the capacitors are constantly charging/discharging, and one way to speed up this action is to shrink the size of the capacitor. Moving to a smaller process enabled ADI to also shrink the capacitor and consequently speed the charge/discharge time. It is one of the ways ADI was able to get to the 2MSPS rate at 18-bits. Additionally, even though this part operates at 2.5V it can accept logic levels to 5V.

Comparatively, the AD7641 is a 2MSPS 18-bit SAR ADC and the closest competitor offers an 18-bit SAR ADC at 500 kSPS. Additionally, this part provides 800 kSPS at the typical +/-2 LSB INL. Competitive products only offer 7 LSB INL. This chip dissipates 100 mW at 2MSPS compared to 110mW at 500kSPS from competitors. A mere six years ago it was considered pretty good if you dissipated about 100 mW at 100 kSPS so this is a 20x improvement while dissipating the same power. However, the increase in accuracy is probably the most important improvement of the part. For example, to improve accuracy of a 16-bit SAR A/D converters designers would have to use a programmable gain amplifier (PGA) preceding the A/D converter to improve the accuracy to 18-bits of resolution. Unfortunately, adding those PGAs also requires external resistors, capacitors, more board space and of course, more expense. This ADI 18-bit converter removes all those components and expense from the signal chain. Finally, the 4x sampling rate improvement over previous 18-bit A/D converters saves money that designers would typically spend in multi-channel applications. That means fewer A/D converters per channel are used with increased speed but with the same throughput.

The package size of the AD7641 is also something that ADI can crow about because it's available in a quad flat pack, and a lead frame chips scale package (LFCSP) which is 7 mm x 7 mm and is 40% smaller than typical package types. ADI also integrated a reference with a very low 3ppm typical drift, and guaranteed at 10ppm to 15 ppm drift. The converter is guaranteed to provide 18-bits no missing code over the full dynamic range which is better than 1 LSB differential nonlinearity (DNL) from -40C to +85 degrees C. Competitive parts have only been able to offer 17-bits no missing code for the full dynamic range and 18-bits no missing code for half the dynamic range. ADI also guarantees the AC performance of 93dB signal-to-noise ratio (SNR) and -100dB total harmonic distortion.

Applications for the AD7641 include medical instrumentation because of the need for high channel count and the requirement for high resolution/accuracy. Examples include CT scanners, MRI, digital X rays, and data acquisition. ADI thinks that other applications include gas and liquid chromatography, spectrophotometers, and even spectrum network analysis, now that the converter offers 2 MSPS.

The AD7760 Sigma Delta A/D converter architecture is typically used in applications where customers want to acquire a signal with the best accuracy. For the AD7760 sigma-delta architecture the traditional use has been very low power and low bandwidth for applications like temperature measurements in industrial controls, as well as audio codecs. The sigma-delta architecture usually provides very high SNR and allows you to digitize a signal very accurately. Typical sigma-delta converters have a slick feature that over-samples the bandwidth and pushes the noise out of the band of interest. The combination of these two allow for very high SNR. The downside is that these converters are used at relatively slow speeds (kHz speeds typically are the high end).

The 7760 converter simplifies antialiasing filters on the front end with the high level of oversampling, and that means less complex and less costly filtering. By improving the speed of the sigma delta converter ADI is making it available to a much broader range of applications. Previously, customers that needed higher performance than what was available would have to do some complex analog front-end signal preconditioning with some offset calibration, gain calibration and even over range signaling. Additionally, post processing would also be required at the output of the converter. This 7760 converter simplifies the signal acquisition by simplifying the antialiasing front end filtering and bringing the gain and offset registers on-chip.

Vibration analysis would be a good fit for the 7760. Vibration typically uses signals of 300 kHz to 500 kHz and needs to acquire the signal very accurately because these customers are looking for changes in the frequency.

The 7760 block diagram shows the buffer and differential amplifiers for the signal conditioning and also the buffering the signal and the modulator. The part also has a digital filter after the modulator, which allows you to filter the output of the modulator. ADI made the decimation filter programmable so the user can change the default characteristics by tweaking the coefficients of the filter to fine tune to fit the needs of the application. ADI supplies default coefficients with the part and the user can change it using three different options that the user can download. The user can also get more information from ADI on the exact coefficients and the impact on the filter.

Block Diagram
AD7760 Block Diagram

The AD7641 is priced at $32.95 per unit in 1,000-piece quantities and is packaged in a 48-lead LFCSP (lead frame chip scale package) or LQFP (low profile quad flat pack). The AD7760 comes in 64-lead TQFP (thin quad flat pack) and 48-lead LFCSP and is priced at $34.95 per unit in 1,000-piece quantities. Both products are sampling now with full production expected in October 2004 for the AD7641 and February 2005 for the AD7760.

For more information, please visit:
www.analog.com/AD7641

www.analog.com/AD7760

0 comments on “Two fast, two accurate A/D converters help eliminate front-end conditioning

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.