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Flea-Power 24-Bit A/Ds Tout Microvolt Noise Levels

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Analog Devices Low-Power 24-Bit Sigma-Delta Data Converter Sets New Standard in Power Consumption and Performance, Ideal for Portable Instruments Norwood, Mass.--Analog Devices Inc., a global leader in high-performance semiconductors for signal-processing applications and the market share leader in data converters, introduced a 24-bit sigma-delta A/D converter with the industry's lowest power consumption and leading-edge accuracy. Consuming only 65 µA, this new analog front-end outperforms its closest competitor with 25 percent lower power dissipation and six times the resolution. Available in a tiny MSOP-10 package and at a competitive price, it also includes an on-chip buffer and an internal clock generator, making this the most integrated sigma-delta A/D converter on the market. Sigma-delta A/D converters are ideal for portable instrumentation; sensor, temperature, and pressure measurement; and weigh-scale applications. "As the cost of high-resolution sigma-delta converters continues to drop, the technology is becoming more accessible, and chip manufacturers are called upon to provide more options for designers of high-resolution measurement systems in which performance, space, and power consumption are critical," said Mike Britchfield, product line director for sigma-delta converters, Analog Devices. "With this new device, Analog Devices raises the standard for noise performance, and continues to push the envelope when it comes to power consumption." New Ultra Low-Power Sigma-Delta A/D Converters The new 24-bit sigma-delta A/D converter, the AD7791, also comes in a 16-bit version, the AD7790. Both accept differential inputs, which can be buffered or unbuffered; output data rates are software-programmable. Parts without the buffer, and with a fixed 16.6 Hz data rate, are also available in 16- and 24-bit versions (AD7788/AD7789). All parts are low-power analog front-ends for low-frequency measurement applications. Each consumes 65 µA when operated with a 3 V supply, and 75 µA when operated with a 5 V supply (buffer disabled). More About the AD7791 Featuring rms noise of only 1.5 µV, the AD7791 operates from an internally generated clock, eliminating the need for a user-supplied clock source. Its output data rate is software-programmable, allowing rates up to 120 Hz (at the default conversion rate of 16.6 Hz, the AD7791 provides simultaneous 50 Hz and 60 Hz rejection notches, making a piece of equipment suitable for both U.S. and European systems). The p-p resolution from the part varies with the programmed output data rate. The device operates from a single 3 V or 5 V power supply, and supply-voltage monitoring is included. It can be used in single- or continuous-conversion modes. Pricing and Availability The new sigma-delta A/D converters come in 10-lead MSOP packaging and are priced as follows in 1,000-piece quantities: 24-bit AD7791—$3.83 16-bit AD7790—$2.95 24-bit AD7789—$2.95 16-bit AD7788—$1.99 Samples and evaluation boards are available now. For more information, please visit www.analog.com/AD7790/91 and www.analog.com/AD7788/89. Analog Devices, Inc. Ray Stata Technology Center 804 Woburn St. Wilmington, MA 01887 Phone: (800) ANALOGD        or (800) 262-5643 Tech Support: (781) 937-1428 FAX: (781) 937-1021

If you're setting up circuitry to gather high-resolution data from slowly changing sensors, check out Analog Devices Inc.'s (ADI) latest sigma-delta A/D converters. These analog-to-digital ICs carve out new territory as low-noise analog front ends in low-bandwidth precision data-conversion subsystems such as those used to monitor pressure or temperature. These days, such signal-conditioning circuits can be found in battery-powered gear (i.e., nomadic medical equipment), as well as in fixed systems such as high-resolution digital weigh scales. Some mainstream process-control systems also benefit from 24-bit sampling. The surface-mountable 10-lead AD7791 is now at the top end of ADI's low-bandwidth sigma-delta family tree (the AD7790 is the 16-bit version). The '7791 also represents the next generation of ADI's sigma-delta converters, supplanting the company's venerable AD7714. The AD7714 has enjoyed a 10-year run in a competitive marketplace, where it competes with devices from Cherry Semiconductor and Linear Technology Corp. ADI hasn't been letting any grass grow under its silicon feet—it has released other sigma-delta slow-speed A/Ds during this period, but they were primarily differentiated by their feature sets. This latest chip is notable for its low power and very low noise. The Competition ADI's press statement mentions a competitor for its new 24-bit devices, but doesn't provide details. What the release refers to obliquely is Texas Instruments' Type ADS1244 chip. Like ADI's chip, it's a 24-bit self-calibrating delta-sigma A/D that's part of TI's Burr-Brown product line. Like the AD7791, the ADS1244 is almost dirt cheap, selling for just under $3 a pop in OEM quantities. The graph here shows how some of the popular competing ICs compare. In addition to the senior players, the table includes the AD7791, as well as the TI/Burr-Brown ADS1244. Notice that noise (measured in microvolts), is specified as rms. More on that in a moment. click for full-size graph Also in a 10-lead surface-mount package, the TI/Burr-Brown silicon draws 90 µA, as opposed to ADI's 65 µA. It also includes a third-order delta-sigma modulator as well as on-chip digital filtering. Multiple ADS1244s can also be connected together to create a synchronously sampling multi-channel measurement system. Like ADI's new part, TI's chip uses serial communication for data transmission, calibration, and for placing the silicon in Sleep mode. The ADS1244's Sleep mode can also be used to shut down the chip between measurements. As such, the dissipation is typically less than 270 µW in normal operation, dropping to less than a microwatt in Sleep Mode. ADI, on the other hand, says its chip typically dissipates 195 µW. While 75 µW may not seem like a lot to some designers, for those designing battery-powered products every last bit of saved dissipation results in improved battery life. Multi-Mode ICs ADI's new converter also touts a differential input, and that input can be logic-switched to be buffered or unbuffered. If unbuffered, you enjoy lowered dissipation. In the buffered mode, however, you can place source impedances on the front-end of these ICs without contributing gain errors to your overall system. The trade-off is yours. By the way, the lower resolution AD7788 and AD7789s flavors mentioned in ADI's press release pricing paragraph have no buffers at all. Note that ADI's new ICs also operate from internal clocks, so that you don't have to supply a clock line. Also, these ICs can be used in either of two conversion modes. In Single Conversion mode the devices power up and perform a single data conversion, returning to their powerdown mode where they sip a mere microamp. In Continuous mode, the chips can be configured to read continuously, with data placed on the IC's serial bus as soon as a conversion is complete. That way, you don't need to write to the device to read the data. Note that the 3-wire serial bus supports Motorola's SPI (Serial Peripheral Interconnect), QSPI (Quick SPI; SPI-compatible but clocks to 2 MHz), National Semiconductor's Microwire, and similar protocols. The IC's SCLK (Serial Clock) input also uses a Schmitt trigger so that the IC can be used with opto-isolators. The serial clock can also be continuous, with all data transmitted in a continuous train of pulses. Alternatively, it can be a non-continuous clock, with the information being transmitted to or from the A/D converter in smaller packets of data. Resolution Varies With Data Rate The output data rate from these chips is software-programmable, but the peak-to-peak (p-p) resolution varies with the programmed data rate, which can be set between 9.5 Hz and 120 Hz. The '7790 flavor offers a p-p resolution of 16 bits at the default output data rate of 16.6 Hz (the AD7789 flavor, briefly mentioned in the press release, has 19 bits of resolution, which is equivalent to an effective resolution of 21.5 bits at 16.6 Hz). By the way, that magic number of 16.6 Hz is no accident; with a data rate of 16.6 Hz, simultaneous 50 Hz/60 Hz rejection is achieved. Making Comparisons Note the method by which the effective number of bits of low-bandwidth/high-resolution A/D converters such as these is calculated. It may differ from vendor to vendor. When comparing ICs from various chip makers, it isn't necessarily valid to compare numbers directly. "Parts that are specified using peak-to-peak specs may appear worse than those that are specified using effective resolution," says ADI product director Mike Britchfield. "Noise has an impact on performance," points out Britchfield. "Resolution is directly related to noise. Our chip has a 1.5 µV spec, compared to 10 µV for TI's. That equates to 2.5 bits of performance." "Noise performance is going to impact the amount of flicker a user gets. For an A/D converter, its effective resolution may be better than its peak-to-peak resolution by 2.7 bits. A device that has an effective resolution of 22 bits has a flicker-free resolution of 22 minus 2.7, or 19.3 bits." "If you're comparing apples to apples, you need to compare parts at the same update rates," emphasizes Britchfield. "These new chips are very low bandwidth converters, and have update rates from 10 Hz to 120 Hz." "Also, a sigma-delta converter essentially works by shaping noise. White noise is shaped by filtering, and pushed as far as possible out of the band of interest. There is a curve, however, so noise and performance is going to relate to bandwidth. You have to look at relative bandwidth compared to the decimation ratios and filter characteristics." Britchfield also warns designers to be very careful comparing rms noise to p-p noise specs, noting that ADI specifies both rms and p-p to help users make meaningful comparisons. "But," cautions Britchfield, "most competitive parts are specified in rms noise only." LCD Flicker Britchfield explains that if you're digitizing a signal and want to put it on a display, you have to be concerned with how much flicker there might be on a display's least significant bit. "Let's say you're looking at temperature," suggests Britchfield. "At a reading of 25.4545°, for example, you wouldn't want your LCD to show flicker of the last digit." "If you want to see what kind of flicker-free performance you're going to get, you must look at peak-to-peak noise, not rms noise values." ADI's press release states that "this analog front-end outperforms its closest competitor with 25 percent lower power dissipation and six times the resolution." Britchfield re-emphasizes that resolution is directly related to noise performance. "A 6× difference is equivalent to about 2.5 least significant bits of worst effective performance," he says. "You can probably achieve the same performance of the AD7791 with other A/D chips, but you'll need outboard filtering, which means more circuitry and more current draw." On-Chip Voltage Monitoring Lastly, ADI's press release mentions that voltage monitoring is included. In operation, the IC can measure the voltage on its power-supply pin. The monitored voltage is divided and applied internally for A/D conversion using an on-chip 1.2 V reference. That's a nifty feature for battery-powered systems. Although pricing has been established for these newest chips, ADI's Web site only carries preliminary datasheets for them. However, samples are available, and ADI is supplying evaluation boards as well.

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