18-bit A/D packs speed & precision into MSOP

Norwood, Mass. — Analog Devices Inc. said this is the only 18-bit analog-to-digital converter to deliver 400-kilosamples/second in a 3mm x 5mm MSOP.

Targeting battery operated medical equipment, remote and isolated data acquisition systems and industrial smart sensors; the AD7690 was made for tight spaces with speed and accuracy in mind.

In addition to doubling speed and enabling a 40 percent reduction in board space, the AD7690 has a signal-to-noise ratio (SNR) of 102 dB, making the 18-bit device 6 dB more accurate than the closest competitor, said Mike Britchfield, product line director of precision signal processing, Analog Devices.

Furthermore, the AD7690 consumes 80 percent less power than similar converters, according to Britchfield. “At its highest throughput rate of 400 ksamples/s, the AD7690 dissipates 20 mW compared to 110 mW for the closest competing A/D converter,” he said. The device powers down automatically at the end of each conversion, so the power scales linearly with the sampling rate.

The AD7690 is a member of the Analog Devices' PulSAR family of successive approximation A/D converter. The device's differential linearity (DNL) of ±1 least significant bit (LSB) makes it the first A/D converter to offer true 18-bit linearity in an MSOP with zero data latency, Britchfield said. It also achieves a typical integral nonlinearity (INL) specification of ±1 LSB.

The A/D converter provides an on-chip track-and-hold, and exhibits no pipeline delay or latency. It operates from a single 5-V power supply; an optional I/O supply ensures compatibility with 1.8-, 2.5-, 3- and 5-V logic using the SPI-compatible serial interface. The serial interface also enables designers to daisy chain multiple A/D converters on a single three-wire bus. The device also includes an internal conversion clock.

In addition to the AD7690, ADI released the AD7691, which also delivers 18 bit linearity for applications that don't have the same speed requirements or board space limitations. The AD7691 provides throughput of 250 ksamples/s, and operates on a 2.7- or 5-V single power supply.

See related diagram

The AD7690 and AD7691 are sampling now with production quantities slated for February. The AD7690 is available in a 3mm 5mm, 8-lead MSOP for $19.50 in 1,000-unit quantities. The AD7691, available in an 8-lead LFCSP, is priced at $14.50 in same quantities. Click here for the AD7690 data sheet. Click here for the AD7691 data sheet.

Analog Devices , 1-800-262-5643,

ADI's 18-bit SAR A/D converters tout the tinniest leadless packages in the world, as well as the best DC performance. “The AD7690 and AD7691 will not only provide customers with the smallest packages but also the highest performance with typical INL of only 4 ppm of full scale, which is a 50 percent improvement over the closest competitor,” said Wayne Talley, ADI's product marketing manager, precision signal processing group.

And, even though the packages are small; the A/D converters still boast low power consumption — 20 mW at 400 ksamples/s, Talley said. “If you look at similar devices in the market, our part is four times smaller and four times lower in power consumption,” he said.

Another advantage of using these converters is that they have 10 leads, compared to 8-leads for other similar devices, Talley said. “ADI can add more functionality with these two extra leads — but the package size is still smaller than competing devices when comparing the 3mm x 3mm LFCSP,” he said.

For instance, the daisy chain feature, a common feature for many serial parts, and a digital I/O pin are possible via the extra pins. “Although daisy chain is a common feature in serial parts, if you are pin limited, adding this capability is a problem in smaller package sizes,” Talley said.

The digital I/O allows the A/D converter to operate at a different voltage than the digital host. This feature is useful in applications where the A/D converter needs to run at a higher voltage than the digital host in order to improve SNR.

Regarding DC performance, the slide (below) shows INL/DNL performance. “We are about 3 ¼ ± 1 LSB,” Talley said.

See INL/DNL performance chart

This is the best 18-bit DC performance ADI has gotten out of any core, Talley said. He attributes the high performance level partly to the new devices .5 micron CMOS core, versus a .6 micron CMOS core for ADI’s older products.

While some companies offer two grade specs (low and high) for their A/D converters — ADI's latest devices are single-grade A/D converters. “We are confident with a single grade because we know that we can hit our spec. We don’t need to have a lower grade to fall back on,” Talley said

Even though these parts are geared towards DC-type applications like battery operated and hand held devices, because of their performance level, they can also be used in AC-type applications such as analytical instruments and ATE.

One of the reasons why the AD7690 and AD7691 have been able to achieve this level of performance in a small package is because they don't require separate digital and analog supply and ground pins to separate the digital side of the circuit from the analog, according to Talley.

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