New A/D converter packs performance, precision into ‘industry’s smallest’ 18-bit device

By Control Engineering Staff November 18, 2005

Norwood, MA—Analog Devices Inc. (ADI) recently introduced what it calls industry’s first 18-bit analog-to-digital converter (ADC) to deliver 400-kSPS (kilo-samples-per-second) data rates in two chip formats: 3 × 5 mm MSOP (mini-small-outline package) and 3 x 3 mm LFCSP (lead frame, chip scale package). The LFCSP format is said to be the world’s smallest leadless package. Designated AD7690 , the new ADC is intended for board-space-sensitive applications where speed and accuracy cannot be sacrificed. Target markets include remote/isolated data acquisition systems, smart industrial sensors, and battery-operated medical equipment.

Besides doubling speed and enabling 40% less board space, AD7690 offers a signal-to-noise ratio (SNR) of 102 dB, making the 18-bit device 6 dB more accurate than its closest competitor, according to ADI. A member of the company’s PulSAR family of successive approximation register (SAR) ADCs, it also offers cost-effective technology for applications that require precise digital modeling of analog signals.

Sample accurately, conserve power
AD7690’s differential nonlinearity (DNL) ofst measurements. “DNL is a measure of error between adjacent code transitions and can be considered a test to ensure no missing code,” Wayne Talley, ADI product marketing, told Control Engineering. “INL, derived from DNL, is a measure of how close the code is to the ideal transfer function.”

Designers of AD7690 also paid close attention to power management. The chip consumes 80% less power than competing devices due to smaller chip size and ability to power down automatically whenever data conversion (or high-rate conversion) is not going on, explains Talley. This makes the chip a natural fit for use in battery-powered portable and handheld instrumentation. For example, at 400 kSPS (highest throughput rate), AD7690 dissipates 20 mW, compared to 110 mW for the closest competing ADC. At 100 kSPS, typical power consumption is 4.5 mW. Power consumption scales linearly with sampling rate.

AD7690 operates from one 5-V power supply; however, an optional I/O supply ensures compatibility with 1.8-, 2.5-, 3- and 5-V logic using the SPI-compatible serial interface. Moreover, designers can “daisy chain” multiple ADCs via the serial interface, using a single wire. It reduces external component count and wiring connections. Daisy-chaining capability is a differentiator, because competing devices do not have an extra pin available for this feature, adds Talley.

Tight INL/DNL performance of AD7690 analog-to-digital converter is demonstrated at 400 kSPS data rate, operating with 5-V power supply and at 25

A companion device with 18 bit linearity, AD7691, was also introduced to suit applications with lower speed requirements or less-stringent board space limitations. AD7691 provides throughput of 250 kSPS and operates on a 2.7- or 5-V single power supply.

Both ADCs are sampling now, with production quantities available in February 2006. Pricing for AD7690 and AD7691 is $19.50 and $14.50, respectively, in 1,000-unit quantities.

Frank J. Bartos , executive editor