Taking RISC
With its recent introduction of the MAX1460, Maxim Integrated Products has joined the fray. This 16-bit low-power ADC provides on-chip digital correction of the output over the specified temperature range. For correcting sensor offset and errors, it incorporates a rudimentary digital signal processor (DSP) for performing multiply-accumulate (MAC) operations. The MAX1460 also includes 128-bit EEPROM to store the user-programmed compensation and calibration coefficients.
The MAC instructions and code are actually embedded within the chip, eliminating the trouble of programming the DSP for signal processing. An uncommitted op amp is available to filter the analog output or implement a 2-wire, 4- to 20-mA transmitter (Fig. 3). The frontal section of this 16-bit ADC offers a 2-bit PGA and a 3-bit coarse-offset DAC to condition the sensor's output.
The device has built-in testability that integrates three traditional sensor-manufacturing operations into one automated process. These include pretest, calibration and compensation, and final test. Maxim claims that by eliminating manual calibration and allowing verification of transducer calibration and compensation in the pretest socket, the MAX1460 cuts sensor manufacturing costs by a substantial amount. Operating from a single 5-V supply, this part is designed to resolve 1 µV of differential input signal. It consumes only 400 µA of supply current, making it suitable for low-power applications.
Speaking of low power, the available MAX1462 version will run from battery power at 2.35 V. According to Maxim, that device can operate down to 2.2 V.
Other key suppliers simplifying the link to DSPs include Intersil Corp., National Semiconductor Corp., Telcom Inc., and Texas Instruments Inc. These manufacturers are adding all the required bells and whistles to make the high-performance ADC connection seamless to a variety of DSP processors.
Interestingly, manufacturers supplying microcontrollers for data-acquisition systems have expanded their role to compete head on with ADC producers. Producers like Microchip Technology and Atmel Corp. have acquired enough analog expertise to bundle the front-end analog peripherals and converters with their 8-bit controllers. They can now deliver a complete solution to the builders of data-acquisition systems.
Last September, for example, Microchip debuted its 8-bit microcontroller line, PIC16C7xx. It packs a 10- or 12-bit successive-approximation-register (SAR) ADC, enhanced capture/compare/PWM, voltage reference, OTP programmable memory, RAM, oscillator, and timers on the same silicon. The goal is to provide a one-stop shop for embedded-systems designers. Until now, they had to rely on analog houses for the peripherals of the microcontroller-based solution.
Now the company provides both integrated solutions, as well as standalone multichannel 10- and 12-bit ADCs. These include op amps for the designers that prefer to segregate the analog and digital portions of their embedded solution for better accuracy control and noise performance. Like Microchip, Atmel has embedded nonvolatile flash memory, a 6-channel 10-bit ADC, and a host of other peripherals around an 8-bit RISC core. The company hopes to furnish a cost-effective solution to many control applications.
Meanwhile, those flaunting analog strength continue to concentrate on precision, low-noise, micropower, and wide-dynamic-range parts from a tiny low-cost package. Their goal is to make these very-high-accuracy parts extremely simple to use. The calibration is transparent to the user.
Driven by this philosophy, Linear Technology Corp. crafted a 24-bit, third-order Δ−Σ ADC, the LTC2400, nearly 18 months ago (see "High-Resolution ADC Targets Precision Data-Acquisition Applications," Electronic Design, Sept. 1, 1998, p. 68). Since then, the supplier has expanded the line with key enhancements like higher speed, lower noise, and the addition of a multiplexer.
"Our focus has been to enhance the analog part and keep the digital interface simpler," says senior design engineer Mike Mayes. "Unlike others, it does not require any writing into the registers. Nor does it ask the user to go through a complex sequence of steps to calibrate the LTC2400. Its self-calibration is transparent to the user. As it offers no latency, it is easy to multiplex. Latency and redundant data are normally associated with Δ−Σ ADCs."
The family has been expanded with multichannel versions.The latest revision has a fully differential input and differential reference. The micropower 24-bit LTC2410 is designed to maintain the effective number of bits (ENOB) and display extreme stability over the input range of −0.5 to +0.5 V. Its offset error is negligible over the input common-mode voltage of 0 to +5.5 V. This no-latency architecture is crafted to ensure that the linearity doesn't drift over the industrial temperature range of −45° to +90°C while the offset is zero and the drift is minimal.
The sum of this performance is that the total adjusted offset error is within 3 ppm over the differential input, as well as over the temperature range of −40° to +125°C (Fig. 4). The noise should only be 800 nV (rms) over the entire input range. This part has been tailored for simplicity. Sampling now, the LTC2410 is slated for production in a month or two. A multichannel version also is in the works.
On the wideband front, applications demand much higher sampling rates. So mixed-signal IC supplier DataPath Systems has extended its capabilities to design a 5-Msample/s, 16-bit pipelined ADC with an on-chip PGA, a sample-and-hold (S/H) circuit, and a voltage reference. Implemented in 0.5-µm CMOS, the DPS9245 provides user-programmable power dissipation that's dependent on the sampling rate. At 5 Msamples/s, the unit dissipates 400 mW. It drops to 200 mW at 2.5 Msamples/s.
The PGA offers up to 20 dB of gain in seven settings. According to the company, the amplifier's high-resistive input impedance (greater than 1 kΩ) eliminates the need for an external buffer amplifier and complex filtering to handle transients. This integrated converter also boasts a noise density of 8 nV/(check)Hz ±1.0-LSB integral nonlinearity. It has ±0.5-LSB differential nonlinearity. A spurious-free dynamic range (SFDR) of 95 dB can be obtained from this pipeline multistage architecture at a 900-kHz sinusoidal input. The differential input range is 5-V p-p. These specifications are guaranteed over the industrial temperature range.
This part targets ATE, subscriber-line test equipment, scanners, and imaging applications that need the data-acquisition subsystem. According to DataPath's director of marketing, Phil Welsh, "This level of integration at such a high sampling rate is unprecedented."