Octal continuous-time sigma-delta ADC targets ultrasound and industrial applications
By Paul Rako, Technical Editor -- EDN, January 28, 2008
National Semiconductor is the first to bring to market a high-speed, continuous-time sigma-delta converter. The 12-bit ADC12EU050 octal converter operates at 50M samples/sec and uses only 44 mW per channel. Whereas conventional sigma-delta converters use internal switched-capacitor filters, continuous-time converters use op-amp filters that offer resistive input impedances. You face a trade-off for using this type of converter, however: You cannot vary sampling rates over a wide range because the parts must operate near 50M samples/sec. In addition to the resistive inputs, a major benefit of the architecture is low power consumption. Because of the nature of the architecture, it requires no input-sample-and-hold circuit. Another benefit is single-clock recovery time from overloads. SINAD (signal-to-noise-and-distortion) figure is 68 dB. The part operates from 1.2V, and you can drive the resistive inputs to a 2.1V p-p range.
Continuous-time delta-sigma converters have been the subject of academic scrutiny for 15 years. National Semiconductor’s January 2007 acquisition of Xignal Technologies combined that company’s advanced intellectual property with National’s design, test, and manufacturing might. The company produces the part on a fine-line, 0.13-micron CMOS process from TSMC (Taiwan Semiconductor Manufacturing Co). The part is the first in a line that will provide low-power, high-speed 10- and 12-bit converters.
The devices have an internal low-jitter PLL (phase-lock loop) that cleans up and reduces the jitter of the applied clock signal while multiplying it by 16. The device uses this clock, divides it back down to the input frequency, and provides a pin for designers to use as a low-jitter system clock. A member of the company’s Powerwise data-converter line, the part is available for sampling, and high-volume production will begin in the third quarter of this year. It comes in a 10×10-mm, 68-pin LLP package and operates over a −40 to +85°C temperature range. The price is $64 (1000).
