There are three basic A/D architectures, and often the best choice for the application depends on the nature of the input signal. Delta-sigma architectures are usually the best solution for very high-resolution applications (typically 24-bit)-either for low-speed precision measurements or where the input signal is a continuous waveform. Quite often, the group delay of the digital filter does not allow impulse conversions or the multiplexing of numerous input channels.

Successive approximation register (SAR)-architected A/Ds do not have those throughput issues, but they often require a moderately complex anti-aliasing analog input filter to achieve the high resolution (now available up to 18 bits) required for precision measurement. Pipeline-architected A/Ds also offer extremely high speed, with sampling rates in excess of 100 Msamples/second; resolution, however, is limited to 14 bits.

In the case of D/A converters, recent developments in "R-2R" resistor ladders and "string" D/A architectures have resulted in tremendous cost savings, even for resolutions beyond 16 bits. R-2R traditionally has been the architecture of choice for high-linearity applications. Generally these are open-loop applications.

String D/As have extremely good differential-linearity performance, though they're typically used in closed-loop applications. These devices are getting much closer in linearity performance to R-2R types-often at lower cost, because of their much smaller chip size. For very-high-speed applications, current-steering D/As have the fastest settling times and update rates.

Selection criteria for D/A converters include bit resolution, output bandwidth and update rate.

Skip Osgood (osgood_skip@ti.com), development manager for precision data acquisition products, Texas Instruments Inc. (Tucson, Ariz.)