1ppm settling time measurement for a monolithic 18-bit DAC

Performance requirements for instrumentation, function generation, inertial navigation systems, trimming, calibrators, Automated test equipment, medical apparatus, and other precision applications are beginning to eclipse capabilities of 16-bit data converters. More specifically, 16-bit digital-to-analog converters (DACs) have been unable to provide required resolution in an increasing number of ultra-precision applications. New components have made 18-bit DACs a practical design alternative. These integrated circuits provide 18-bit performance at reasonable cost compared to previous modular and hybrid technologies. The monolithic DAC DC and AC specifications approach or equal previous converters at significantly lower cost. DAC DC specifications are relatively easy to verify. Measurement techniques are well understood, albeit often tedious. AC specifications require more sophisticated approaches to produce reliable information. In particular, the settling time of a DAC and its output amplifier is extraordinarily difficult to determine to 18-bit (4ppm) resolution. DAC settling time is the elapsed time from input code application until the output arrives at, and remains within, a specified error band around the final value. To measure a new 18-bit DAC, a settling time measurement technique has been developed with 20-bit (1 ppm) resolution for times as short as 265 ns. The new method will work with any DAC. Realizing this measurement capability and its performance verification has required an unusually intense, extensive and protracted effort. Hopefully, the data converter community will find the results useful.