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ADCs and DACs fundamentals, Part 5

Posted: 21 Nov 2008     Print Version  Bookmark and Share

Keywords:spuriousfree dynamic range  distortion 

The ac specifications that are most likely to be important with DACs are settling time, glitch, distortion, and spuriousfree dynamic range (SFDR). The settling time of a DAC is the time from a change of digital code to when the output comes within and remains within some error band. With amplifiers, it is hard to makecomparisons of settling time, since their error bands may differ from amplifier to amplifier, but with DACs the error band will almost invariably be ±1 or ±1/2 LSB.

The settling time of a DACis made up of four different periods: the switching time or dead time (during which the digital switching, but not the output, is changing), the slewing time (during which the rate of change of output is limited by the slew rate of the DAC output), the recovery time (when the DAC is recovering from its fast slew and may overshoot), and the linear settling time (when the DAC output approaches its final value in an exponential or near-exponential manner). If the slew time is short compared to the other three (as is usually the case with current output DACs), the settling time will be largely independent of the output step size. On the other hand, if the slew time is a significant part of the total, the larger the step, the longer the settling time.

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