ADC home pageIntroductionGoalsWhy digitize?Number RepresentationDigital to AnalogAnalog to DigitalApplicationsLiterature Citations

Flash Analog to Digital Conversion

Choose Subtopic

The first type of analog to digital coverter (henceforth, ADC) we will consider is the fastest: the aptly-named flash converter. This converter uses comparators to determine what potential is instantaneously present on an input, and just as instantaneously to provide a digital output instantaneously reporting the value. No sample and hold is employed.

To keep the situation simple, let's design a 2 bit straight binary flash converter. The output codes can be 00, 01, 10, or 11. If the full range of the converter is 5 V, then the transitions between the codes should be at 1/4, 1/2, and 3/4 of full scale or 1.25 V, 2.5 V, and 3.75 V. If the instantaneous potential is, say, 1.35 V, the output should be 01. We can generate the reference potentials with a resistive divider, and use one comparator for each reference level. That means the analog section of the circuit looks like this:

Flash no logic

The switching speed is the speed of the comparators, and that can be extremely fast as long as the input capacitance of the comparators is small.

   
 

 

DAC Ladder Networks DAC Speed and Glitches Scheeline Group Home Page Univ. of Illinois at Urbana-Champaign Home Page Department of Chemistry Home Page Creative Commons License System Homepage University of Illinois Homepage The Camille & Henry Dreyfus Foundation Homepage Home