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Flash Analog to Digital Conversion

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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.



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