Flash ADC
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A Flash ADC (also known as a Direct conversion ADC) is a type of analog-to-digital converter that uses a linear voltage ladder with a comparator at each "rung" of the ladder to compare the input voltage to successive reference voltages. The output of these comparators is generally fed into a digital encoder which converts the inputs into a binary value (the collected outputs from the comparators can be thought of as a unary value).
[edit] Benefits and drawbacks
Flash converters are extremely fast compared to many other types of ADCs which usually narrow in on the "correct" answer over a series of stages. Compared to these, a Flash converter is also quite simple and only requires logic for the final conversion to binary.
A Flash converter requires a huge number of comparators compared to other ADCs, especially as the precision increases. A Flash converter requires 2n-1 comparators for an n-bit conversion. In place of these comparators, most other ADCs substitute more complex logic which can be scaled more easily for increased precision. The size and cost of all those comparators makes Flash converters generally impractical for precisions much greater than 8 bits (255 comparators).
