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Concept
Flash ADC
Summary
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. Often these reference ladders are constructed of many resistors; however, modern implementations show that capacitive voltage division is also possible. 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).
Flash converters are high-speed compared to many other ADCs, which usually narrow in on the "correct" answer over a series of stages. However, compared to these, a flash converter is also quite simple and, apart from the analog comparators, only requires logic for the final conversion to binary.
For best accuracy, a track-and-hold circuit is often inserted in front of the ADC input. This is needed for many ADC types (like successive approximation ADC), but for flash ADCs, there is no real need for this because the comparators are the sampling devices.
A flash converter requires many comparators compared to other ADCs, especially as the precision increases. For example, a flash converter requires comparators for an n-bit conversion. The size, power consumption, and cost of all those comparators make flash converters generally impractical for precisions much greater than 8 bits (255 comparators). In place of these comparators, most other ADCs substitute more complex logic and/or analog circuitry that can be scaled more easily for increased precision.
Flash ADCs have been implemented in many technologies, varying from silicon-based bipolar (BJT) and complementary metal–oxide FETs (CMOS) technologies to rarely used III-V technologies. This type of ADC is often used as a first medium-sized analog circuit verification.
The earliest implementations consisted of a reference ladder of well-matched resistors connected to a reference voltage.
Official source
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