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Successive Approximations ADC

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From the time of their invention in the 1940s until the turn of the 21st Century, Successive Approximation ADCs were the most common choice for high resolution, low cost, intermediate speed digitization. They remain popular except for the very highest resolution digitization.

It is CRITICAL that a sample and hold stage precede a successive approximations converter! As we will see, the sampled signal must be stable for the entire duration of a conversion.

A successive approximations ADC has much in common with the children's classic, "The Story of the Three Bears." At each stage of the story, results are too hot, too cold, or just right, too big, too small, or just right, etc. Because measurements are hardly ever "just right" (due to noise), a guess at an answer to any question will likely be too high or too low. So if one starts making large amplitude guesses, then takes progressively smaller steps, one will always be too high or too low, but will iterate to "very close to just right."



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