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 Question 1029

 How do analog to digital converters change the analog input signal into a stream of numbers? — RME, Santa Monica, CA
A typical analog-to-digital converter (ADC) uses a process called "successive approximation" to find a binary number that accurately represents the voltage on an input wire. It samples the voltage on the input wire at one moment in time and then gradually constructs a binary number representing that voltage. The ADC tries various binary numbers and uses a digital-to-analog converter to form a voltage from each number. It compares the two voltages, the original and its approximation, to determine how close its current guess is to the correct value. With each successive approximation, it adds a bit a precision to its measurement so that after 16 approximations, it has a 16 bit number that accurately represents the voltage on the input wire.

For applications requiring even faster measurements, there are flash ADCs. These devices synthesize the entire range of possible voltages and then compare the input voltage directly with the complete collection of possible voltages. Since 8 binary bits can represent 256 possible numbers, an 8 bit flash ADC synthesizes 255 different voltages and makes 255 voltage comparisons simultaneously. It instantly determines where among the various voltages the input voltage falls and it reports this value in billionths of a second.