Principles of analog-to-digital conversion

The techniques for analog-to-digital conversion can be classified into one of three fundamental principles they differ in conversion speed and sophistication of physical implementation. 

Flash converters are mainly used in very fast measurement devices with low to medium requirements on resolution (e.g. digital sampling oscilloscopes or video digitizers). 

This technique is called successive approximation as the digitized value is better approximated with each step of conversion. Compared with the word-at-a-time method the method of successive approximation needs only ld(n) reference voltages but it is slower than the above mentioned technique by a factor of ld(n) (n = number of discrete steps, Id = logarithm with basis 2). 

ADCs based on successive approximation are the most popular converters as they are both cheap and fast enough to sample signals in the kHz range. 

Voltage-to-frequencv-converters (V/F). The applied input voltage is converted into an oscillation with a frequency proportional to the input voltage. The resulting frequency is measured within fixed time intervals. 

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