Analog-to-digital converter

An analog-to-digital converter (often referred to using the shorter abbreviations, a-to-d converter, ADC, or A/D) is a device (usually attached to a computer as a peripheral) which takes an analog signal and measures its amplitude (usually the voltage) and converts the measurement to digital form. It can thus convert analog data to digital data.

Implementation basics

Depending on how much resolution (i.e. how many bits in the digital output value) is desired in the reading, and how quickly it is desired to get a reading, there are quite a few potential techniques that can be used. Almost all make use of one or more comparators (a circuit which compares two input voltages, and indicates which one is higher).

The simplest mechanism for performing the A->D function uses a 'reference voltage' and a 'voltage division network' of resistors; the network is wired to produce increasing partial values (1/2, 1/4's, etc) of the reference voltage. A comparator is wired to each, along with the input. This produces a reading quickly, but has the down-side that it requires 2^N comparators, for an N-bit output.

Another approach is to use a variable, controlled, voltage reference; this only needs one comparator, but takes more time (to adjust the reference voltage up and down). The voltage of the reference source is adjusted until it matches that of the input. There are numerous variations within this basic approach, depending on what pattern it uses to adjust the voltage (binary search, etc).

See also

• Digital-to-analog converter

Further reading

• Barbera W. Stephenson, Analog<—>Digital Conversion Handbook, E-5100, Digital Equipment Corporation, Maynard, 1964 - covers a very early state of the technology