Analog-to-Digital Converter

An analog-to-digital converter is device for converting the information contained in the value or magnitude of some characteristic of an input signal, compared to a standard or reference, to information in the form of discrete states of a signal, usually with numerical values assigned to the various combinations of discrete states of the signal.

Analog-to-digital (A/D) converters are used to transform analog information, such as audio signals or measurements of physical variables (e.g., temperature, force, or shaft rotation) into a form suitable for digital handling, which might involve any of these operations:

• processing by a computer or by logic circuits, including arithmetical operations, comparison, sorting, ordering, and code conversion
  
  
• storage until ready for further handling
  
  
• display in numerical or graphical form
  
  
• transmission

If a wide-range analog signal can be converted, with adequate frequency, to an appropriate number of two-level digits, or bits, the digital representation of the signal can be transmitted through a noisy medium without relative degradation of the fine structure of the original signal.

ABOVE: A three-bit binary representation of a range of input signals.

Conversion involves quantizing and encoding. Quantizing means partitioning the analog signal range into a number of discrete quanta and determining to which quantum the input signal belongs. Encoding means assigning a unique digital code to each quantum and determining the code that corresponds to the input signal. The most common system is binary, in which there are 2n quanta (where n is some whole number), numbered consecutively; the code is a set of n physical two-valued levels or bits (1 or 0) corresponding to the binary number associated with the signal quantum.

The illustration shows a typical three-bit binary representation of a range of input signals, partitioned into eight quanta. e.g., a signal in the vicinity of 3/8; full scale (between 5/16 and 7/16) will be coded 011 (binary 3).

Also see: Digital Filter, Digital-to-Analog Converter, Anti-Aliasing