The Role of the DAC
Real-world analog signals such as temperature, pressure, sound, or images are routinely converted to a digital representation that can be more easily stored, processed, or transmitted. In many systems, however, digital information must be reconverted back to the analog domain to carry out some real-world function. The components that perform this step are digital-to-analog converters (DACs), and their outputs are used to drive a variety of devices. Loudspeakers, motors, RF transmitters, and temperature controls are just a few diverse examples. Often, DACs are incorporated into data systems in which real-world signals are digitized by analog-to-digital converters (ADCs), processed, and then converted back to analog by DACs. In these systems, the performance required of the DACs will be influenced by the capabilities and requirements of the other components in the system (see "Real-world sampled data systems").
A DAC produces a quantized (discrete step) analog output in response to a binary digital input code. The digital input may be TTL, ECL, CMOS, or LVDS, while the analog output may be either a voltage or a current. To generate the output, a reference quantity (either a voltage or a current) is divided into binary and/or linear segments. Then the digital input drives switches that connect an appropriate number of segments to the output. The number of segments reflects the number of possible digital input codes, which is a function of converter resolution or the number of bits (N) in the input code. For N bits, there are 2 N possible codes.