i) Analog-to-digital Converters: In a digital computer control system, the plant output (or controlled variable) is usually an analog signal, which is measured by sensors (or instruments). This analog signal has to be converted into digital signal before it is fed to the computer. The conversion of analog measurements to digital measurements involves three operations, namely sampling, quantization and encoding.
ii) Digital-to-analog Converters: In computer-controlled processes, the output of the digital controller (manipulated variable) is in digital form and must be converted to analog form before it can be communicated to the analog plant. The device that accomplishes this is called digital-to-analog converters. DAC provides reconstruction of discrete-time digital signals into continuous-time analog signals for computer interfacing output data recovery purposes such as actuators, displays, and signals synthesizers. DAC is considered prior to ADC because some ADC circuits require DACs in their implementation. A DAC may be considered a digitally controlled potentiometer that provides an output voltage or current normalized to a full-scale (FS) reference value.