Because most signals in the real world are analog but digital computers work on discretizations, it is necessary to convert between digital and analog signals. As mentioned above, this is done through digital-to-analog and analog-to-digital converters. Most approaches to digital-to-analog conversion use summers, where the voltages representing the digital bits are applied to input resistors, either directly or indirectly through switches gated on by the digital bits which change the input resistance fed by a dc source.
In design documentation, a document showing in conventional notation the components of an article or installation and the connections or links between components. Diagrams are usually drawn without regard to scale or to the actual spatial arrangement of the components. They may be classified according to the type of components and connections between components as electrical, pneumatic, hydraulic, kinematic, or combination diagrams. They may also be classified according to the purpose for which they will be used.
Since active devices usually supply signal energy to an electronic circuit, and since energy can only be transformed and not created, a source of energy is needed when active devices are present. This energy is usually obtained from batteries or through rectification of sinusoidal voltages supplied by power companies. When inserted into an electronic circuit, such a source of energy fixes the quiescent operation of the circuit; that is, it allows the circuit to be biased to a given operating point with no signal applied, so that when a signal is present it will be processed properly. To be useful, an electronic circuit produces one or more outputs; often inputs are applied to produce the outputs. These inputs and outputs are called the signals and, consequently, generally differ from the bias quantities, though often it is hard to separate signal and bias variables. Biasing of electronic circuits is an important, non- trivial, and often overlooked aspect of their operation.
In basic diagrams, the standard symbols for the circuit components are arranged in correspondence with the operational principle of the device, as well as to facilitate the reading of the diagram. In design diagrams, all or some of the components are represented as equivalent circuits; in such diagrams, sources of electromotive force, current, resistance, inductance, capacitance, and so on are assumed to be components with lumped parameters.