Analog output is a continuously variable electrical signal that represents the magnitude of a physical quantity or process variable. In instrumentation and control systems, an analog output is typically generated by a device such as a controller, programmable logic controller (PLC), distributed control system (DCS), or signal conditioning module to drive a final control element or to transmit measured data to another device. The defining characteristic of an analog signal is that it can assume any value within a specified range, as opposed to a discrete or digital signal which is limited to defined states (e.g., 0 or 1).
In industrial practice, the most common forms of analog output are voltage and current signals. Standard voltage outputs include ranges such as 0–10 V or 1–5 V. Standard current outputs are typically 0–20 mA or 4–20 mA, with 4–20 mA being widely adopted in process control due to its improved noise immunity and the ability to detect open-circuit faults (indicated by current dropping below 4 mA). The output signal is scaled linearly (unless otherwise specified) to correspond to a defined engineering range. For example, a 4–20 mA output may represent 0–100% valve position or 0–200 °C temperature, where 4 mA corresponds to the lower range value and 20 mA corresponds to the upper range value.
Analog outputs are produced using digital-to-analog conversion (DAC) when the originating system processes information digitally. The DAC converts discrete numerical values into proportional continuous voltage or current levels. The resolution and accuracy of the analog output depend on the DAC resolution (e.g., number of bits), reference stability, calibration, and overall system design.
From an engineering perspective, proper specification of analog output requires consideration of signal range, load impedance, wiring distance, electrical noise environment, grounding strategy, isolation requirements, and accuracy class. These factors directly affect signal integrity and measurement reliability.
