What are analog inputs and outputs?

An analog input is an interface that reads an incoming measurement signal. An analog output is an interface that sends a measurement or control command to another device. Put simply, an input receives information, and an output transmits it. Direction depends on the device being considered: A sensor's analog output can connect to a PLC's analog input.

An analog signal represents intermediate values within a range, rather than just an on/off state. In industrial automation, AI stands for analog input, and AO stands for analog output. An analog signal does not have to keep changing; a steady pressure measurement can produce a steady signal.

What does an analog input do?

A sensor detects a physical quantity. A transmitter or signal conditioner, where required, converts it into a suitable electrical signal. The analog input reads that signal. For example, a pressure sensor can represent 0–10 bar with 4–20 mA. The PLC input's analog-to-digital converter (ADC) converts the electrical signal into a raw numerical value; the program scales this into engineering units such as bar, °C, or metres.

Not every sensor produces a standard voltage or current signal. A load cell may need a bridge measurement input or amplifier, while RTDs and thermocouples require compatible temperature inputs. For temperature measurement, select the sensor, wiring arrangement, and input module together.

What does an analog output do?

The PLC program calculates a command. A typical analog output module uses a digital-to-analog converter (DAC) and output circuitry to generate a voltage or current signal. This can set a drive's speed reference, a proportional valve's position command, or a power controller's demand. Analog current and analog voltage are two common signal formats.

For example, if the PLC calculates a 40% heating demand, linear scaling can produce 4 V on a 0–10 V output or 10.4 mA on a 4–20 mA output. The power controller interprets that command and supplies energy to the heater. A 40% command does not necessarily mean exactly 40% instantaneous electrical power; the controller's operating method and configuration determine the result.

Measurements and commands are different

Measurement example: If 0–100 °C is mapped linearly to 4–20 mA, 12 mA represents 50 °C. Command example: If 0–100% demand uses the same signal range, 12 mA represents 50% demand. The meaning of a current depends on its scaling and purpose. Choosing 0–10 V or 4–20 mA is only part of the setup; the lower and upper limits must also be defined correctly.

How do inputs and outputs work together?

In PID temperature control, a sensor reports the actual temperature. The controller compares it with the setpoint and calculates an output demand. The power controller drives the heater, and the sensor measures the resulting temperature. This forms a closed loop. PLC-based PID control can use the same approach for pressure, level, and flow control.

Resolution, accuracy, and speed

Resolution concerns the size of the distinguishable steps; accuracy concerns how close the result is to the true value. An ideal 12-bit converter provides 4096 codes. Across a full 0–10 V range, its ideal step is approximately 2.44 mV. This calculation does not establish measurement accuracy: Sensor error, noise, temperature effects, and module errors also matter.

Sample rate describes how often the input is measured; update rate describes how often the output is refreshed. The PLC cycle and filtering affect overall response. Stronger filtering can smooth a reading while increasing delay. Consider time delays and PID tuning together.

Selection and commissioning

Signal compatibility: Check whether the input and output support 0–10 V, ±10 V, 0–20 mA, or 4–20 mA. Current and voltage interfaces are wired differently. Supply and load: Check the current loop's supply requirements, active/passive arrangement, and allowable load. Select the power supply with the receiver's input requirements in mind.

Wiring and isolation: Route signal cables appropriately away from strong interference sources. Follow the equipment documentation for shielding and grounding. Scaling and calibration: Apply known low, middle, and high values and compare both the electrical signal and displayed engineering units. Do not use calibration adjustments to conceal incorrect scaling.

Fault behaviour: Define what the output should do when a cable breaks, a sensor fails, the PLC stops, or power is lost. 0 V or 4 mA does not automatically represent a safe state. The required state depends on the equipment and process.

Technical references: NI: analog module specifications · Siemens: analog and PWM heating outputs