What is the difference between analog input and digital input on a DDC?

• A. Analog inputs measure continuous signals (e.g., 0-10 V, 4-20 mA).
• B. Digital inputs detect discrete states.
• C. Analog inputs are always 0-5V.
• D. Digital inputs require calibration.

Answer: (A)

The essential distinction is between how the signals are represented: analog inputs read continuous, variable signals, while digital inputs read discrete, on/off states. In a DDC, an analog input connects to sensors that can output a value that can vary smoothly across a range—for example, a voltage from 0 to 10 volts or a current in the 4–20 mA range. This lets the controller measure precise quantities like temperature, pressure, or level and respond with proportional control or data logging.

Digital inputs, on the other hand, detect simple binary conditions such as a switch being open or closed, or a relay status being high or low. They don’t convey a range of values, just a single state.

So the best answer highlights that analog inputs measure continuous signals, whereas digital inputs deal with discrete states. The notion that analog inputs are always 0–5 V is too limiting because sensor ranges vary, and digital inputs don’t typically require calibration, which isn’t the core distinction here.