UK What an analogue signal is
An analogue signal is one whose voltage or current varies continuously and can take any value within its range. It is an analogue, a direct copy, of the quantity it measures: as a tank fills, a level sensor’s output rises smoothly with it. In US English it is spelled analog signal. Sound from a microphone, the output of a thermocouple and a 4–20 mA pressure transmitter are all analogue.
Because every value carries meaning, anything that disturbs the signal changes the reading. Electrical noise from motors and drives, resistance in long cables and poor earthing all add error, so analogue wiring in factories uses shielded cable, careful grounding and, often, current rather than voltage.
What a digital signal is
A digital signal has a fixed set of levels, almost always two: high and low, on and off, 1 and 0. The receiver only has to decide which side of a threshold the signal is on, so small amounts of noise make no difference at all.
That is the main advantage of digital: a signal can be copied, stored and passed through repeaters with no loss, because each stage rebuilds the clean levels. It is also why computers, networks and almost all modern communication work digitally, and why analogue measurements are converted to numbers as early as possible.
Industrial networks are digital too. RS-485, CAN bus and industrial Ethernet send measurements as numbers rather than as a voltage, so a reading arrives exactly as it was sent. Some instruments combine both: HART superimposes a small digital signal on a 4–20 mA loop, so one pair of wires carries the analogue reading plus diagnostics and configuration data.
Analogue vs digital compared
Most real systems use both. Sensors measure an analogue world, computers process digital data, and the I/O hardware in between converts from one to the other.
| Property | Analogue signal | Digital signal |
|---|---|---|
| Values | Any value within a range | A fixed set of levels, usually two |
| Shape | Smooth, continuous waveform | Square steps between levels |
| Effect of noise | Adds directly to the reading | Ignored until it crosses a threshold |
| Over long distances | Degrades gradually, amplifiers boost noise too | Regenerated exactly by repeaters |
| Industrial examples | 4–20 mA, 0–10 V, thermocouple, RTD | 24 V on/off inputs, relay contacts, pulse counts |
| Computer interface | ADC for inputs, DAC for outputs | Digital input and output lines |
| Typical module spec | Resolution in bits, sample rate, range | Channel count, voltage, isolation |
Neither is better in general. Analogue suits quantities that change smoothly and need a continuous reading, such as temperature or flow. Digital suits states and counts, and it is the only practical way to send data a long way without loss. The skill is choosing the right I/O for each signal on the machine.
Converting between analogue and digital
An analogue-to-digital converter (ADC) measures a signal at regular intervals and turns each sample into a number. Two specifications decide how faithful the result is:
-
//
Resolution. The number of bits per sample. A 16-bit ADC splits its range into 65,536 steps, so on a ±10 V input each step is about 0.3 mV. A 12-bit converter gives 4,096 steps, around 4.9 mV on the same range.
-
//
Sample rate. How many readings per second. The Nyquist rule says it must be more than twice the highest frequency in the signal, and in practice several times higher is safer.
-
//
Input range and type. Single-ended or differential inputs, and voltage or current ranges, set what the converter can connect to without extra conditioning.
-
//
Digital-to-analogue output. A DAC works the other way, turning numbers into a voltage or current to drive valves, speed controllers or test signals.
Analogue and digital signals in industrial I/O
In automation, analogue signals carry measurements and setpoints: 4–20 mA and 0–10 V transmitters, thermocouples and RTDs for temperature, and strain gauges for force. Our analogue I/O modules read and drive these signals, including 16-bit PC/104 modules with autocalibration, analogue output boards from 4 to 16 channels and a rugged PCIe MiniCard.
Digital signals carry states and events: limit switches, push buttons, relay contacts, alarms and pulse counts from flow meters or encoders. Industrial inputs usually work at 24 V, well above logic levels, and are often optoisolated to protect the computer from spikes on the plant wiring. Our digital I/O modules range from 16 to 96 channels, with optoisolated inputs, relay outputs and counter/timers on some models.
Boards and edge computers often also carry a few general-purpose I/O pins at logic level. Our guide to the differences between DIO and GPIO explains when those are enough and when isolated industrial I/O is needed.
Quick answers on analogue and digital signals
Analogue and digital signals: common questions
What is an analogue signal?
A signal that varies continuously. Its voltage or current can take any value within a range and follows the quantity it represents, such as temperature, pressure or sound, smoothly over time.
What is a digital signal?
A signal limited to fixed levels. Usually just two, high and low, representing 1 and 0. Information is carried by the pattern and timing of those levels rather than their exact value.
Can an analogue signal be converted to digital?
Yes, with an analogue-to-digital converter. An ADC samples the signal at regular intervals and turns each sample into a number. A DAC does the reverse for outputs.
Is Wi-Fi analogue or digital?
Digital data on an analogue carrier. Wi-Fi, 4G and 5G send digital data by modulating radio waves, which are analogue. The receiver turns them back into bits.
Why is 4–20 mA still used?
It is simple, robust and self-checking. Current is not lost over long cable runs the way voltage is, and because the scale starts at 4 mA, a reading of zero shows a broken wire rather than a real value.
We fit and configure analogue and digital I/O modules in industrial and embedded computers as standard, wiring and testing each channel against the signals on site before the system ships.