US What signal conditioning means
Signal conditioning is everything done to a sensor’s electrical signal between the sensor and the converter or controller that reads it. Sensors rarely produce a clean signal in the range an input expects. A thermocouple gives a few millivolts, a strain gauge needs a supply before it gives anything, and a long cable picks up mains hum on the way. Conditioning turns that raw signal into a stable voltage or current that the measuring input can digitize accurately.
The conditioning stage can be a few components on a measurement board, a plug-in module in a data acquisition chassis, or a separate DIN-rail signal conditioner in a control cabinet. The functions are the same in each case; only the packaging changes.
The main signal conditioning functions
Most conditioning circuits combine several of these functions, chosen to suit the sensor.
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Amplification. Boosts millivolt signals so they use the full range of the ADC, which improves effective resolution and the signal-to-noise ratio. Instrumentation amplifiers reject noise common to both input wires.
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Attenuation. Scales down voltages too large for the input, such as a 48 V supply or a mains-derived signal, using resistor dividers or isolated dividers.
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Filtering. Low-pass filters remove noise above the frequencies of interest, including the anti-aliasing filter that must sit before any sampled input. Notch or averaging filters reject 50 or 60 Hz mains interference.
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Isolation. Breaks the electrical path between sensor and measuring system with a transformer, optocoupler or capacitive barrier. It removes ground loops, blocks common-mode voltages and protects the computer from surges.
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Excitation. Supplies the voltage or current that passive sensors need, such as the bridge voltage for a strain gauge or the small constant current through an RTD.
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Linearisation and compensation. Corrects sensors whose output is not a straight line, and adds cold-junction compensation for thermocouples. This is often done in software after the ADC.
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Conversion. Turns one signal type into another, for example a 4–20 mA loop into 1–5 V across a 250 ohm resistor, or a frequency into a voltage.
Signal conditioning by sensor type
The sensor sets the conditioning. This table shows the common industrial sensors, what they produce and what has to happen before the signal can be measured.
| Sensor | Raw output | Conditioning it needs |
|---|---|---|
| Thermocouple | Microvolts per °C (about 41 µV/°C for type K) | Amplification, cold-junction compensation, linearisation, filtering |
| RTD (Pt100) | Resistance change of about 0.385 Ω per °C | Excitation current, 3- or 4-wire lead compensation, linearisation |
| Strain gauge or load cell | A few millivolts per volt of excitation | Bridge excitation, bridge completion, high-gain amplification |
| 4–20 mA transmitter | Current loop | Current-to-voltage conversion, loop power, isolation |
| IEPE accelerometer | AC voltage on a DC bias | Constant-current supply, AC coupling, anti-aliasing filter |
| LVDT | AC signal that varies with position | AC excitation, demodulation, filtering |
| 24 V digital sensor | On/off voltage | Level shifting, optoisolation, debounce |
Values are typical. Always take the excitation level, range and wiring from the sensor datasheet, and match the conditioning to the input of the analog I/O module or controller that will read it.
Signal conditioner vs signal conditioning module
A signal conditioner is a stand-alone device, often DIN-rail mounted, that takes one sensor signal and outputs a standard one, such as 4–20 mA or 0–10 V, usually with isolation. Process plants use them to protect PLC inputs and to convert signals between systems.
A signal conditioning module plugs into a data acquisition system or terminal board and conditions one channel or a group of channels for a particular sensor type, such as thermocouple, strain gauge or isolated voltage inputs. A transmitter goes a step further and puts the conditioning at the sensor head, sending a 4–20 mA or digital signal back to the cabinet.
Digital inputs need conditioning too. A 24 V proximity sensor has to be level-shifted, filtered against contact bounce and usually optoisolated before it reaches the logic of a computer, and the input must suit the sensor’s output type, as our explainer on NPN vs PNP sensors covers.
Where conditioning sits in a measurement system
In a complete measurement chain, conditioning sits between the sensor and the analogue-to-digital converter. It is the step that most often decides accuracy: a 16-bit converter gives little benefit if the signal only uses a tenth of its range or carries more noise than its smallest step. Our explainer on analog vs digital signals covers what the converter does next.
On embedded computers, the conditioning is often built into the I/O module itself: input ranges, filtering and optoisolation are part of the board. Our data acquisition and I/O modules include analog modules with standard voltage ranges, optoisolated digital inputs and isolated CAN interfaces, with external conditioners added where a sensor needs excitation or high-voltage isolation.
Quick answers on signal conditioning
Signal conditioning: common questions
What is a signal conditioning module?
A plug-in conditioning stage for one sensor type. It sits between the sensor wiring and a data acquisition or control input, and provides the amplification, filtering, isolation or excitation that sensor needs.
Does a thermocouple need signal conditioning?
Yes. Its output is only tens of microvolts per degree, so it needs a high-gain amplifier, a filter against mains noise, cold-junction compensation and linearisation before the reading is accurate.
What is the difference between a signal conditioner and a transmitter?
Where the conditioning happens. A transmitter conditions the signal at the sensor and sends a standard signal, usually 4–20 mA. A signal conditioner does the same job in the cabinet, at the receiving end.
What does a load cell signal conditioner do?
It powers the bridge and amplifies its output. It supplies a stable excitation voltage, amplifies the millivolt bridge output, filters it and converts it to a voltage, current or digital value proportional to load.
Is isolation part of signal conditioning?
Yes, and often the most important part. Isolation removes ground loops and common-mode voltages that would otherwise corrupt the reading, and it protects the computer from surges on long field cables.
We specify the I/O modules and any external signal conditioners together with the computer as standard, wire the channels to the sensors you list, and check each input against a known signal before the system ships.