A temperature measurement is only as accurate as the input module that reads it. An RTD sensor capable of ±0.1°C repeatability, connected to a PLC input module with inadequate channel-to-channel isolation, produces readings that drift with every change in the adjacent channel — the neighboring thermocouple switches from 200°C to 800°C, and the RTD reading shifts by 0.5°C. This is not a sensor error; it is a module design error. Choosing between RTD and thermocouple input modules is not just about matching the sensor type — it is about understanding the electrical environment the module operates in, the isolation topology that protects each channel, and the noise rejection mechanisms that separate signal from interference.
RTD Input Modules: Excitation Current, Lead Wire Compensation, and Self-Heating
An RTD input module works by passing a precision excitation current — typically 0.5–1.0 mA for a Pt100 — through the platinum sensing element and measuring the resulting voltage drop. The module must perform two error corrections simultaneously: compensating for the resistance of the lead wires connecting the sensor to the module (which adds in series with the RTD element), and managing the self-heating effect of the excitation current itself (which raises the sensor temperature by roughly 0.05–0.1°C per milliwatt of power dissipation).
Three-wire RTD connection is the minimum acceptable configuration for industrial applications — it cancels lead-wire resistance by measuring the voltage drop across one lead and subtracting it from the total. Four-wire connection eliminates lead-wire error entirely by separating the current-carrying and voltage-sensing paths, but requires an input module designed with four dedicated terminals per channel. The module selection criterion is not just "supports RTD" — it is whether the module provides the wire-count configuration your installation requires.
Thermocouple Input Modules: Cold Junction Compensation and Microvolt Sensitivity
A thermocouple input module faces a fundamentally different measurement challenge. The thermocouple generates a microvolt-level signal — a Type K thermocouple produces roughly 41 microvolts per degree Celsius — that must be amplified by a factor of 100–1,000 before the analog-to-digital converter can resolve it. The module must also compensate for the cold junction: the point where the thermocouple wires connect to the module terminals creates an unintended thermocouple at the terminal block, and the module must measure the terminal temperature and add the equivalent voltage to the measured signal.
The quality of cold junction compensation (CJC) is the single largest source of measurement error in thermocouple input modules. A module with a CJC sensor buried inside the PCB, 2 cm from the terminals, can see a 2–3°C offset between the actual terminal temperature and the measured reference temperature. High-performance modules place the CJC sensor directly at the terminal block, reducing this offset to under 0.3°C. When comparing modules, the CJC accuracy specification — not the ADC resolution — determines real-world measurement performance. Browse temperature sensors for compatible RTD and thermocouple options.
Channel-to-Channel Isolation: The Specification That Separates Precision from Drift
Channel-to-channel isolation is the most overlooked specification in temperature input module selection. In a non-isolated or group-isolated module, channels share a common ground reference. A ground potential difference of even 50 mV between two thermocouple channels — caused by different cable lengths, different conduit paths, or different proximity to motor leads — appears as a differential signal at the ADC input. For a Type K thermocouple at 41 microvolts per degree, 50 mV of common-mode noise translates to approximately 1.2°C of apparent temperature shift.
Channel-to-channel isolated modules place an isolation barrier — typically an isolated DC-DC converter and a digital isolator or optocoupler — between each channel, allowing each input to float independently. This eliminates common-mode noise between channels and also protects the module from a wiring fault: if a thermocouple on channel 1 shorts to a 24 V DC power line, channel-to-channel isolation prevents the fault from damaging channels 2 through 8. The cost premium for channel-to-channel isolation is typically 30–50% per channel, but this pays for itself the first time a wiring fault on one channel would have taken down all eight.
When Each Sensor Type Justifies a Dedicated Module
| Application | Sensor | Module Priority |
|---|---|---|
| HVAC room temperature, ±1°C accuracy | RTD Pt100, 3-wire | Group-isolated acceptable |
| Pharmaceutical reactor, ±0.2°C accuracy | RTD Pt100, 4-wire | Channel-to-channel isolation required |
| Furnace control, 200–1,200°C range | Type K thermocouple | Channel-to-channel isolation + CJC at terminals |
| Multi-zone plastic extrusion, 8 zones | Type J thermocouple | Channel-to-channel isolation mandatory |
| Cryogenic storage, -196°C | RTD Pt100, 4-wire | Channel-to-channel isolation + low excitation current |
The sensor tells you what the temperature is; the input module determines whether you can trust that number when the channel next to it changes state. For multi-zone temperature control, channel-to-channel isolation is not an upgrade — it is the minimum spec that makes the measurement independent of its neighbors.
