Match RTD Lead Wiring to the Measurement Circuit

A connector with four pins does not automatically provide a four-wire RTD measurement. The excitation and voltage-sense paths must remain separate to the intended sensing points. Likewise, a three-wire circuit compensates lead effects only under the assumptions of its particular topology and measurement sequence.

Draw the complete resistance path

Include the RTD element, cable conductors, connector contacts, protection resistors and PCB traces. Mark where excitation current flows and where voltage is measured. In a two-wire arrangement, lead resistance contributes to the measured path. Avoid subtracting one nominal cable resistance without considering temperature and connection changes.

Texas Instruments’ guide to RTD measurements compares two-, three- and four-wire configurations and explains lead-resistance compensation. In three-wire implementations, matching assumptions matter. Use the equations for the selected circuit rather than assuming all three-wire interfaces cancel any cable resistance.

Check connector and protection symmetry

Verify pin assignments against the sensor cable drawing, not just wire colors remembered from another probe. Record which conductors terminate at the same end of the RTD. A swapped connection can produce a plausible reading at one condition while breaking the intended cancellation.

Inspect filters and protection components in each relevant path. Resistance mismatch, leakage and current-source mismatch can influence the result depending on the topology. Components intended to protect the ADC should be included in the error analysis, not treated as electrically invisible.

Exercise the actual measurement sequence

If the design switches excitation sources or measurement inputs, document the order and settling interval. Confirm that software associates each conversion with the correct state. A static schematic review will not catch a conversion taken before the new current or input path settles.

Use a suitable resistance standard or simulator to separate interface behavior from sensor temperature uncertainty. Add controlled lead resistance to the appropriate conductors and compare the measured change with the circuit’s predicted behavior. For a three-wire design, test unequal lead resistances as well as equal ones.

Include faults and usable limits

Test open leads, shorted leads and out-of-range resistance using conditions allowed by the interface. Define how the product reports invalid measurements. A numerical temperature produced from a saturated ADC code should not be mistaken for a valid extreme temperature.

Check excitation compliance and input common-mode limits over the resistance range. Also evaluate RTD self-heating under the actual excitation schedule. Preserve the cable configuration, compensation equations, component assumptions and test results together. That evidence makes a later cable extension or protection-network change reviewable instead of relying on the label ‘three-wire RTD’ as a complete design description.