3-Wire RTD Measurement Using Polarity Inversion

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Solution Overview

Problem

Existing temperature sensing circuits using 3-wire RTDs face inaccuracies due to mismatched parasitic resistances in lead wires, requiring complex matching of current sources to achieve accurate resistance measurements.

Innovation Solution

A temperature sensing circuit method that uses a switch circuit to make resistance measurements in opposite polarity configurations, eliminating the need for matched current sources by deriving the RTD resistance from two voltage measurements across the RTD and sense resistor, implemented using pass transistors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If matched current sources are used to compensate for parasitic lead resistances, then measurement precision is improved, but device complexity increases

Engineering Contradiction:

Improvetemperature measurement accuracyVSAvoidcurrent source matching complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

Instead of matching current sources to compensate for lead wire resistance errors, the patent inverts the approach by switching the measurement polarity. By measuring voltage across the RTD with reversed current direction and using the difference between two measurements, the method eliminates the need for matched current sources while achieving accurate resistance measurement.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent introduces a switch circuit as an intermediary component that enables polarity reversal without requiring matched current sources. The switch circuit allows the measurement system to alternate between two configurations, using the voltage difference to cancel out parasitic lead resistance effects.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If matched current sources are implemented, then measurement precision is improved, but ease of manufacture deteriorates

Engineering Contradiction:

Improveresistance measurement accuracyVSAvoidmanufacturing simplicity

Core Design Contradiction:

Measurement precisionVSEase of manufacture

Solution Approach 1:

The patent eliminates the complex manufacturing process of matching current sources by inverting the measurement approach. Instead of designing matched current sources, the system uses a switch circuit to reverse polarity and calculates resistance from the voltage difference, significantly simplifying manufacturing.

Inventive Principle:

Principle #13The other way round (Inversion)

3Measurement precision

If matched current sources are used, then measurement precision is improved, but testing complexity increases

Engineering Contradiction:

Improvetemperature sensing accuracyVSAvoidtesting complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent simplifies testing by eliminating the need to verify current source matching. Instead, the test procedure involves switching the polarity and measuring voltage differences, which is a simpler and more straightforward testing process that inherently compensates for lead wire resistance.

Inventive Principle:

Principle #13The other way round (Inversion)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach provides accurate temperature sensing independent of current source matching, simplifying the design, testing, and manufacturing processes by eliminating the complexity associated with matching current sources.

Implementation Method 1

The resistance temperature detector (RTD) is a common temperature sensor that varies its resistance with temperature

Methodology Applied

Scientific EffectResistive temperature detection: Electrical Resistance

Implementation Method 2

connecting a first terminal of the RTD device to a first current source and connecting a second terminal of the RTD device to a second current source; measuring a first voltage across the RTD device

Methodology Applied

Scientific EffectElectrical conduction: Conduction (electrical)

Data Source

PatentEP2921832B13-wire resistance temperature detector and method of measuring said resistance

Publication Date: 2016.06.15 LINEAR TECHNOLOGY CORP

Figure 1~2b

AI summary

In a temperature sensing circuit, a method for measuring a resistance of a RTD device to sense temperature includes (a) connecting a first terminal of the RTD device to a first current source and connecting a second terminal of the RTD device to a second current source; (b) measuring a first voltage across the RTD device; (c) connecting the second terminal of the RTD device to the first current source and connecting the first terminal of the RTD device to the second current source; (d) measuring a second voltage across the RTD device; and (e) deriving the resistance of the RTD device based on the first voltage measurement and the second voltage measurement. The RTD device may be connected in series with a sense resistor to ground.