Constant-Current vs Ratiometric RTD Measurement: Accuracy, Drift and Error Sources

Constant-current RTD measurement calculates resistance from a known excitation current and measured voltage, while ratiometric measurement compares the RTD voltage with a precision reference resistor. Ratiometric designs reduce sensitivity to excitation-current drift, but neither method is inherently more accurate; performance ultimately depends on component stability, measurement-path matching, wiring, calibration and installation.

How Constant-Current RTD Measurement Works

A precision current source supplies a known excitation current to the RTD. The instrument measures the resulting voltage and calculates resistance using Ohm’s law:

R_RTD = V_RTD ÷ I_exc

This method does not require a separate reference-resistor measurement. Its accuracy depends on the excitation current, voltage-measurement gain and analogue-to-digital converter reference. Initial errors can be corrected during calibration, but temperature-dependent and long-term drift after calibration can still affect the result.

A 1% error in the assumed excitation current produces approximately a 1% resistance error. Constant-current excitation nevertheless provides a relatively simple architecture and can achieve high accuracy when the current source and voltage-measurement path are stable.

How Ratiometric RTD Measurement Works

In a ratiometric system, the same current flows through the RTD and a precision reference resistor:

V_RTD = I × R_RTD

V_ref = I × R_ref

The RTD resistance is calculated from the voltage ratio:

R_RTD = R_ref × (V_RTD ÷ V_ref)

The excitation current cancels mathematically. If it changes by 1%, both voltages change by approximately 1%, while their ratio remains substantially unchanged.

Accuracy depends primarily on the reference resistor and how closely both measurement paths track each other. The reference resistor should have low tolerance, low temperature coefficient, low long-term drift and minimal self-heating.

Ratiometric measurement is often preferred for high accuracy and long-term stability because it reduces sensitivity to excitation-current drift. In a fully ratiometric converter architecture, reference-voltage and shared gain changes can also cancel.

Constant-Current vs Ratiometric RTD Measurement: Which Is More Accurate?

Ratiometric measurement is not automatically more accurate. A poor reference resistor or unequal measurement paths can limit performance, while a well-designed constant-current system can provide excellent accuracy.

High-Accuracy RTD Measurement with the Gantner Instruments Q.series X A105

The Gantner Instruments A105 is designed for precise and stable RTD and resistance measurement. It provides four galvanically isolated channels for Pt100, Pt1000 and resistance sensors, helping prevent electrical interaction between measurement channels. Its 24-bit conversion supports high measurement resolution.

The module combines a specified measurement accuracy of 0.05 °C with temperature stability of 0.02 °C per 10 °C change in ambient temperature. This makes it particularly valuable where small temperature differences must remain trustworthy despite changing environmental conditions. Filtering, linearisation, minimum and maximum detection, and alarm functions can be configured independently for each channel.

Neither measurement principle automatically eliminates lead-wire resistance, RTD self-heating, noise, sensor tolerance or installation errors. Suitable two-, three- or four-wire connections, appropriate excitation and careful calibration remain essential.

RTD Measurement Error Sources

Accurate RTD temperature measurement depends on more than the excitation method. Whether a constant-current or ratiometric circuit is used, several error sources can affect the final temperature result:

• Excitation-current drift: Changes in excitation current can introduce resistance errors in constant-current measurements. Ratiometric designs largely cancel this effect by measuring the RTD and reference resistor with the same current.
• Lead-wire resistance: Cable resistance adds measurement error, particularly with 2-wire RTDs. Three-wire compensation reduces this effect, while 4-wire measurement provides the best compensation for precision applications.
• RTD self-heating: Excitation current dissipates power in the RTD, causing its temperature to rise slightly above the actual measured temperature. Lower excitation currents help reduce this error.
• Reference and component drift: Reference resistors, ADCs, amplifiers, and other analog components can change with temperature and time. Low-temperature-coefficient components and calibration help minimize these errors.
• Electrical noise: Electromagnetic interference, ground loops, long sensor cables, and inadequate shielding can introduce unstable readings. Proper wiring, shielding, filtering, and grounding are essential in industrial environments.
• Sensor tolerance and installation: RTD accuracy is also influenced by sensor class, calibration, thermal coupling, response time, and installation conditions.

For high-accuracy Pt100 and Pt1000 measurements, the complete measurement chain, including the sensor, wiring, excitation source, reference components, ADC, calibration, and installation, must therefore be considered when evaluating total measurement uncertainty.

Frequently Asked Questions About RTD Measurement 

What is the difference between constant-current and ratiometric RTD measurement?

Constant-current measurement determines RTD resistance from the measured voltage and a known excitation current using Ohm's law. Ratiometric measurement compares the voltage across the RTD with the voltage across a precision reference resistor carrying the same current, allowing excitation-current variations to largely cancel.

Is ratiometric RTD measurement more accurate than constant-current measurement?

Not automatically. Ratiometric measurement can improve long-term stability by reducing sensitivity to excitation-current drift, but accuracy still depends on the reference resistor, measurement-path matching, temperature coefficients, ADC performance and calibration. A well-designed constant-current measurement system can also achieve high accuracy.

Why does excitation-current drift affect RTD measurement accuracy?

In a constant-current system, RTD resistance is calculated using the assumed excitation current. Therefore, an error in that current directly produces a corresponding resistance error. In a ratiometric system, the same current passes through the RTD and reference resistor, so changes in excitation current largely cancel in the voltage ratio.

Does ratiometric measurement eliminate RTD lead-wire and self-heating errors?

No. Ratiometric measurement reduces specific electronic errors but does not inherently eliminate lead-wire resistance, RTD self-heating, sensor tolerance, electrical noise or installation errors. Appropriate 2-, 3- or 4-wire connections, excitation levels and calibration remain important.

Which Gantner Instruments module is designed for high-accuracy Pt100 and Pt1000 measurement?

The Q.series X A105 family supports Pt100, Pt1000 and resistance measurement with 2-, 3- or 4-wire connections. Current product specifications include four analog input channels, 24-bit conversion, up to 0.05 °C maximum Pt100 measurement error and 0.02 °C per 10 °C temperature drift.