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Baker's Best: Designing with temperature sensors, part three: RTDs

The temperature coefficient of an RTD (resistance-temperaturedetector) element is positive. Most stable, linear, and repeatable RTDs are of platinum-metal construction. You can use the constant 0.00385Ω/Ω/°C to approximate the resistance change over temperature for the platinum RTD element. In contrast, the NTC (negative-temperature-coefficient) thermistor has a negative change with increasing temperature. Figure A is a comparison of resistance and temperature performance for RTD sensors and NTC thermistors.

The RTD element’s resistance is much lower than that of an NTC thermistor element, which ranges to 1 MΩ at 25°C. Typical specified 0°C values for RTDs are 25Ω to 1 kΩ. Of these options, the 100Ω platinum RTD is the most stable over time and linear over temperature.

An RTD element must be excited with a stable current reference at a level that does not create an error due to self-heating. A current source that is 1 mA or less is usually adequate. Under this circumstance, the accuracy of an RTD can be ±4.3°C over its temperature range of −200 to +800°C. If higher accuracy is required, you can use the Callendar-Van Dusen equation to generate a look-up T_Able: R_RTD(TA)=R_RTD(T0) [1+aT_A+bT_A ²+cT_A ³(100−T_A)], where R_RTD(TA) is the resistance of the RTD at ambient RTD temperature; RRTD(T0) is the value of the RTD at 0°C; and a, b, and c are constants, supplied by the RTD vendor.

You can implement an RTD signalconditioning circuit in a number of ways. Figure 1 shows an example that uses four OPA₃34 amplifiers, an REF5025 voltage reference, an ADS8634 ADC, and an MSP430C1101 microcontroller, all from Texas Instruments, as well as a PT100 RTD (Reference 1). In this figure, a 2.5V reference, A₁, A₂, and five resistors generate a 1-mA current source.

The signal-conditioning portion of the circuit includes A₃ and A₄. A₃ senses the voltage drop across the RTD element and cancels the RTD wire resistance errors: R_W1, R_W2, and R_W3. A₄ provides gain, and a lowpass filter, such as TI’s FilterPro, provides the RTD’s output voltage (Reference 2). In this circuit, the RTD element has a value of 100Ω at 0°C. If this RTD senses temperature over its entire range of −200 to +600°C, the RTD would provide a nominal 23 to 331Ω range of resistance. You can use TINA-TI to simulate the analog portion of this circuit (Reference 3). Within TINA-TI’s examples, under the Files tab, a PT100 RTD element accurately simulates the correction of the nonlinearity of the RTD.

The circuit in Figure 1 generates a current source that is ratiometric to the voltage reference. The ADC uses the same voltage reference to provide a ratiometric digital output. Over temperature, the ADC digitizes the changes in the RTD resistance. Although an RTD requires more circuitry in the signal-conditioning path than a thermistor or a silicon temperature sensor requires, it ultimately provides a high-precision, relatively linear result over a wider temperature range. If you use the Callendar-Van Dusen equation, this RTD circuit can achieve ±0.01°C accuracy.

Read the first two parts of this series;

www.edn-europe.com/article.asp?articleid=5211 www.edn-europe.com/article.asp?articleid=5261

REFERENCES

Kuehl, Thomas, “Developing A Precise Pt100 RTD Simulator For SPICE,” Texas Instruments. www.en-genius.net/ includes/files/acqt_052807.pdf FilterPro Active Filter Design Application, Texas Instruments. www.ti.com/tool/filterpro TINA-TI SPICE-Based Analog Simulation Program, Texas Instruments. www.ti.com/tool/tina-ti