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Designing with temperature sensors, part three: RTDs

01 Dec 2011  | Bonnie Baker

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The temperature coefficient of an RTD (resistance-temperature-detector) 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. See a comparison of resistance and temperature performance for RTD sensors and NTC thermistors in the figure below.

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 1kΩ. 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 1mA 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 table:

RRTD(TA)=RRTD(T0) [1+aTA+bTA2+cTA3(100−TA)], where RRTD(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 signal-conditioning circuit in a number of ways. Figure 1 shows an example that uses four OPA334 amplifiers, an REF5025V 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 1mA 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: RW1, RW2, and RW3. A4 provides gain, lowpass filter, such as TI's FilterPro, and 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.

References
Kuehl, Thomas, ""Developing A Precise Pt100 RTD Simulator For SPICE,"" Texas Instruments.

FilterPro Active Filter Design Application, Texas Instruments.

TINA-TI SPICE-Based Analog Simulation Program, Texas Instruments.




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