Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
You need to apply some formula on the measured result in order to get the real temperature. Usually these calculations are floating point so the lookup table is faster and also smaller as size approach.
You need to apply some formula on the measured result in order to get the real temperature. Usually these calculations are floating point so the lookup table is faster and also smaller as size approach.
It's not necessary to have floating point lookup table to get floating point output results. Normally you measure with 10 bit ADC and your range consist from about 300-500 different values which will be placed into look up table. These values use usually piecewise linearization - after the look up table you get linear results and you need some simple division / multiplication to get the floating point data. If you apply oversampling you can move inside these values and you can get 0.01 deg or even better resolution on the price of slower response time
Without showing the source code, the post seems unrelated to the discussion. You didn't even tell a word about the implemented temperature calculation. It's no problem for me, because i know how to. But it's unkind to edaboard members seeking for help.
P.S.: By running the simulation, I realized that you are using a linear voltage to temperature function, as previously suggested. If you increase the temperature to 400 °C, the deviation from real Pt100 characteristic becomes obvious. But it's just O.K. for the limited temperature range. You can increase the resolution by scaling the tempearture range of interest to 0..5V ADC range. It would be also highly suggested to use the same reference voltage for Pt100 current source and ADC. In this case no voltage regulation is required, the problem reduces to a simple ratiometric measurement.
You need to apply some formula on the measured result in order to get the real temperature. Usually these calculations are floating point so the lookup table is faster and also smaller as size approach.
You need to apply some formula on the measured result in order to get the real temperature. Usually these calculations are floating point so the lookup table is faster and also smaller as size approach.
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 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 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 OPA335 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: RW1, RW2, and RW3. 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.
---------- Post added at 10:10 ---------- Previous post was at 10:10 ----------
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 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 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 OPA335 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: RW1, RW2, and RW3. 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.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
