0 Preface
The metrological verification instrument is mainly used to verify and calibrate whether the sensors used in daily scientific research and industrial production meet the standards of use. In essence, metrological verification instruments are standard instruments with higher accuracy and resolution than the instruments used today. At present, the platinum resistance verification and calibration device used by the provincial metrological verification department, which undertakes a large number of industrial enterprises and some scientific research institutes, has an accuracy of about 0.1 degrees and a resolution of 0.01 degrees. As the requirements for temperature measurement in the product development or scientific research process of some enterprises are getting higher and higher, the measurement accuracy of the current verification device can no longer meet the requirements. Moreover, the structure of the verification device is large and complicated, and it is difficult to be used as a transferable metrological verification. The device provides the user with high-precision temperature measurement or verification service. In order to meet the requirements of scientific research and industrial production for temperature measurement accuracy and the miniaturization and easy transfer of platinum resistance measurement thermometers, this paper proposes a design scheme of platinum resistance thermometer based on TI's ADS1247 chip.
1 Measuring principle
There are generally two ways to convert a resistance value of a platinum resistance as a function of temperature into a voltage signal: a bridge method and a constant current source method. Since the bridge method has unavoidable nonlinearities and requires multiple resistors to match, systematic errors are introduced in the measurement. The constant current source method has the characteristics of simple structure and good linearity. However, due to the fluctuation of the output current of the constant current source, a random error is introduced in the measurement process, and a proportional measurement form of a standard resistor in series in the measurement path is usually adopted. To eliminate the fluctuation of the constant current source. If the output of the constant current source is stable, then the voltage drop of the platinum resistor can be directly measured without using a series standard resistor for temperature measurement.
The ADS1247 is a high performance, high precision 24-bit analog to digital converter from TI. The ADS1247 monolithically integrates a low noise programmable gain amplifier, a precision (ΔΣ) ADC with a single-cycle low-pass digital filter and an internal clock. Built-in 10mA low drift power reference and two programmable current-mode digital-to-analog converters (DACs). Through the program settings, within the output voltage margin, the DACS can provide a constant current of various strengths for the outside, and the current intensity can be set to 50uA, 100uA, 250uA, 500uA, 750uA, 1000uA, 1500uA. The absolute error of the output current is related to the output current intensity. The smaller the intensity, the smaller the error. The current fluctuates with temperature by about 100 ppm/°C.
The ADS1247 also features a programmable amplifier (PGA) with amplification factors of 1, 2, 4, 8, 16, 32, 64, 128. The principle of temperature measurement with the ADS1247 is shown in Figure 1:
In Figure 1, the purpose of using a matching resistor is to operate the constant current source in the optimum linear region. The configuration of the ADS124 and the reading of the test data are performed by the microcontroller via the SPI bus.
2 Implementation methods and experiments
When the ADS1247 adopts the internal reference voltage, the data sampling rate is 5SPS, and the PGA amplification factor is 128 times, the internal noise voltage peak-to-peak value can be less than 0.05uA. The resistance range of Pt100 in the range of -30-300 °C is 88.22-212.05 ohms. Therefore, in order to achieve the lowest noise conditions, the ADS1247 should be configured to have a constant current of 100uA. In order to make the constant current output constant current have high stability and linearity, the output voltage range of the pin should be in the range of 3.5-4V. Therefore, a resistance of 37.4K ohm is connected in series with pt100. . The ADS1247 uses an internal reference voltage with an analog supply voltage (AVDD) and a digital supply voltage (DVDD) of 5V, separated by a 0 ohm resistor. All pins connected to analog ground (AGND) are single-ended to ground, and the analog ground and digital ground (DVDD) are also isolated by a 0 ohm resistor.
After receiving the measurement command input by the keyboard, the MCU starts the ADS1247 and configures the ADS1247. After the MCU delays for 100ms, it starts a measurement and collects 30 data. According to the Pt100 index table, the voltage range corresponding to each degree Celsius is an interpolation interval, and the range is one ten thousandth of a degree Celsius. The unit is linearly interpolated. The temperature data obtained after the interpolation is divided into three groups according to the chronological order of the collection, and each group has 10 data. According to the formula (1), the variance sum of the temperature data in each group is calculated.
The variance reflects the degree of data fluctuation. In the three sets of data, the variance and the smallest set of data, that is, the group with the smallest fluctuation of the measured value, are used as the effective test data. Further, test data having the smallest variance within the set of data is selected as the final measured value.
The system size can be controlled within 80 & TImes; 80 mm. It is powered by a ±15V DC regulated power supply and tested in low temperature and constant temperature oil baths with a temperature accuracy of 0.01 °C and a resolution of 0.001 °C. 100 temperature points were tested in the range of -30-300 °C. By analyzing the test data, the maximum error is not more than 0.05 ° C, and the test resolution is up to 0.004 ° C.
3 Conclusion
The ADS1247's on-chip integrated constant current source, programmable amplifier (PGA) and 24-bit AD converter provide excellent hardware support for temperature measurement based on platinum resistance. Processing temperature data according to the principle of minimum variance can further improve the accuracy of temperature measurement. The designed platinum resistance thermometer is small in size, high in precision and convenient to transfer.
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