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Overview:
In view of the current complex and low precision of platinum resistance measurement in the range of -30-300 °C, this paper proposes a design method of platinum resistance thermometer for small metrology verification based on TI company ADS1247.

This design uses the programmable constant current of the ADS1247 output as a platinum resistance excitation source. During the measurement, the ADS1247 integrated programmable amplifier amplifies the voltage drop of the platinum resistor and converts the amplifier output signal to 24-bit AD conversion. Through experimental tests, the ADS1247-based platinum resistance thermometer can achieve an accuracy of 0.05 ° C and a resolution of 0.004 ° C.

0 Introduction Metrology and calibration instruments are mainly used to verify and calibrate the sensors used in daily scientific research and industrial production to 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 method of platinum resistance thermometer based on TI's ADS1247 chip.

1 Measurement principle There are usually two ways to convert the resistance value of platinum resistance as a function of temperature into a voltage signal: the bridge method and the 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 setting, within the output voltage margin, the DACS can provide a constant current of various strengths for the external. The current intensity can be set to 50uA, 100uA, 250uA, 500uA, 750uA, 1000uA, 1500uA. The absolute error of the output current It is related to the output current intensity, and 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.
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