Commonly used platinum thermal resistance, that is, as the temperature increases, the resistance of the sensor itself changes in a positive correlation, as shown in Figure 1.1, when the temperature of the measured object rises by 1 °C, the resistance of the PT100 increases by 0. 385 , and at - 200 ° C ~ In the range of 850 ℃, the good linearity of platinum thermal resistance PT100 provides developers with a guarantee of high-precision measurement, which is one of the reasons why PT100 is widely used.
PT100 can be used as a high-precision temperature sensor with excellent functions, but in the case of complex industrial environments, the lengthy wires of the sensor will bring about line impedance. For 0.385, if you want to ensure the accuracy of 0.1 degrees, you need to ensure that the line impedance leads to The error must be less than 38.5, and in order not to introduce the error caused by the thermal effect of the sensor PT100 itself, the constant current source current of the PT100 is generally set at 0.1mA~1mA. If the constant current source is set at 1mA, it can be calculated by formula 1.1. Must be less than 38.5uV, which is an estimate excluding the influence of noise.
Therefore, in order to obtain a high-precision temperature measurement effect, the error effect caused by the sensor line impedance must be eliminated. There are three general wiring methods in the industry, as shown in Table 1.1.

Table 1.1 General connection method of PT100 It can be seen from Table 1.1 that the two-wire connection method has the largest normal line impedance error and cannot be ruled out; the four-wire connection method, also known as Kelvin connection method, is the most accurate and simple, but requires more interface resources ; The three-wire connection method can eliminate the line error through calculation and save the interface resources, but it requires three lines of equal impedance, which is also relatively easy to achieve.

In view of the complex industrial environment and the customer's demand for high-precision temperature measurement products, our company has launched a dual-channel high-precision thermal resistance PT100 isolation temperature measurement module TPS02R, which can achieve a measurement accuracy of 0.02 in the range of -200℃~850℃ %0.1℃, 10ppm low temperature drift index.

As shown in Figure 1.2, the three-wire connection method collects the voltage signal, and the back-end PGA provides gain amplification. After the ADC analog-to-digital conversion, the digital signal is output to the MCU for processing. Because the A, B, and C three wires all have line impedance, . Here, a standard three-wire PT100 can be installed to ensure that the constant current source flows through the common mode resistor R to provide a certain common mode voltage for the ADC. The ADC differential input channels measure the voltages at both ends of AB and AC respectively. A known

Combined with Equation 1.2, through calculation and processing through Equation 1.4, the PT100 measurement value that completely eliminates the line impedance error can be obtained, and high-precision temperature measurement can be achieved. The three-wire connection method can achieve the same measurement accuracy as the four-wire system, while saving a terminal, which is very advantageous for users to develop miniaturized products. The three-wire connection method only needs to ensure A, B, C and other impedances, namely It can be ensured that the introduced error of the line impedance is basically negligible.

Our company TPS02R optimizes the constant current source on the basis of the three-wire connection method, uses the negative feedback principle of the operational amplifier to improve the stability of the constant current source, and provides a certain common mode voltage for the ADC input to provide a guarantee for the differential measurement of the ADC , and give monitoring and alarm function. The specific measurement data are shown in Table 1.2