The WZP platinum resistance temperature sensor Pt1000 is a device that uses the characteristic of electromagnetic parameters of components changing with temperature to detect temperature related parameters. Due to its relatively good linearity, strong oxidation resistance, and wide temperature range, its temperature measurement range is from -200 ℃ to 650 ℃, and it is currently widely used in industrial production and scientific research. The signal processing circuit of the sensor needs to convert the resistance change signal related to temperature into a unified voltage signal.
Output characteristic curve of Pt1000
Place Pt1000 into a high and low temperature experimental chamber, set the temperature to -30 ℃~70 ℃, and measure the resistance values of the temperature sensor at different temperatures. The measured data is shown in Figure 1. Within the temperature measurement range, the output resistance value of Pt1000 is proportional to the temperature. However, under high and low temperature conditions, there is a certain deviation that requires temperature compensation in the conditioning circuit.
2 Basic constant current source circuits
The signal processing circuit of the platinum resistance temperature sensor can use a constant voltage source or a constant current source. Through research on the constant voltage source, it has been found that there are problems such as instability and low accuracy in practical applications. The reason is that when working at constant voltage, in addition to the nonlinear error of the platinum resistance itself, there will also be inherent errors in the constant voltage working circuit, which will increase the system error of the entire circuit. Therefore, this article adopts the method of constant current source in its design.
The basic constant current source circuit is shown in Figure 2. Using a platinum resistor RT instead of the Rf of the inverting amplifier, according to the formula of the inverting amplifier, we can obtain:
After Vi and R1 are fixed, the current flowing through RT remains constant, and Vo is proportional to RT. The corresponding voltage change can be obtained from the change in RT, thus achieving voltage output while maintaining linearity.
3 temperature processing circuits with in-phase input
The ideal temperature sensing circuit outputs a voltage of 0 V at 0 ℃, while in Figure 1, RT=1000.8 Ω. Substituting it into equation (1), Vo is not 0, so the voltage needs to be zeroed. The implementation method is to add an input voltage to the in-phase terminal of the operational amplifier in Figure 2 for adjustment, as shown in Figure 3.
So as long as k can be adjusted to the appropriate value, the signal can be zeroed. But because this is only a theoretical calculation, the actual operational amplifier is not ideal, and the resistance values of various resistors may not fully match the nominal resistance values due to temperature and other factors. So the sizes of R2 and R3 are not fixed, and variable resistors are used for fine-tuning in practice.
The addition of voltage adjustment at the in-phase input terminal ensures that even if the temperature increases, the output voltage will decrease. Therefore, in order to ensure a linear output of -300~700 mV of the signal at -30 ℃~70 ℃, an amplification circuit is used to adjust the amplification factor after zeroing. As shown in Figure 4.
The inverting amplifier circuit shown in Figure 4 can not only achieve an amplification effect of A=R5/R4, but also invert the voltage Vo1 that has been inverted by the previous stage operational amplifier, which becomes a positive phase voltage that meets the requirements.
4 Linearization Supplementary Processing
The relationship after passing through operational amplifier 1 is shown in equation (2). After linear amplification by operational amplifier 2, if RT is constant, Vo and Vi are proportional, so it is necessary to maintain linearity. Vi is preferably a constant value, otherwise Vi and RT will appear in the functional relationship of Vo, and accurate temperature measurement cannot be achieved. The voltage fluctuation range given by the design is ± 10% VCC, and RT is a constant value at a certain temperature. Therefore, the final Vo also has a fluctuation of ± 10%. Using a unified R-T relationship to judge, the temperature error obtained within the measurement range becomes ± 10 ℃, which is unacceptable. The circuit must have a reliable high-precision voltage regulator, so in actual operation, there is no need for self connected power supply. The high-precision voltage reference MAX6025 can be used to provide a standard 2.5 V voltage reference for powering components and circuits.
In addition, according to the relationship between the resistance value R of Pt1000 platinum and temperature T, it can be seen that Pt1000 has a high sensitivity of 3.786 59 Ω/℃ within the measurement temperature range, so only a general-purpose operational amplifier needs to be selected. When matching the temperature coefficient of the resistor used, nonlinear errors can also be ignored.
5. Actual temperature sensing signal processing circuit
The actual temperature sensing signal processing circuit is shown in Figure 5. The capacitor C in the circuit diagram is a noise reducing capacitor, with an actual value of 1 μ F. The 2.5 V voltage reference is provided by MAX6025. According to the given parameters in the figure, before conducting actual temperature testing, it is necessary to first zero and fill it up. Replace Pt1000 with a precision adjustable resistor and connect it to the circuit. Change the resistance value to make it equal to the equivalent resistance of 1000.8 Ω at 0 ℃. Adjust the variable resistor R5 to make the output voltage 0. Then change the precision adjustable resistor value to 1265.8 Ω at 70 ℃. Adjust the variable resistor R6 to make the output 700 mV, completing zero and full adjustment. The circuit has been experimentally tested and achieved excellent temperature sensing performance.

---Source: sooro.com