Design of PT100 platinum thermal resistance temperature measurement system

Temperature measurement and control are extremely important topics in all aspects of industry, agriculture, scientific research, national defense and people's daily life. Temperature measurement systems are widely used in single-chip microcomputer system design. According to the different design requirements of single-chip microcomputer systems, the design of temperature measurement systems is also different. Some use integrated chips, and some use constant current source devices and constant voltage source devices. This system uses PT100 platinum thermal resistor as a temperature signal acquisition element to design a temperature measurement system.
1 Basic working principle
The resistance of PT100 platinum thermal resistor changes with the change of temperature. This feature is used to collect temperature signals and convert the collected signals into voltage signals; then they are converted into digital signals through A/D and read by the single-chip microcomputer system; the single-chip microcomputer system identifies and processes the read digital signals and converts them into digital signals corresponding to the temperature, and finally the output temperature value is displayed by the LCD display.
2 Hardware design
The hardware mainly includes five components: constant current source circuit, voltage amplifier, A/D conversion interface circuit, optocoupler isolation circuit, and LCD display circuit.
2.1 Constant current source circuit
The constant current source circuit is shown in Figure 1. The chip OP07 is an operational amplifier, which forms a constant current source circuit with 5 resistors, outputting a working current of 1 mA at VIN+. In the figure, DGND=5 V, VMC=0 V, and there are 4 nodes, namely NET1, NET2, NET3, and NET4. Let the current flowing through R110 be Ia, and the current flowing through R114 be Ib, in mA, and both directions are to the right.
According to the virtual open and virtual short of the op amp, we have the equation:
DGND-(R111+R110)×Ia+R114×Ib-R113×((DGND-R111×Ia)／R112)-(VDGND-R111×Ia)=0
Substituting the data, we have:
5-(10+1)×Ia+1×Ib-2×((5-10×Ia)／10)-(5-10×Ia)=0
It can be calculated that Ia+Ib=1, and Ia+Ib is the required current I, which is 1 mA.
According to the equation, to get Ia+Ib as a constant, it must satisfy:
R113×R111／R112-R110=R114
Therefore, the condition for this circuit to become a constant current source is:
R111／R112=(R110+R114)／R113
If R111=R112, then R110+R114=R113 must be. At this time, the constant current value is I=DGND×R113／R112／R114.
Among them, J110 is used to connect the PT100 platinum thermal resistor.

2.2 Voltage amplification and A/D conversion interface circuit
The voltage output from one end of the PT100 platinum thermal resistor is very small. If it is directly connected to the A/D converter, the conversion data deviation is large; so in this design, the voltage output from one end of the PT100 platinum thermal resistor is amplified 10 times and connected to the voltage follower, and then A/D conversion is performed, so that a better conversion effect can be obtained, as shown in Figure 2. The precision amplifier INA118 and the voltage reference chip MC1403 form an amplifier circuit, VIN+ is the voltage value output from one end of the PT100 platinum thermal resistor; WIN- is the voltage value output by the reference voltage source MC1403; VOUT is the output voltage value after amplification. The calculation formula is: VOUT=G×((VIN+)-(VIN-)), where the size of G is determined by the resistor R120, G=1+50 kΩ／R120. The chip OPA277 and the peripheral resistor form a voltage follower.