Single-power signal conditioning circuit for RTD Pt100

Abstract: This paper introduces an excitation and signal amplification circuit for a resistive temperature sensor Pt100 from a single-supply CMOS quad operational amplifier MCP604. Among them, Pt100 is excited by the floating current source formed by two amplifiers in the MCP604; the lead compensation is performed by the 3-wire system by the lead compensation circuit composed of the operational amplifier: the compensated voltage signal is amplified by the other two amplifiers in the MCP604 The electrical network is sampled and amplified.

Resistive Temperature Detector (RTD) is a sensor whose resistance changes with temperature, so the temperature measurement can be done by measuring the resistance value. There are generally three methods for measuring the resistance value: 1 ohm meter: 2 measuring the voltage drop across the resistor under constant current excitation; 3 measuring the current flowing through the resistor under constant voltage excitation. Method 2 is often used in temperature sensor circuits to perform temperature measurements. The bridge is the most common method, but its output signal is nonlinearly related to the bridge arm resistance. It needs subsequent linearization, such as using hardware circuits, or using a lookup table to correct the results through software.

This paper proposes a non-bridge electric hoof form, which not only performs constant current excitation on the sensor, but also the linear relationship between the circuit output signal and the sensor resistance value. The circuit is applied in the "substation operation monitoring system" to realize the detection of the three-way temperature signal, and the operation effect is good.

The excitation and sampling conditioning circuits of one of the temperature sensors are discussed below.

The resistance of metal platinum (Pt) varies with temperature and has good reproducibility and stability. A sensor made using this physical property of platinum is called a platinum resistance temperature sensor, such as Ptl00, Ptl000. Not only are they widely used in industrial temperature measurement, but they are also made into a variety of standard thermometers (covering national and world reference temperatures) for metering and calibration. Pt100 is more accurate and more linear than thermocouple in the range of -200 ° C ~ +850 ° C. The voltage drop generated at both ends is much larger than the output of the thermocouple, and no cold junction compensation is required. In addition, Ptl00 has the advantages of anti-vibration, anti-corrosion, good interchangeability, reproducibility and stability, and is often used in low and medium temperature measurement.

The resistance value of Ptl00 as a function of temperature

When Pt100 is actually used, it has the following application points:

(1) The selection should be based on the actual temperature measurement range, accuracy, size and mounting method.

(2) The current allowed to flow should be less than 5 mA. When performing current source excitation Pt100 (especially in the form of a circuit using a bridge), it should be noted that this condition is met, otherwise Pt100 self-heating will affect the measurement accuracy. The general excitation current is lmA, even smaller

(3) The circuit access mode generally has two-wire system, three-wire system and four-wire system. The schematic diagram of the lead-out line is shown in Figure 1. The two-wire sensor resistance value and the connection wire resistance value together constitute the total resistance value, so the additional error caused by the wire resistance will make the actual measurement value higher, generally suitable for low measurement accuracy and short lead wire; three-wire system requirements The cross-sectional area and length of the three leads are the same, and the influence of the lead (positive and negative line) resistance can cancel each other. In the industry, the three-wire system is generally used; the four-wire system is used in the case where the measurement accuracy is high and the change is small, because in these In the case of the lead resistance, a significant error may be introduced, in which case two of the leads may be used to provide a constant current, and the other two leads provide a voltage drop of an unknown resistance, and a four-wire system is also a common method.

(4) Error.

The main error sources of the temperature sensor in the measurement process are the error of the sensor on the indexing table; the error caused by poor insulation; the error caused by the line resistance; the error of the measuring instrument and the heat transfer error, dynamic corresponding error, and interference error. Some of these errors occur only under certain conditions and can be reduced or eliminated by certain measures. In addition to the three-wire access method to reduce the influence of lead resistance, a second-order active filter amplifier circuit is also used to reduce the error caused by external interference.

The complete circuit of single-supply temperature detection by MCP604 is shown in Figure 2. The circuit consists of three parts: precision floating current source, lead compensation and second-order low-pass active filter amplifier circuit.

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