Commonly used temperature measurement technology and its interface circuit

Temperature is a parameter that often needs to be tested in practical applications. From steel manufacturing to semiconductor production, many processes rely on temperature. Temperature sensors are the bridge between application systems and the real world. This article briefly summarizes different temperature sensors and introduces the interface between them and circuit systems.

Temperature measurement is widely used. Not only does the production process require temperature control, but some electronic products also need to measure their own temperature. For example, computers need to monitor the temperature of the CPU, motor controllers need to know the temperature of the power driver IC, and so on. Here are some commonly used temperature sensors.

Thermistor

There are many types of sensors used to measure temperature, and thermistors are one of them. Many thermistors have a negative temperature coefficient (NTC), which means that their resistance increases when the temperature drops. Of all passive temperature sensors, thermistors have the highest sensitivity (i.e., the change in resistance for each degree change in temperature), but the resistance/temperature curve of thermistors is nonlinear.

Table 1 is a typical NTC thermistor performance parameter. These data are measured for Vishay-Dale thermistors, but they also represent the overall situation of NTC thermistors. The resistance value is given in the form of a ratio (R/R ₂₅ ), which represents the ratio of the resistance at the current temperature to the resistance at 25°C. Usually, thermistors in the same series have similar characteristics and the same resistance/temperature curve. Taking the thermistor series in Table 1 as an example, a resistor with a resistance of 10KΩ at 25°C has a resistance of 28.1KΩ at 0°C and a resistance of 4.086KΩ at 60°C; similarly, a thermistor with a resistance of 5KΩ at 25°C has a resistance of 14.050KΩ at 0°C.

Figure 1 is the temperature curve of thermistor. It can be seen that the resistance/temperature curve is nonlinear. Although thermistor data here is in increments of 10℃, some thermistors can be in increments of 5℃ or even 1℃. If you want to know the resistance value at a certain temperature between two points, you can use this curve to estimate it, or you can directly calculate the resistance value. The calculation formula is as follows:

Here T refers to the absolute temperature in Kelvin, and A, B, C, and D are constants that vary according to the characteristics of the thermistor. These parameters are provided by the thermistor manufacturer.

Thermistors generally have an error range that specifies the consistency between samples. Depending on the materials used, the error value is usually between 1% and 10%. Some thermistors are designed to be interchangeable during application and are used in situations where field adjustments cannot be made, such as an instrument where the user or field engineer can only replace the thermistor but cannot calibrate it. This type of thermistor is much more accurate than ordinary thermistors and is much more expensive.

Figure 2 is a typical circuit for measuring temperature using a thermistor. Resistor R1 pulls the voltage of the thermistor up to the reference voltage, which is generally consistent with the reference voltage of the ADC. Therefore, if the reference voltage of the ADC is 5V, V _ref will also be 5V. The thermistor and resistor are connected in series to produce a voltage divider. The change in their resistance value causes the voltage at the node to change. The accuracy of the circuit depends on the error of the thermistor and resistor and the accuracy of the reference voltage.

◆Self-heating problem

Since the thermistor is a resistor, a certain amount of heat will be generated when current flows through it, so the circuit designer should ensure that the pull-up resistor is large enough to prevent the thermistor from excessive self-heating, otherwise the system will measure the heat generated by the thermistor instead of the temperature of the surrounding environment.

The effect of thermistor energy consumption on temperature is expressed by the dissipation constant, which refers to the number of milliwatts required to raise the temperature of the thermistor by 1°C above the ambient temperature. The dissipation constant varies depending on the thermistor package, pin specifications, encapsulation materials and other factors.

The amount of self-heating and current-limiting resistors allowed by the system are determined by the measurement accuracy. A measurement system with a measurement accuracy of ±5°C can withstand greater thermistor self-heating than a measurement system with an accuracy of ±1°C.

It should be noted that the pull-up resistor value must be calculated to limit the self-heating power dissipation over the entire measurement temperature range. Given the resistance value, the dissipated power will vary at different temperatures due to the variation in the thermistor resistance.

Sometimes it is necessary to calibrate the thermistor input to get the right temperature resolution. Figure 3 shows a circuit that extends the 10-40°C temperature range to the entire 0-5V input range of the ADC. The operational amplifier output formula is as follows:

Once the thermistor input is calibrated, the actual resistance vs. temperature can be plotted. Since thermistors are nonlinear, a plot is needed, and the system needs to know what the ADC value is for each temperature. Whether the accuracy of the table is in 1°C increments or 5°C increments depends on the specific application.

◆Cumulative error

When measuring temperature with a thermistor, the sensor and other components in the input circuit should be selected to match the required accuracy. Some applications require resistors with an accuracy of 1%, while others may require resistors with an accuracy of 0.1%. In all cases, a table should be used to calculate the impact of the cumulative error of all components on the measurement accuracy, including the resistor, the reference voltage, and the thermistor itself.

If you need high accuracy but want to spend less money, you need to calibrate the system after it is built. Since the circuit board and thermistor must be replaced on site, it is generally not recommended to do so. In the case that the equipment cannot be replaced on site or the engineer has other ways to monitor the temperature, you can also let the software build a table of temperature corresponding to ADC changes. At this time, you need to use other tools to measure the actual temperature value so that the software can create the corresponding table. For some systems where thermistors must be replaced on site, the components to be replaced (sensors or the entire analog front end) can be calibrated before leaving the factory and the calibration results can be saved on disk or other storage media. Of course, after the components are replaced, the software must be able to know and use the calibrated data.

In summary, thermistors are a low-cost method for measuring temperature and are simple to use. Below we introduce resistance temperature detectors and thermocouple temperature sensors.

Resistance Temperature Detector

A resistance temperature detector (RTD) is actually a special wire whose resistance changes with temperature. Common RTD materials include copper, platinum, nickel, and nickel/iron alloys. The RTD element can be a wire or a thin film, which is coated on a ceramic substrate by electroplating or sputtering.

The resistance value of RTD is nominally at 0℃. The resistance of 100Ω platinum RTD at 0℃ is usually 100.39Ω at 1℃ and 119.4Ω at 50℃. Figure 4 compares the resistance/temperature curve of RTD with the resistance/temperature curve of thermistor. The error of RTD is smaller than that of thermistor. For platinum, the error is generally 0.01% and for nickel, it is generally 0.5%. In addition to the smaller error and resistance, the interface circuit of RTD and thermistor is basically the same.

Thermocouple

Thermocouples are made of two different metals. When heated, they will generate a small voltage. The voltage depends on the two metal materials that make up the thermocouple. Iron-Constantan (J type), copper-Constantan (T type) and chromium-aluminum (K type) thermocouples are the three most commonly used.

The voltage generated by a thermocouple is very small, usually only a few millivolts. The voltage change of a K-type thermocouple is only about 40μV for every 1°C change in temperature, so the measurement system must be able to measure a 4μV voltage change to achieve a measurement accuracy of 0.1°C.

Since two different types of metals come together to create a potential difference, the connection between the thermocouple and the measurement system will also generate a voltage. The connection point is usually placed on an insulating block to reduce this effect, so that both nodes are at the same temperature, thereby reducing errors. Sometimes the temperature of the insulating block is also measured to compensate for the temperature effect (Figure 5).

The gain required to measure the thermocouple voltage is generally 100 to 300, and the noise picked up by the thermocouple is amplified by the same factor. An instrumentation amplifier is often used to amplify the signal because it removes the common-mode noise in the thermocouple wiring. Thermocouple signal conditioners are also commercially available, such as the AD594/595 from Analog Devices, which can be used to simplify the hardware interface.

Solid-state thermal sensors

The simplest semiconductor temperature sensor is a PN junction, such as the PN junction between the base and emitter of a diode or transistor. If a constant current flows through a forward-biased silicon PN junction, the forward voltage drop will decrease by 1.8mV for every 1°C change in temperature. Many ICs use this property of semiconductors to measure temperature, including Maxim's MAX1617, National Semiconductor's LM335 and LM74, etc. Semiconductor sensors have various interfaces, ranging from voltage output to serial SPI/Microwire interfaces.

There are many types of temperature sensors. By correctly selecting software and hardware, you can definitely find a sensor that suits your application.