The implementation of cold junction compensation in thermocouple applications Because the thermocouple is a differential temperature measurement device, the cold junction is used as a reference point when processing the thermocouple signal. Considering the voltage of the non-zero Celsius cold junction, the thermocouple output voltage must be applied. Perform cold junction compensation. In this paper, several cold junction compensation devices are compared, and three application design methods and measurement results are introduced by taking silicon temperature sensor detection IC as an example. There are many types of sensors in temperature measurement applications. Thermocouples are the most commonly used ones and can be widely used in automobiles, homes, etc. Compared to resistive temperature detectors (RTDs), thermoelectric regulators, and temperature sensing integrated circuits (ICs), thermocouples are capable of detecting a wider temperature range and are more cost effective. In addition, the robustness, reliability and fast response time of thermocouples make them the first choice for a wide range of operating environments. Of course, thermocouples also have some drawbacks in temperature measurement, such as poor linearity. In addition, RTDs and temperature sensor ICs offer higher sensitivity and accuracy, making them ideal for precision measurement systems. Thermocouple signal levels are low and often require amplification or high resolution data converters for processing. If the above problems are excluded, the low temperature, easy to use, and wide temperature range of the thermocouple can make it widely used. Thermocouple and cold junction compensation thermocouples are differential temperature measurement devices consisting of two different wires, one for the positive junction and the other for the negative junction. Table 1 lists the four most common types of thermocouples, the metals used, and the corresponding temperature measurement ranges. The two different metal wires of the thermocouple are welded together to form two junctions. As shown in Figure 1a, the loop voltage is a function of the temperature difference between the two junctions. This takes advantage of the Seebeck effect, which is often described as the process of converting thermal energy into electrical energy. The Seebeck effect is the opposite of the Peltier effect, which is the process of converting electrical energy into thermal energy. Typical applications are thermoelectric coolers. As shown in Figure 1a, the measured voltage VOUT is the difference between the junction voltage (hot junction) junction voltage and the reference junction (cold junction) junction voltage. Since VH and VC are produced by the temperature difference between the two junctions, VOUT is also a function of the temperature difference. The scaling factor corresponds to the ratio of the voltage difference to the temperature difference, called the Seebeck coefficient. Figure 1b shows one of the most common thermocouple applications. A third metal (intermediate metal) and two additional junctions are introduced in this configuration. In this example, each open junction is electrically connected to a copper wire. These wires add two additional junctions to the system. As long as the junction temperatures are the same, the intermediate metal (copper) does not affect the output voltage. This configuration allows the thermocouple to be used without a separate reference junction. VOUT is still a function of the temperature difference between the hot junction and the cold junction, and is related to the Seebeck coefficient. However, since the thermocouple measures the temperature difference, in order to determine the actual temperature of the hot junction, the cold junction temperature must be known. The coldest junction temperature is 0 ° C (freezing point) is the simplest case, if TC = 0 ° C, then VOUT = VH. In this case, the hot junction measurement voltage is a direct conversion of the junction temperature. The National Bureau of Standards (NBS) provides a look-up table of voltage characteristic data and temperature correspondence for various types of thermocouples, all based on a 0°C cold junction temperature. Using the freezing point as a reference point, the hot junction temperature can be determined by looking up the VH in the appropriate table. In the early days of thermocouple applications, the freezing point was used as the standard reference point for thermocouples, but it is not realistic to obtain a freezing point reference temperature in most applications. If the cold junction temperature is not 0 ° C, the cold junction temperature must be known in order to determine the actual hot junction temperature. Considering the voltage at the non-zero cold junction temperature, the thermocouple output voltage must be compensated, the so-called cold junction compensation. Figure 1: a. The loop voltage is generated by the temperature difference between the two junctions of the thermocouple. b. A common thermocouple configuration is connected by two wires to a junction, and the open junction of each line is connected to a copper thermostat. Figure 2: The local temperature sensing IC (MAX6610) determines the cold junction temperature. The thermocouple and cold junction temperature sensor output voltages are converted by a 16-bit ADC (MAX7705). Figure 3: The remote junction diode is mounted near the cold junction to detect the temperature. The MAX6002 provides a 2.5V reference for the ADC. Figure 4: ADC with integrated cold junction compensation to convert thermocouple voltage to temperature without external components. Table 1: Several common thermocouple types. Table 2: Measurements taken from cold junctions and hot junctions in different ovens Point temperature. The cold junction temperature range is -40 ° C to 85 ° C, and the hot junction temperature is maintained at 100 ° C. Table 3: Measurements taken from cold junction and hot junction temperatures in different ovens. The cold junction temperature range is -40 ° C to 85 ° C, and the hot junction temperature is maintained at 100 ° C. The hot junction measurements in the table are compensated. Table 4: Measurements taken from cold junction and hot junction temperatures in different ovens. Cold junction temperature range: 0 ° C to 70 ° C, the hot junction temperature is maintained at 100 ° C. The hot junction measurements in the table are the decimal numbers provided by the circuit. Choosing a Cold Junction Junction Temperature Measurement Device To achieve cold junction compensation, the cold junction temperature must be determined, which can be achieved with any type of temperature sensing device. In general temperature sensor ICs, thermostats, and RTDs, different types of devices have different advantages and disadvantages and need to be selected according to the specific application. For applications where accuracy is critical, calibrated platinum RTDs maintain high accuracy over a wide temperature range, but at a high cost. When the accuracy requirement is not very high, the thermistor and silicon temperature sensor IC can provide high cost performance, the thermistor has a wider temperature range than the silicon IC, and the temperature sensor IC has higher linearity, so the performance The indicator is better. Correcting the nonlinearity of the thermistor will consume more microcontroller resources. The temperature sensing IC has excellent linearity, but the temperature range is narrow. Therefore, the cold junction temperature measurement device must be selected according to the actual needs of the system. Careful consideration should be given to accuracy, temperature range, cost and linearity index in order to obtain the best cost performance. Lookup Table Method Once you have established a cold junction compensation method, the compensated output voltage must be converted to the corresponding temperature. A simple method is to use a lookup table from the NBS. Implementing lookup tables in software requires memory for storage, but these tables provide a fast and accurate solution when continuous testing is required. Two other methods for converting the thermocouple voltage to temperature require more than just a look-up table: a linear approximation of the polynomial coefficients and an analog linearization of the thermocouple output signal. Software linear values ​​are popular because no storage is required other than the predefined polynomial coefficients. A disadvantage of this approach is the processing time problem associated with multiple-order polynomial. For more order polynomials, the processing time is further increased. For temperature measurement applications that require multiple polynomials, the lookup table may be more efficient and accurate than the linear approximation method. The analog linearization method is often used before software is used to convert voltage to temperature (except for manual search lookup tables). This hardware-based approach uses analog circuitry to correct the nonlinearity of the thermocouple response. The accuracy is determined by the order of approximation correction. This method is still widely used in multimeters that receive thermocouple signals. Application Circuits Three typical applications for cold junction compensation using silicon sensor ICs are discussed below. Three circuits are used to address cold junction temperatures with narrow temperature ranges (0°C to 70°C and -40°C to 85°C). Compensation, accuracy is within a few degrees Celsius. The first circuit uses a temperature-sensing IC to determine its temperature near the cold node; the second circuit contains a far-node diode temperature detector consisting of a diode-connected transistor (connector directly connected to the thermocouple) ) Provides a test signal for it; the analog-to-digital converter (ADC) in the third circuit has built-in cold junction compensation. All three circuits were temperature measured using a K-type thermocouple consisting of a nickel-chromium alloy and a nickel-based thermocouple alloy. 1. Typical Application In the circuit shown in Figure 2, the 16-bit ADC converts the low-level thermocouple voltage into a 16-bit serial data output. The integrated programmable gain amplifier helps to improve the resolution of the A/D conversion, which is necessary to handle the small signal output of the thermocouple. The temperature sensing IC is mounted close to the thermocouple connector and is used to measure the temperature near the cold junction. This method assumes that the IC temperature is approximately equal to the cold junction temperature. The cold junction temperature sensor output is digitally converted by channel 2 of the ADC. The 2.56V reference inside the temperature sensor saves an external voltage reference IC. When operating in bipolar mode, the ADC can convert the positive and negative signals of the thermocouple and output them on channel 1. Channel 2 of the ADC converts the single-junction output voltage of the MAX6610 into a digital signal that is provided to the microcontroller. The output voltage of the temperature sensing IC is proportional to the cold junction temperature. In order to determine the hot junction temperature, the cold junction temperature is first determined, and then the cold junction temperature is converted to the corresponding thermoelectric voltage by a K-type thermocouple lookup table provided by NBS. This voltage is added to the PGA gain-calibrated thermocouple reading, and finally the summation result is converted to temperature by a look-up table, and the result is the hot junction temperature. Table 2 lists the temperature measurements. The cold junction temperature varies from -40 ° C to 85 ° C and the hot junction is maintained at 100 ° C. The accuracy of the actual measurement depends to a large extent on the accuracy of the local temperature sensing IC and the oven temperature. 2. Typical application 2 In the circuit shown in Figure 3, the junction temperature detection IC measures the cold junction temperature of the circuit. Unlike the local temperature detection IC, the IC does not need to be installed close to the cold junction, but is externally connected to form a diode. The transistor measures the cold junction temperature. The transistor is mounted directly at the thermocouple connector. The temperature sensing IC converts the measured temperature of the transistor into a digital output. Channel 1 of the ADC converts the thermocouple voltage to a digital output, channel 2 is not used, and the input is directly grounded. An external 2.5V reference IC provides the reference voltage for the ADC. Tables 2 and 3 list the temperature measurements. The cold junction temperature range is -40 ° C to 85 ° C and the hot junction is maintained at 100 ° C. The accuracy of the actual measurement depends to a large extent on the accuracy of the remote junction diode temperature sensing IC and the oven temperature. 3. Typical application The 12-bit ADC in the circuit of Figure 4 has a temperature-sensing diode. The temperature-sensing diode converts the ambient temperature into a voltage. The IC calculates the compensated thermal junction by processing the thermocouple voltage and the diode's detection voltage. temperature. The digital output is the result of compensating the thermocouple test temperature. The device temperature error is kept within 9LSB from 0°C to 700°C. Although the device has a wide temperature range, it cannot measure temperatures below 0 °C. Table 4 shows the measurement results of the circuit shown in Fig. 4. The cold junction temperature varies from 0 °C to 70 °C, and the hot junction temperature is maintained at 100 °C. (This article Source: Alibaba)
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