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Select the cold end temperature measuring devices
As mentioned above, in order to realize the cold end compensation, must determine the cold end temperature, this can be achieved by any type of temperature detection device. In gm's temperature sensor IC, and RTD, thermistor, different types of devices have different advantages and disadvantages, choose should be carried out according to the specific application. Device for precision is very high, after calibration the platinum RTD can in wide temperature range with high precision, but the cost is very high. Accuracy is not high, the silicon thermistor and temperature sensor IC can provide high cost performance, thermal resistance than silicon IC has a wider temperature range, and the sensor IC has higher linearity, thus better performance indicators. Correction of thermistor micro controller of nonlinear will occupy more resources. The IC temperature sensor has good linearity, but very narrow temperature range. Anyway, must according to the actual needs of the system of the cold end temperature measuring device, need to carefully consider precision, temperature range, cost, and the linear index, in order to get the best cost performance.
Factors to consider
Once, a cold junction compensation method is established to compensate the output voltage must be converted to the corresponding temperature. A simple way is use a lookup table provided by the NBS, software is used to implement a lookup table to memory, but a lookup table for continuous repetitive queries provide a rapid and precise measurement scheme. The thermocouple voltage is converted into a temperature value of two other solutions than a lookup table is more complex, the two methods are: 1) the use of polynomial coefficients for the linear approximation, 2) simulated linearization of the thermocouple output signal. Software linear approximation is only need to determine the coefficient of polynomial in advance, don't need to store, so it is a more generic solution. Disadvantage is need a long time to solve many order polynomials, the higher order polynomial, the longer the processing time, especially in the case of wide temperature range. Higher order polynomial, the lookup table provides a relatively higher precision and more effective temperature measurement scheme. Appear before software test plan, simulated linearization is often used to measure the voltage conversion into temperature (except artificial lookup table retrieval). This kind of method based on hardware use correction thermocouple response of nonlinear analog circuit. The accuracy depends on the modified approximate polynomial order number, in the the multimeter to test thermocouple signal is still using this method.
Application circuit
Are discussed below three kinds of the use of silicon sensor IC for cold junction compensation of the typical applications, three circuits are used to solve temperature range is relatively narrow (0 ° C to 70 ° C and + - 40 ° C to + 85 ° C) of the cold junction compensation, precision within a few degrees Celsius. The second circuit includes a remote diode temperature detector, by connecting a diode transistor with test signal. The third circuit of d/a converter (ADC) built-in cold junction compensation. All three circuit adopts K type thermocouple (composed of nickel chromium alloy and nickel base thermocouple alloy) for temperature measurement. Example # 1 in the circuit shown in figure 2, 16 Σ - Δ ADC to convert low level thermocouple voltage to 16 bits serial data output. Integration of programmable gain amplifier is helpful to improve the resolution of the ADC, it is necessary for processing small thermocouple signal output. Temperature detection IC near the thermocouples installed, is used to measure the temperature of cold end near. This method assumes that the IC temperature is approximately equal to the cold end temperature. Cold end temperature sensor output digital conversion by ADC channel 2. Temperature sensor internal 2.56 V reference benchmark IC saves an external voltage.