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ADUCM360 Arkusz danych(PDF) 3 Page - Analog Devices |
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ADUCM360 Arkusz danych(HTML) 3 Page - Analog Devices |
3 / 5 page Circuit Note CN-0221 Rev. 0 | Page 3 of 5 Code Description The source code used to test the circuit can be downloaded as a zip file from the ADuCM360 product page. The UART is configured for a baud rate of 9600, 8 data bits, no parity, and no flow control. If the circuit is connected directly to a PC, a communication port viewing application, such as a HyperTerminal, can be used to view the results sent by the program to the UART, as shown in Figure 3. Figure 3. Output of HyperTerminal Communication Port Viewing Application To get a temperature reading, measure the temperature of the thermocouple and the RTD. The RTD temperature is converted to its equivalent thermocouple voltage via a look-up table (see the ISE, Inc., ITS-90 Table for Type T Thermocouple). These two voltages are added together to give the absolute value at the thermocouple. First, the voltage measured between the two wires of the thermocouple (V1). The RTD voltage is measured, converted to a temperature via a look-up table, and then, this temperature is converted to its equivalent thermocouple voltage (V2). V1 and V2 are then added to give the overall thermocouple voltage, and this is then converted to the final temperature measurement. 20 0 –20 –40 –60 –80 –100 –210 –140 –70 0 70 140 210 280 350 TEMPERATURE (°C) Figure 4. Error When Using Simple Linear Approximation Initially, this was done using a simple linear assumption that the voltage on the thermocouple was 40 µV/°C. It can be seen from Figure 4 that this gives an acceptable error only for a small range, around 0°C. A better way of calculating the thermocouple temperatures is to use a six-order polynomial for the positive temperatures and a seventh-order polynomial for the negative temperatures. This requires mathematical operations that add to computational time and code size. A suitable compromise is to calculate the respective temperatures for a fixed number of voltages. These temperatures are stored in an array, and values in between are calculated using a linear interpolation between the adjacent points. It can be seen from Figure 5 that the error is drastically reduced using this method. Figure 5 gives the algorithm error using ideal thermocouple voltages. 0.30 0.25 0.20 0 0.05 0.10 0.15 –0.05 –210 –140 –70 0 70 140 210 280 350 TEMPERATURE (°C) Figure 5. Error When Using Piecewise Linear Approximation Using 52 Calibration Points and Ideal Measurements |
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