CN-0221 Datasheet, PDF (3/5 Pages) Analog Devices – USB-Based Temperature Monitor Using the ADuCM360 Precision Analog Microcontroller

Circuit Note
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.
CN-0221
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
0.15
0.10
0.05
0
–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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