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