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Theory of Operation
5.2.12 Piecewise Linear Approximation Method
RTD manufacturers usually provide a lookup table that offer excellent accuracy for linearization of a
specific type of thermocouple. The granularity on these lookup tables is also very precise—approximately
1°C for each lookup value. To save microcontroller memory and development time, an interpolation
technique applied to these values can be used. An example of this method when converting from voltage
to temperature with eight lookup table entries is shown in Figure 48.
To perform a linear interpolation using a lookup table, first compare the measured RTD resistance to
values given in the lookup table, until the lookup table value exceeds the measured value that is being
converted.
Then, use Equation 32 to convert the measured RTD resistance to temperature, where R LT is the
resistance lookup table array, and T LT is the temperature lookup table array. This operation involves four
additions, one multiplication, and one division step, respectively. This operation can be done easily on
most 16- and 32-bit microcontrollers.
Figure 48. V-to-T Conversion Block Diagram for RTD
T TLT n 1 TLT n TLT
n 1
LT
LT
RIN
R n 1
RLT
n R n 1
There is a trade-off between memory size and the piecewise linear approximation method’s accuracy. The
more entries on the lookup table, the more accurate the results are. However, the larger number of entries
on the lookup table uses more memory space.
(32)
Figure 49. Temperature Error versus Temperature Using Different Sizes of lookup Table
TIDU271–May 2014
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Temperature Sensor Interface Module for Programmable Logic Controllers
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