Table 3. Design Requirements

10.2.2.1 Temperature Algorithm Selection

Of the three algorithms presented in this datasheet, linear interpolation, second order transfer function or third order transfer function, the one selected will be determined by the users microcontroller resources and the temperature range that will be sensed. Therefore, a comparison of the expected accuracy from the LMT70 is given here. The following curves show effect on the accuracy of the LMT70 when using each of the different algorithms/equations given in LMT70 Output Transfer Function. The first curve (Figure 28) shows the performance when using linear interpolation of the LUT values shown in Electrical Characteristics Temperature Lookup Table (LUT) of every 10°C and provides the best performance. Linear interpolation of the LUT values shown in Electrical Characteristics Temperature Lookup Table (LUT) is used to determine the LMT70 min/max accuracy limits as shown in the Electrical Characteristics and the red lines of Figure 28. The other lines in the middle of Figure 28 show independent device performance. The green limit lines, shown in the subsequent figures, apply for the specific equation used to convert the output voltage of the LMT70 to temperature. The equations are shown under each figure for reference purposes. The green lines show the min/max limits when set in a similar manner to the red limit lines of Figure 28. The limits shown in red for Figure 28 are repeated in all the figures of this section for comparison purposes.

Temp (°C)		VTAO (mV)		Local Slope (mV/°C)
	MIN	TYP	MAX
20	994.367	995.050	995.734	-5.171
30	942.547	943.227	943.907	-5.194
40	890.423	891.178	891.934	-5.217
50	838.097	838.882	839.668	-5.241

Figure 28. LMT70 Performance Using LUT and Linear Interpolation

TM = -1.809628E-09 (VTAO)3 – 3.325395E-06 (VTAO)2 – 1.814103E-01(VTAO) + 2.055894E+02

Figure 30. Using Third Order Transfer Function Best Fit -10°C to +110°C

TM = -7.857923E-06 (VTAO)2 – 1.777501E-01 (VTAO) + 2.046398E+02

Figure 32. Using Second Order Transfer Function Best Fit -10°C to 110°C

TM = -1.064200E-09 (VTAO)3 – 5.759725E-06 (VTAO)2 – 1.789883E-01(VTAO) + 2.048570E+02

Figure 29. Using Third Order Transfer Function Best Fit -55°C to +150°C

TM = -8.451576E-06 (VTAO)2– 1.769281E-01 (VTAO) + 2.043937E+02

Figure 31. Using Second Order Transfer Function Best Fit -55°C to 150°C

10.2.2.2 ADC Requirements

The ADC resolution and its specifications as well as reference voltage and its specifications will determine the overall system accuracy that you can obtain. For this example the 12-bit SAR ADC found in the MSP430 was used as well as it's integrated reference. At first glance the specifications may not seem to be precise enough to actually be used with the LMT70 but the MSP430 ADC and integrated reference errors are actually measured during production testing of the MSP430. Values are then provided to user for software calibration. These calibration values are located in the MSP430A device descriptor tag-length-value (TLV) structure and found in the device-specific datasheet. The MSP430 Users Guide includes information on how to use these calibration values to calibrate the ADC reading. The specific values used to calibrate the ADC readings are: CAL_ADC_15VREF_FACTOR, CAL_ADC_GAIN_FACTOR and CAL_ADC_OFFSET.