SLOA247C July   2017  – March 2019

1.   Frequently asked questions for RF430FRL15xH devices
2.   Revision History

#### 4.1 How can the SD14 ADC result be converted to temperature?

The ROM library in the RF430FRL15xH does not calculate out the temperature. The device only provides the raw reference resistor and thermistor results as they come from the SD14 ADC. These actual results of the raw values are expected to be calculated on a device with more processing power and program memory space available, or with a look-up table.

The following example shows how the temperature is calculated for the RF430FRL152HEVM by an external reader. The equation in step 3 came from the thermistor data sheet. This equation converts from raw ADC result to temperature.

As part of the calculation of the temperature, the formula to determine the thermistor's resistance is required. This formula is provided in the thermistor resistance calculation section in the RF430FRL15xH firmware user's guide.

Definitions:

• refValue is the reference resistor ADC result.
• thermValue is the thermistor ADC result.
• refResistance is the resistance value of the reference resistor, which for the RF430FRL152HEVM is 100 kΩ.
1. tempConv = (thermValue / refValue) × refResistance
2. Using the resistance from the previous equation, the temperature in Celsius is calculated.
3. Temp in C = (1.0 / ((((Math.Log10(tempConv / 100000.0) / Math.Log(2.718))) / 4330.0) + (1.0 / 298.15))) – 273.15
4. Temperature in °F = (Temperature in °C × 9 / 5 + 32)

Notice that the reference resistor result is also used in the calculation. This is needed to properly measure the current through the thermistor and reference resistor. When the current source is known, using the thermistor ADC result, the temperature can be calculated.