1.4.5 Temperature Sensor
The internal temperature sensor is linear, and is intended to give a rough approximation of the ambient temperature (not a PT100 sensor replacement). The target value at 0K is 0 mV from the analog sensor, resulting in “0x00” from the ADC (plus ΔV from single-ended measurements – refer to the device manual/data sheet for this value). An approximate linear line can be made from the 0K point to the production calibration value in the signature row. This value is stored in the signature row and corresponds to a temperature measurement done at 85°C (358K) with a typical accuracy of ±15°C. The inaccuracy will result in some gain error when measuring temperatures.
The measurement stored in the signature row is done in an unsigned mode with 12 bits resolution with the internal 1V reference. VCC is 3.2V and the ADC clock is 62.5 kHz. The ADC setup has to be the same if this value is going to be used for calibration in the application.
The best way to get more accurate results is to do a 2-point calibration to get the incline of the curve. To do a 2-point calibration, select two temperatures where you can do the measurements and where the temperatures are known and accurate. Do a measurement at both temperatures with the wanted setup of the ADC (mode, sample rate/frequency with a maximum of 125 kHz, resolution). When you have these values, you can calculate the coefficient for the curve and you can use this in your application. The formula below can be used to calculate the temperature change per bit:
For a single point calibration, use the same formula, but replace TempLow and ValueLow with 0°K and “0x00” result from the ADC (plus ΔV).