Forum Post: RE: Internal temperature sensor in CC2540

The internal temp sensor is connected to the ADC input internally.

Internal Reference = 1.24V  Temperature coefficient: 4.5bits /1°C at 12-bit resolution. (as shown in the datasheet )

So if the internal reference is connected to the ADC, the ADC readout should be

= (2^{(n bit – 1)}– 1)/1.24

= (2¹¹ - 1)/1.24

= (2048 – 1)/1.24

= 2047/1.24

= 1650.81

Now, when the temp sensor is connected, the temperature coefficient can then be calculated as

= 4.5bits /1°C

= 4.5 (internal reference voltage/Max ADC readout for 12bit resolution)

= 4.5 (1.24/2047)

= 2.7259 mV/1°C

Now, from the number above, you’d see that 10°C requires 27.26mV. In the internal structure, one can have offsets of 5mV or so, which might further reduce the accuracy of the internal temp sensor. Also remember, the internal chip temperature may differ from the ambient temperature, that is if the chip is actively chugging away.

To do the single point, use the following app note as a reference. The SoC ADC is similar, so the steps should be to

http://www.ti.com/litv/pdf/swra101a

Essentially,

1. Set room temperature to 25°C
2. Select the temp sensor as an ADC input
3. Measure the ADC readout (The ADC value always resides in MSB section of ADCH:ADCL)