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CC2650: CC2650 internal ADC measuring thermistors

Brian Antheunisse
Expert 5305 points

Part Number: CC2650

Hi team,

Not sure if this is a question better suited for analog forum, but here it goes.

I have a customer who wants to use the CC2650 and it's internal ADC to measure a couple of thermistors. I imagine the 12 bits are plenty, and they could just mux in the two thermistors at a low sampling rate. There are all sorts of methods of measuring a thermistor with higher and higher accuracy, but the simplest one that would require the fewest number of components would likely be this one:

Figure 1.

This method would be ratiometric (cancelling any Vref error out of the temperature measurement), and should work if the following are true:

  • R1 is temp stable
  • Vref feeding the ADC and the Vcc of the thermistor is the same

Then you should be able to create a lookup table based on your thermistor's specs. 

My question is this:

Can you route the internal Vref of the CC2650's ADC out through a GPIO in order to achieve this? And if yes, how much current can this Vref source to the resistor divider circuit in this scenario? (100uA - 1mA should be plenty enough to excite the thermistor)

Alternatively, do you guys know of any reference designs (using any MCU with an internal ADC, doesn't even have to be CC26xx) that show in detail how to measure a thermistor with an MCU's internal ADC?

Thanks,

Brian