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Regarding CC2640 Module and LMT85 Temperature Sensor

Other Parts Discussed in Thread: LMT85, CC2640

Hi Ti Experts

We are making a BLE beacon project which uses LMT85 temperature sensors.

I have few questions regarding CC2640 Module and LMT85 temperature sensor

1)How does the +/- 0.4 degrees accuracy of the sensor compare with the ADC accuracy over the sensor OUT voltage range?  What accuracy should we expect with a single ADC sample?

2)  As the battery decays in voltage, will the ADC reading of the sensor need to be adjusted to get an accurate reading (see figure 5-18 in CC2640 datasheet)?  How much variance is this?

3) As the temperature of CC2640 changes, ADC output changes (see figure 5-19 in CC2640 datasheet) - how much variance is this in our temperature reading?  We expect to read values -10C to 50C, maybe up to 85C.

4)  Does connection mode or OTA require sleeping while maintaining a connection?  For transmit only, are there any power benefits for using an external 32kHz crystal rather than internal oscillator?  I.e. parts of CC2640 can sleep rather than generating oscillator?

Plz reply ASAP.

  • For the 2nd question, you should ensure the used battery voltage can always generate proper ADC reference voltage which is only 1.xV in CC2640. So, a 3V battery is still no problem even drop to 2V.


  • Hi Ti Experts,
    I didn't get it.
    Can anybody explain the answer of all question I have asked because these questions are very important for us.
  • In addition to your questions. I am wondering why you need an external temperature sensor? You should be able to use the CC2640 built-in temperature sensor, right?
  • First thing Internal temperature sensor has accuracy of ±5% which is not in our use.
    Second thing we want to measure ambient temperature not IC temperature .That's why we need through hole temperature sensor which will come out from mechanical for measuring ambient temperature
  • OK. For your questions,

    (1) ~ (3): CC2640 ADC is 12 bits, but the effective bits is around 10 depending on the setting. You can use over-sampling if more bits are required. Figures 5-18 & 5-19 already answer your questions. Yes, you need to check the LMT85 analog output range of -10 ~ 85C and then calculate the mapped ADC output code, respectively. In this way, you will know if the ADC full-scale and the variation due to VDDS and temperature is good enough for your required precision. Btw, you need to ensure the battery supply always meets CC2640's operating range. Please note that ADC reference voltage is 1.4x V. But there are two modes, with or without scaling. Please read CC26xx User's Guide, the AUX chapter has detailed description about ADC.

    (4) Not sure, need TI guys to answer.
  • Hi Jasbeer,

    (4) If you are advertising only and will never enter a connection you can use the internal oscillator for sleep. This will reduce the Standby power consumption by approximately 100 nA.