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CC2640: ADC

Dhananjay Yadav
Intellectual 280 points
Part Number: CC2640

Hello,

We are using a Wheatstone bridge based pressure sensor as shown in below image by R1,R2,R3 and R4 piezo resistive arms. When bridge is balanced i.e. when there is no pressure input max value of each of these resistors will be 6K neglecting the offset. This pressure sensor gives differential output voltage in mV range proportional to input pressure. When there is pressure input, one of the diagonal arm(R1 and R4) will change by positive delta amount and other diagonal arm will change by same negative delta amount.

So new bridge resistor values will be as below:

R1_new=R1+ delta_R, R4_new=R4 + delta_R, R2_new=R2 - delta_R, R3_new=R3 - delta_R. Due to this, node “pos” becomes positive with respect to node “neg”, giving positive differential o/p proportional to input pressure. Considering sensor datasheet max value of “delta_R” can be 200 Ohms.

Earlier we were using differential amplifier to amplify this low level signal and feed to integrated ADC of CC2640. Now we want to eliminate the opamp and want to explore below solution and want to confirm feasibility of this from your side.

As shown in below image, 2 ADC channels of CC2640 are used. Node “pos” and “neg ” are given to different ADC channels. RADCx and CADCx shows sample and hold block resistor and capacitor.  Both sensor and CC2640 chip is powered by 3V coin cell battery.

Please find below queries and let me know answer to them asap.

 

  1. Can you please let me know if this solution is feasible? As coin cell battery voltage will vary w.r.t. time as it is discharged, same battery voltage can be given to ADC reference.  
  2. R1 and R2 resistors will come in picture while charging the hold capacitor. Considering max value of R1, R2, what will be new value of max. sampling frequency?
  3. Can you please specify RADCx and CADCx values for all ADC channels of CC2640?
  4. Can you please specify Total ADC error as it is not mentioned in Datasheet?

Regards,

Dhananjay Yadav

  • 0
    TI__Guru**** 317180 points

    The ADC block diagram looks like the following: 

    This means that RADC and CADC are equal for all ADC channels, the difference is the mux resistance. The mux resistance is low and will therefore not contribute to the total. 

    Use the internal ADC reference to avoid VDDS dependency. 

    I have not used a Wheatstone bridge so I can't tell if it's possible to use a single ended option here. It will be a time difference between when you sample the POS and NEG node, will this have an impact? 

  • 0 in reply to TER
    Intellectual 280 points
    Hello,

    Thanks for the response.
    Yes, While rate of change of pressure is high there would be some impact as there will be time difference between sampling of these two nodes. But this can be eliminated by averaging more number of samples and sampling rate is high compared to change of pressure. This is OK but my query was if we consider 6Kohm value of R1,R2 bridge resistances, what will be the new sampling frequency?
    Now in this cased CADCx will be charged through R1 or R2, charging current will be taken from coin cell battery and peak current would be 3V/6Kohm = 0.5mA. So this is also ok for coin cell battery.

    Another query was will above configuration degrade ADC performance other than reduction of sampling frequency?

    Regards,
    Dhananjay
  • 0 in reply to Dhananjay Yadav
    TI__Guru**** 317180 points
    Not sure if I follow you, why would the peak current be 0.5 mA? You would have a C in the system that limits the current.

    By using a launchpad and the adc driver examples it would be easy to do a quick test.
  • 0 in reply to TER
    Intellectual 280 points
    Hello,

    OK. Can you please tell me how to calculate reduced sampling frequency if I connect external resistor at ADC pin as RADC and CADC is unknown in CC2640?

    Regards,
    Dhananjay Yadav
  • 0 in reply to Dhananjay Yadav
    TI__Guru**** 317180 points
    The input impedance to the ADC is given in the datasheet.
  • 0 in reply to TER
    Intellectual 280 points

    Hello,
    Thanks for reponse!

    Please find below screenshot from datasheet of CC2640. Using 1.48 V ADC reference disables input voltage scaling as shown in red color box. Does it mean on chip battery monitor is disabled in this case?

    Regards,
    Dhananjay Yadav

  • 0 in reply to Dhananjay Yadav
    TI__Guru**** 317180 points
    The on chip battery monitor uses a separate 8 bit ADC used for the bat mon only.
    => Input scaling does not impact the battery monitor.