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BQ756506-Q1: GPIO for NTC sampling

wang lei
Prodigy 100 points
Part Number: BQ756506-Q1

Hello experts,

About BQ756506-Q1, when we use GPIO inputs for thermistor connections for temperature measurements like below:

I have two questions and need your help.

1、BQ main ADC Effective number of bits is 16bit ,but GPIO resolution (Main & AUX ADC) is 152.59 μV/LSB.

      From my side, for a 16bit ADC powered by 5V supply, the resolution should be 5V/(216-1)=76.3 μV/LSB.  How to understand this difference?

2、What's the meaning of “VACC_MAIN_GPIO_RATIO”  ?  Does this parameter  include the error of TSREF? As we know, if we want to calculate the accuracy of NTC circuit, we need to take both ADC Sampling error(ADC self) and supply error（TSREF） into consideration.

 

  • 0
    TI__Mastermind 23410 points

    Hi Wang,

    wang lei said:

    BQ main ADC Effective number of bits is 16bit ,but GPIO resolution (Main & AUX ADC) is 152.59 μV/LSB.

          From my side, for a 16bit ADC powered by 5V supply, the resolution should be 5V/(216-1)=76.3 μV/LSB.  How to understand this difference?

    The 152.59 uV / LSB comes from 5V/(215-1). Due to the fact that the Main ADC reports in two's complement, the factor is one degree less than the number of bits. 

    wang lei said:
    What's the meaning of “VACC_MAIN_GPIO_RATIO”  ?  Does this parameter  include the error of TSREF? As we know, if we want to calculate the accuracy of NTC circuit, we need to take both ADC Sampling error(ADC self) and supply error（TSREF） into consideration.

    This does take into account the TSREF. You can see that in the test conditions it is (measured GPIO from Main ADC) / (measured TSREF from AC). Any supply error will be within the `measured TSREF from Main ADC` portion of the test conditions. 

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 100 points

    Hello experts,

    About the first question:

    "Due to the fact that the Main ADC reports in two's complement, the factor is one degree less than the number of bits." 

    Could you please explain this for detail.

  • +1 in reply to wang lei
    TI__Mastermind 23410 points

    Hi Wang,

    Certainly!

    You sighted the following for your resolution calculation: 

    wang lei said:
    From my side, for a 16bit ADC powered by 5V supply, the resolution should be 5V/(216-1)=76.3 μV/LSB

    This would normally be true in the case that all bits are used to represent value measured by the ADC.

    The main ADC in the BQ75606 can represent both positive and negative values through the method of two's complement.

    In this case, one bit (the MSB) is used in part to signify the sign (positive or negative) of the reading. That is why effectively the V/LSB is 5V/(215-1). 

    Best Regards,

    Anthony Pham

  • +1 in reply to Anthony P Pham
    Prodigy 100 points
    Anthony P Pham said:

    Hello experts,

        Got it.

        Thank you for your support.