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BQ76952: ADC Gain when using ADCIN functionality

Aman Bhasin
Prodigy 20 points
Part Number: BQ76952


Hi,

We're working with the BQ76952 in a battery management system application. We've connected NTCs to the following inputs for use as thermistors for cell temperature measurement:

  1. TS1
  2. TS3
  3. DCHG
  4. DDSG

All inputs are configured in the following manner:

Field  Setting  Description 
OPT5-OPT4 0 selects 18-kOhm pull-up for thermistor measurement
OPT3-OPT2 3 no polynomial is used, raw ADC counts are reported
OPT1-OPT0 2 thermistor temperature measurement, reported but not used for protections
PIN_FXN1-PIN_FXN0 3 ADC Input or Thermistor

When we apply a known voltage to these inputs, we observe that the value reported by the AFE is incorrect. Following are a few examples:

Value measured at NTC pin (mV) Value reported by AFE (mV) Comments
1760 1957 This is when no NTC is connected, so entire voltage on ADC input is from REG18 (1.8 V)
173 264
730 826

We read the configured ADC Gain value in the part under test, and it was 4040. Using the above values, we calculated the ADC gain (using directions mentioned in BQ76952 TRM, page 27). he calculated value was 5714.

Why are we unable to read the correct voltage reading at these pins? 

  • 0
    TI__Guru* 81035 points

    Hi Aman,

    I think the problem is that the combination of options you are using will not work - it looks like your settings for these registers is 0x37 from your description. The attachment below demonstrates some of the valid options as examples. I think your OPT1-OPT0 settings may be causing an issues since it is selecting to report temperature (but a polynomial is needed to report temperature).

    BQ769x2_TemperatureRawCounts_Instructions.pdf

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt,

    Thanks for your response.

    > it looks like your settings for these registers is 0x37

    It is 0x3B.

    > I think your OPT1-OPT0 settings may be causing an issues since it is selecting to report temperature (but a polynomial is needed to report temperature)

    My intention is to configure for temperature measurement (enable 18K/180K pull up) but read the millivolt value as read by the ADC as I want to handle the NTC transfer function in software. Is this not recommended using this AFE?

  • 0 in reply to Aman Bhasin
    TI__Guru* 81035 points

    Hi Aman,

    I do not really recommend this configuration. OPT1-OPT0 should select ADC measurement if you want to read millivolts.

    However, when you use the normal temperature measurement with the polynomial, it is going to be more accurate. The internal pullup resistor is connected to the internal REG18 LDO and the thermistor voltage is digitized using the internal ADC which is also using the REG18 LDO as its reference. This provides a ratiometric measurement, so any inaccuracy of the REG18 voltage is removed.

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt, 

    Thanks for your response again. I have a few follow up questions.

    1. Is my understanding correct: Selecting a polynomial in OPT3-OPT2 ensures that the ADC reference is selected as REG18 LDO and selecting no polynomial would have the reference as VREF1, correct?
    2. I've been following instructions from BQ769x2_TemperatureRawCounts_Instructions.pdf, (selecting a custom polynomial to report raw counts) but I'm still getting an error of around 10-16 mV between the value reported by the AFE and the value measured at across the input resistance. Is this a question of tuning the Adc0 and Rc0 values or am I missing a step to get more accurate readings?
    3. Where can I find the actual polynomials used for conversion? Is there documentation available?

    Thanks!

  • 0 in reply to Aman Bhasin
    TI__Guru* 81035 points

    Hi Aman,

    You can calibrate the ADCIN measurement if needed by following the steps in this application report: https://www.ti.com/lit/an/sluaa32a/sluaa32a.pdf 

    We do not have documentation we can share on the actual polynomials. This would show proprietary information on the device. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 20 points

    Hi Matt,

    I did a few tests and saw that following trend in error values 

    Measured value Reported value Delta
    36 68 32
    111 143 32
    208 236 28
    364 389 25
    393 411 18
    451 466 15
    516 532 16
    615 629 14
    725 737 12

    I don't think a constant offset wouldn't help in eliminating the error.

    Do we have information on the expected maximum error in thermistor measurements?

  • +1 in reply to Aman Bhasin
    TI__Guru* 81035 points

    Hi Aman,

    There is a ADC Gain parameter for calibration. I am not sure how much it will help with the results you are showing.

    There are multiple sources of thermistor error: the accuracy of the thermistor itself, the variation of the pullup resistor value, the pullup resistor variation over temperature, the ADC accuracy, ADC noise. When using the internal pullup resistor, the measurement is ratiometric so the gain error of the ADC should not be a factor if using the internal thermistor and polynomial.

    Regards,

    Matt