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AM3358: Internal 1.8V band gap reference specifications

Mike Pettus
Prodigy 210 points

Part Number: AM3358

What is the stability and accuracy specification for the internal band gap 1.8V reference?

  • 0
    TI__Guru 64525 points

    I'm trying to understand why you are asking this question. The internal bandgap performance should not be relevant for anything outside of the device since it was designed to perform as necessary to achieve overall device performance as defined in the datasheet.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 210 points

    One simple reason: We need to understand the internal 1.8V reference that is used for the ADC.  We understand the ADC is 12-bit resolution.  But to determine the analog measurement accuracy and the accuracy stability over temperature, it's a normal specification expectation to know the ADC voltage reference in terms of accuracy (+/- percentage) and stability (PPM/degC).  We've checked both the data sheet and the technical reference documents and cannot find the internal 1.8V specifications for these two parameters.

  • 0 in reply to Mike Pettus
    TI__Guru 64525 points

    The ADC does not use the internal bandgap as a reference. The ADC produces a 12-bit value that scales the applied voltage relative to voltage applied to external reference signals VREFP and VREFN.  Assuming you have VREFP connected to VDDA_ADC and VREFN connected to VSS, the accuracy of the ADC conversion is a function of your power supply.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 210 points

    Paul,

    My misunderstanding.  I was told by a colleague that there was an "internal reference", and I assumed it was the band gap.  Thank you for clearing this up.  The VDDA_ADC is derived from our PMIC, which has a known accuracy and line/load tolerance for the 1.8V source.  Most likely in our system, we'll use an external voltage reference to the ADC for better measurement accuracy.

    Appreciate the clarification!

    Mike

  • 0 in reply to Mike Pettus
    TI__Guru 64525 points

    The internal reference block shown in the FAE block diagram in the Touchscreen Controller Chapter of the TRM is misleading. If you use an external voltage reference to source the VREFP pin, make sure it can source a peak current of 20mA. The VREFP/VREFN pins must have a common mode voltage of approximately 1/2 of VDDA_ADC. For example, the potential you apply to VREFN but be offset from VSS the same amount as VREFP is offset from VDDA_ADC. If VREFN is not connected to VSS, its source needs to be able to source and sink a peak current of 20mA. The VREFP supply only needs to source current.

    Regards,
    Paul