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TMS320F2812: ADC EXTREF

Martin Kempf
Prodigy 50 points
Part Number: TMS320F2812


Hi community

I have following question:

In the initialization of the ADC from the derivate TMS320F2812 I recognized that the Bit ADCTRL3.EXTREF is set to 1 (means the ADC reference works with the external voltage source) (image "Register ADCTRL.jpg").

The Pins for the external reference voltage source "ADCREFP" and "ADCREFM" are connected like in the image "ADC Pin Conn with internal reference.jpg".

Now my question is, how the analog to digital conversion works correctly without a external reference voltag source? So what happens, when the ADCTRL3.EXTREF is set to 1 and no external reference voltage source is connected? Is this allowed?

In the image "ADC Pin Conn with internal reference.jpg" in the section "Note:" is just the case described, if an external reference voltage source is connected and the bit ADCTRL3.EXTREF is set to 0.

Thanks for your help.

  • 0
    TI__Guru 62975 points
    Martin,

    The ADC will not function properly if the external reference voltages are not provided when the ADC is configured to operate in external reference mode. The resulting conversion values would be undefined.

    Are you asking about a specific reference design and example?

    -Tommy
  • 0 in reply to tlee
    TI__Guru 62975 points

    To elaborate some more, when the ADC is configured for internal reference mode, the device will use the on-chip reference voltages which are also driven out to the ADCREFx pins. In this configuration, the system should not apply any external voltage to the ADCREFx pins or else there will be contention between the external and internal reference sources.

  • 0 in reply to tlee
    Prodigy 50 points
    Hi tlee
    Thanks for your reply.
    So to explain my problem more accurate. I have thousands of devices in use and they are all configured like the explanation ADCTRL3.EXTREF set to 1 (use external reference voltage supply), but the ADC conversion works. If I take one of these devices and configure it with the ADCTRL3.EXTREF set to 0 (use internal reference voltage supply), then the ADC results change just about 10 to 20 digits. Since I can not change the software of these devices, I'd like to know why the ADC conversion works properly? Or is this just a coincidence?

    -Martin
  • 0 in reply to Martin Kempf
    TI__Guru 62975 points
    Martin,

    Can you confirm that there are no external reference voltages present on the pins? I would not expect the system to work when configured for external reference mode with no reference voltages applied externally.

    -Tommy
  • 0 in reply to tlee
    Prodigy 50 points

    Hi Tommy

    Yes, I can confirm that. Below is my schematic from the ADC module.

    Best Regards

    Martin

  • 0 in reply to Martin Kempf
    TI__Guru 62975 points
    Martin,

    I discussed this with a colleague and we can't figure out how the ADC is working without an external reference.

    It looks like there might be probe points on REFM (P126) and REFP (P127)? Can you measure the voltage on those pins?

    -Tommy
  • 0 in reply to tlee
    TI__Guru 62975 points
    Martin,

    Are you still investigating this? If not, please mark the thread as resolved.

    -Tommy
  • 0 in reply to tlee
    Prodigy 50 points

    Hi Tommy

    Sorry, for my late answer.

    I measured the voltage over the two capacitors C124 (ADCREFP) and C123 (ADCREFM).

    The voltages are:

    ADCREFP --> 1.933 V

    ADCREFM --> 0.965 V

    Martin

  • 0 in reply to Martin Kempf
    TI__Guru 62975 points
    Martin,

    Can you confirm that the ADCREFP and ADCREFM voltages are stable during operation? The voltages on the pins would explain how the ADC is able to convert properly.

    The source of the voltage is unknown to us. However, if C123 and C124 are 10uF as shown in the schematic, and the voltage is stable during ADC operation, then the voltage source (wherever it is from) is strong enough to act like an external reference.

    Officially, we cannot guarantee or support this use case of configuring the ADC for external reference mode without supplying the external reference voltages. However, I suspect that you now have enough information to make an educated risk assessment.

    -Tommy
  • 0 in reply to tlee
    Prodigy 50 points
    Hi Tommy

    I have to do some more measurements if it is stable over a longer time.
    Thanks for your help.

    Martin