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TDA3MV: Analog input pull down

Behnam Eskandariun1
Prodigy 90 points
Part Number: TDA3MV

Hi, 

We have recently realized there is a recommendation in the user manual that all the unused analog input pins need to be pulled to GND via 10K resistor.

Since, we have already freezed our design, I was wondering what would be the probable issues if the analog inputs aren't pulled low. Also is there any software configuration that can avoid any problem in case the pull down on the analog inputs are missing. 

Thanks

Behnam

  • 0
    TI__Mastermind 18010 points

    Hi Behnam,

    Which are the analog pins in question?

    Stan

  • 0 in reply to Stanislav Stilyanov
    Prodigy 90 points

    Hello Stan, 

    The question is about ADC input pins as shown below: 

    My concern is about the information in the SPRS964H –JUNE 2016–REVISED FEBRUARY 2020, Chapter 4.1.1 Table 4-1. 

    Thanks 

    Behnam

  • 0 in reply to Behnam Eskandariun1
    TI__Mastermind 18010 points

    Behnam,

    I see no issue. ADC is powered off at reset anyway and obviously you are not planning to enable it.

    Just make sure you add resistors for next revision. You can even short all ADC_in pins and pull them down with only one resistor saving the rest 7.

    Regards,

    Stan

  • 0 in reply to Stanislav Stilyanov
    Prodigy 90 points

    Stan, 

    Thanks for getting back to me. 

    Actually, the chance of having another revision is very low since we are in middle of Product Validation. Unless, leaving these ADC input pins float is causing some serious issues in the functionality of the SoC or can have high impact on the Internal circuit of the TDA3.

    Could you please confirm by turning off the ADC module during startup and keeping it off, we will not have any malfunction related to this floating pins?  

    Thanks,

    Behnam

  • 0 in reply to Behnam Eskandariun1
    TI__Mastermind 18010 points

    Behnam,

    I'm sorry, I cannot confirm nothing will happen. Since pulldowns are a requirement stated in the datasheet,  violating it, user is doing it at his own risk.

    My personal opinion is still that ADC pins are unconnected internally after POR and any voltages will not propagate, but I don't think this was tested (the floating pins).

    I would recommend you to perform the following test, provided all ADC power supplies and VREF  are powered:

    - Load the software driver for ADC

    - Continously monitor the 8 ADC channels for voltages and log

    - Run different use cases on your device several times (more is better)

    - if ADC readings are somewhere between GND and VREF, then it is within the ADC range. If redings are very close to or equal to VREFP then there are high voltages induced on the pins.

    If ADC pins are surrounded by GND and/or there is no high-speed signals passing in proximity, then most probably you will observe readings close to GND.

    Seeing high voltages however will indicate that pulldowns are really needed.

    I hope this will be of some help.

    Regards,

    Stan