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ADS8686S: Do I need to use resistor when grounding unused parallel lines (Dx)

Andreas Nestler
Intellectual 370 points
Part Number: ADS8686S


How shall unused parallel lines (Dx) be handled if ADC is configured in serial mode? Datasheet (SBAS905C) says : "In serial mode, these pins must be connected to DGND." (e.g. for DB0, 1, 2 3 on page 4). The question raised up, since DBx pins are of DIO type. Is there a possibility that the pins are actively driven even if ADC is configured in Serial Mode (SER/PAR* tied to +3.3V).

Q1: Can we directly tie the pins to DGND without any series resistors?

Q2: If for safe operation resistors are required: what is the proposed value range?

  • +1
    TI__Expert 4740 points

    Hello Andreas, 

    For DB pins that only have 1 functionality, DB[3:0], [8:6], they should be directly tied to DGND or tied to logic low.  Unless there is a need to use the device in a different mode other than serial, it should be safe to tie them to DGND. 

    For the DB pins with alternate functionalities, then the alternate functionality should be considered. Many of which require either a fixed logic high or low depending on configuration. If the configuration is fixed, then the other DB pins can also have a fixed DGND or digital supply/logic high would be acceptable. 

    If it is preferred to have a  resistor just in case, a low current resistor could be used. We used 100kΩ in the EVM and would recommend that, but a 10kΩ should work too. 

    Best regards, 

    Yolanda

  • 0 in reply to Yolanda Gonzalez
    Intellectual 370 points

    So, to be safe. DB pins with 1 functionality will never be driven active in serial mode. So I guess internal logic will first evaluate the state of SER/PAR* pin and only drive DB pins in parallel mode. Can you confirm?

  • 0 in reply to Andreas Nestler
    TI__Expert 4740 points

    Yes, you are correct Andreas

    Best regards, 

    Yolanda 

  • 0 in reply to Yolanda Gonzalez
    Intellectual 370 points

    Ok, thanks for clarification.