AD7768 external clock check

Hi,

Would you help me about the Register 0x09[2] NO_CLOCK_ERROR function of AD7768.
If the MCLK is not applied correctly to the ADC at power-up, this bit is set.
Does this function work if the MCLK becomes abnormal after the MCLK is correctly applied and the ADC operates during power-up?
Or does it only work on startup?

Regards,
HM

  • 0
    •  Analog Employees 
    on Sep 17, 2021 4:14 AM

    Hi, 

    I think it depends on the status of the CLK_QUAL_DIS bit in the general configuration register. So if this bit is set then the qualification will no longer perform and the ADC will accept an invalid external clock. 

    There is also an error_flagged in the header status bits that could also flag this. And In general, when the external clock is invalid then the conversions will just output all zeros regardless of voltage input applied. So it could also be a sign for you to check or enable the Clock qualification to verify that the external clock is causing the issue. 

    Thanks, 

    Jellenie 

  • Hi, Jellenie,

    Thank you for your answer.
    If the CLK_QUAL_DIS bit is enabled and the ADC works correctly and then the quality of the clock becomes abnormal, will the clock error bit be set?
    Also, if the CLK_QUAL_DIS bit is disabled, will the ADC operate using that input clock?

    Regards,

    HM

  • 0
    •  Analog Employees 
    on Sep 17, 2021 8:01 AM in reply to HM72

    Hi, 

    Okay I see I think I have missed answering that original question. So upon power up/reset the ADC uses the internal clock and then transfer the control of clocking to the external clock once a valid reference is used. Now if upon initialization the external clock is valid then I guess it will transfer the control successfully, but once an invalid clock is present during the normal operations I think that question is valid since the DS does not explain it clearly. but one thing I just noticed here is if check is only upon initialization/power up/reset then there's no way you can disable the qualification bit during initialization stage.

    So my thought on this is this should be a continuous check. Though I need to confirm this with product owner just to be sure and will come back to you.  

    For the second question, No, if the clock qualification is disabled and an invalid external clock (<1.15MHz) is present then the ADC will not forced to use the internal clock and just force the AD7768 to accept and hand control over to an external clock source with a lower frequency. 

    Another thing to note here, is even if the ADC is using the internal clock when an invalid clock is detected the conversion will always be 0 if an invalid external clock is detected. So by checking the conversion result, it can be diagnosed already.

    Thanks,

    Jellenie 

  • +1
    •  Analog Employees 
    on Sep 20, 2021 9:41 AM in reply to JellenieR

    Hi, 

    Apologies. I have confirmed that the clock check is only happening upon power up. 

    There is no way to monitor the clock during operation. If there is no clock at all, you would see no DRDY is pulsing so that's a way to realize that clock is not present. If the clock becomes unstable or becomes very low frequency then the DRDY will also pulse at different intervals as normal as well. 
    Thanks,
    Jellenie
  • HI, Jellenie,

    Thank you for confirmation.

    I understand that the clock is only checked at startup.

    Regards,

    HM