AD7811 - DOUT stucks to fixed value no matter Vin value

Hi,

We meet some trouble concerning AD7811.

Sometimes during running, ADC stucks to fixed value (decimal = 82) independ of input voltage on all channel.

We need to power down AD7811 to clear this behavior.

AD7811 is connected directly to FPGA.

Could you help us ?

BR,

Benjamin

  • 0
    •  Analog Employees 
    on Sep 16, 2021 4:04 PM

    Hi Benjamin,

     

    Thank you for your question. To help resolve this issue, could you provide me a few more details:

    1. What is your device control word? Specifically, how are you setting the PD bits?
    2. Is the appearance of the 82d output occurring randomly or periodically? Does it occur after a certain time period or event during the operation time?
    3. Which operating mode are you using, Mode 1 for high speed sampling or Mode 2 for automatic power down?


    Thank you,

    Kiriaki

  • Hi,

    For normal operation, our device control word is : 

     - 0x85 : for channel 2 acquisition

     - 0x89 : for channel 3 acquisition

     - 0x8D : for channel 4 acquisition

     - 0x81 : for channel 1 acquisition

    We are using  Mode 2 with automatic power down.

    Actually, this behavior appear randomly but when it occurs AD7811 still on this status.

    During anormal operation, device control word stay the same, however AD7811 send always the value of channel 3

    Unfortunatly, we can't arrive to catch the event when AD7811 enter on anormal mode.

    We have verfiry all timings and don't find violations.

    Attached, frames decoding during normal and anormal operation.

    BR,

    Benjamin

     

  • 0
    •  Analog Employees 
    on Sep 17, 2021 6:45 PM in reply to B.KOSICKI

    Hi Benjamin,

     

    Thank you for sharing the additional details. It does not seem like there are any errors with the configuration of the device or timing between the ADC and FPGA. I think that there may be an issue occurring where the device does not correctly reset after power cycling. This FAQ describes a similar situation and describes the cause(s) and their solution(s): https://ez.analog.com/data_converters/precision_adcs/w/documents/2975/ad7811-incorrect-o-p-code

     

    I would recommend trying the solutions mentioned in the FAQ to see if that resolves the error that you are observing.

     

    Best,
    Kiriaki

  • Hi,

    Thanks you for your explanations.

    However, if we study all cases in details concerning this note.

    Power Supply Reset Error

    We can clear this situation thanks to power down control. In our case, AD7811 stay stuck with conversion result define to 0x82 for all channel. Between each conversion, control register is set to PD1=1 & PD0=0, so AD7811 enter on full power down after conversion....

    In conclusion, this case should be clear by the reset of AD7811 with power down mode.

    CMOS Latch-up

    This case is under investigation, however as the previous case, power down control should be clear this situation.

    Power supply glitch

    This case seems not occur, conversion result obtained is 0x82 and not 0xFF.

    Any others ideas ? 

    BR,

    Benjamin

  • 0
    •  Analog Employees 
    on Sep 20, 2021 4:15 PM in reply to B.KOSICKI

    Hi Benjamin,

    I wanted to clarify on the Power Supply Reset Error. In this error case, while powering on the internal reset signal may not be issued. If the internal reset is not issued, when the device powers back on, it is in an error state which may be why the output code gets stuck. To force the internal reset, the PD bits need to be cycled. Once the PD bits have been cycled, the device should be in a normal operating state. 

    Best,

    Kiriaki