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ISOW7841: Using ISOW7841 and ISO7841 to control 2 ADS129* Using SPI

James Hatswell
Prodigy 150 points
Part Number: ISOW7841

Hi,

Currently we are facing an issue when using 2 isolator parts in parallel and trying to control 2 ADS129* parts in Daisy Chain Mode.

We are finding some issues as follows:

1, When we power up the board, we supply 3v3 to VCC1 of the ISOW part, and can read 3v3 on VCC2. As soon as we try write data to the ISO and ISOW parts, the voltage on VCC2 drops to 2V and never recovers, until we powercycle the board

2, When we try and write commands across either ISO part (ISO and ISOW) we do not read any data on the output of the isolator devices. We only get data across when we remove the parts and manually bridge the gaps with wire

3, I don't know if this is relevant to the Isolation forum, but our ADS129* are not working, as we do not get any activity on the DRDY pin when we remove the isolators.

Many thanks,

James

  • 0
    TI__Mastermind 49495 points

    Hi James,

    Thanks for reaching out to us. I remember us discussing a related topic in the past, I am copying below the other related E2E post you had posted earlier. I believe you had mentioned that there was something wrong with the schematic and you will be working to fix the same, were you able to fix the schematic and make the isolators communicate normal?

    VISO voltage can get disturbed for the following reasons,

    1. If the decoupling capacitors aren't appropriate. From our past discussion, I think the capacitors are appropriately placed.
    2. If there is an output overload. The device supports 75mA of output current in 3.3V-3.3V configuration. Any current drawn more than this can put VISO at risk.
    3. If the input current limit is very low. For 3.3V-3.3V configuration, the typical efficiency is going to be about 47% but the worst-case efficiency that depends on many factors can be much lower. Considering these factors, an input supply current limit of >200mA is necessary, better if it is higher. The device does consume short duration transient currents of upto 600mA, if the input decoupling capacitor is placed right then the device should be able to work fine with >200mA of input supply current limit.

    Please do let us know if your application violates any of the above points listed. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 150 points

    Hi Koteshwar,

    Thanks for your reply, to answer your points:

    1. If the decoupling capacitors aren't appropriate. From our past discussion, I think the capacitors are appropriately placed.
      1. We have a 10uF and 0.1uF cap at the start of each VCC input/output placed very close to the ports so I don't think this is the problem, unless we need a higher overall capacitance (I think I remember reading maybe needing 100uF?)
    2. If there is an output overload. The device supports 75mA of output current in 3.3V-3.3V configuration. Any current drawn more than this can put VISO at risk.
      1. I don't think we get anywhere near to this so it shouldn't be a problem
    3. If the input current limit is very low. For 3.3V-3.3V configuration, the typical efficiency is going to be about 47% but the worst-case efficiency that depends on many factors can be much lower. Considering these factors, an input supply current limit of >200mA is necessary, better if it is higher. The device does consume short duration transient currents of upto 600mA, if the input decoupling capacitor is placed right then the device should be able to work fine with >200mA of input supply current limit.
      1. I think we have enough current going into the device so this should not be a problem.

      Do you have any other suggestions on why it would not be working?

  • 0 in reply to James Hatswell
    TI__Genius 17100 points

    Hi James,

    While Koteshwar responds, below are some common debug E2E posts you can reference regarding the ADS129x silence on /DRDY pin. The PADC team informed me this resembles a startup issue:






    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 150 points

    Hi Manuel,

    thanks for the reading material! If I had a question about our setup, would it be best asking it on this thread or shall I start a new one? ( I don't know if there is a link between our problems hence why I included the ADS129*)

  • 0 in reply to James Hatswell
    Prodigy 150 points

    Hi guys,

    We have managed to fix the problem, it turns out that our /RESET was being pulled low so the device was not able to communicate at all.

    Thanks for all your help guys!

    James

  • 0 in reply to James Hatswell
    TI__Genius 17100 points

    Hi James,

    You're welcome! We're glad to have helped and wish you success on your projects.


    Thank you for posting to E2E,
    Manuel Chavez