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ISO7731: I2C isolation circuit not working

Ben-478
Prodigy 110 points
Part Number: ISO7731

Hi,

I am trying to use the circuit provided on the PDF entitled "Designing a reinforced isolated I2C bus interface by using digital isolators" but am unable to get it working. I have built the "Final isolator circuit" shown in fig 5 using the exact values given for components and have double checked connections etc, however, for the chip I have used a different isolator (as couldnt get a TI part) and have realised that it uses a different method for isolation. The circuit is built to be used with the ISO7731 and I have used the ADUM131D Analog devices part, the TI uses capacitive isolation whereas the Analog uses transformer based technology. As I cant find any other reason why its not working, I wonder if the difference between the 2 chips would explain it?

The PDF seems to suggest that the circuit will work with any digital isolator, providing it has 2 channels in one direction and 1 channel in the other, but I wonder if it is only known to work with TI isolator chips? 

Any help would be greatly appreciated,

Ben

  • 0
    TI__Guru 53615 points

    Hi Ben,

    Sorry to hear about the issue.

    We do expect the I2C circuit in the referred document to work with most digital isolators but I will not be able to guarantee that it will work with ADuM131D for sure.

    Could you please elaborate more on the issue? When you say not working, what exactly is the behavior do you observe? When SDA1 is toggling, does SDA2 remain 0V? Could you please share input and output waveform? Please also share your schematic along with part numbers for the diodes and transistor used in the circuit. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 110 points

    Thankyou Koteshwar,

    Im afraid I have limited test equipment and cannot look at both sides together but there does appear to be a signal at both sides on the data and clock lines so things are getting through in one direction, it seems to be the return of data that is the issue. The transistor and diodes used were the same as in the TI circuit.

    Unfortunately I cant send the schematic due to IT issues with my computer here at work, but I copied the circuit exactly as it appeared on the PDF. 

    Im sorry this reply isnt that helpful (to allow you to help me) but if there are any common issues, or mistakes made by users, with the circuit it would be handy to know, if anything comes to mind.

    Thanks,

    Ben

  • 0 in reply to Ben-478
    TI__Guru 53615 points

    Hi Ben,

    Thanks for the update.

    I understand that SCL data transmission is working fine while SDA data transmission is only working in one direction. Could you please confirm which direction data transmission is not working? Is it from "SDA1 to SDA2" or "SDA2 to SDA1"?

    Please also note that the circuit is designed to operate with the I2C devices that have a maximum LOW output voltages (VOLmax) of 0.4V. Hence, please do check the VOLmax spec of the devices that are connected to ISO7731 / ADuM131D on both sides.

    Can I reach out to you over your email to discuss further and try my best to help you with this issue?


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 110 points

    Hi Koteshwar,

    I think the data is getting through from SDA1 to SDA2 because data would not be transmitted from the slave without it receiving the instruction to do so. I therefore believe that the SDA2 to SDA1 transmission is not working. 

    I have checked the maximum voltage for a LOW state on both devices and according to the datasheets they are below 0.4V, so that should be OK. 

    By all means get in touch via ben071509@gmail.com to discuss further.

    Thanks very much,

    Ben

  • 0 in reply to Ben-478
    TI__Guru 53615 points

    Hi Ben,

    I have reached out to you over email. Since we are continuing the discussion over email, I will go ahead and mark this E2E thread as closed. Thanks.


    Regards,
    Koteshwar Rao