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ISO1540: I2C Isolator output impedance

Timothy Tran2
Intellectual 420 points
Part Number: ISO1540

We noticed that when bias supply VCC2 is not present (0V), the I2C lines (SCL, SDA) output appear to be in LOW impedance as opposed to HIGH impedance as stated in Table 2.   Could you please check and confirm?   Thanks!

  • 0
    TI__Intellectual 2325 points
    Hi Timothy,

    Thanks for posting on TI E2E.

    1) The image you posted doesn't seem to be loading. Could you please try and re-attach it?
    2) To clarify a little bit, Table 2 is meant for DC values only. Can you please tell us more details on how you are testing this ?

    Sorry for the questions, but we'll work to get this clarified for you !

    Regards,
    Abhi
  • 0 in reply to Abhi Aarey
    Intellectual 420 points
    The Table 2 is on page 20 under the Section 8.5 Device Functional Modes) in the attached ISO154x datasheet link below.
    www.ti.com/.../iso1540.pdf
  • 0 in reply to Timothy Tran2
    TI__Guru 55815 points
    Hi Timothy,

    Thanks for pointing us to Table 2 in ISO1540 datasheet.

    Like Abhi asked for clarification in his post,
    1. Could you please let us know how did you check the impedance between SDA/SCL lines and respective GND?
    2. Did you measure the impedance using a multi-meter?
    3. Could you please share what is the value of impedance that you read?
    4. What value of pull-up resistors are you using?

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Intellectual 420 points

    Hi Koteshwar,

    Though I don't have all detail measurements and tests, I was told that there a few other I2C devices were on the same bus and  when the bias supply 3.3V was not present at VDD2 pin, the I2C's of all other devices were pulled down (lost communication).

    The I2C bus is pulled up at 3.3V @ 2K Ohms.  This is a separate 3.3V from the 3.3V at VDD2 pin.

    should we simply disconnect all other devices on the bus and measure impedance between VDD2 and GND2 with a multi meter?  if so, what impedance should it be?

    Likewise, is it the same for VDD1?

    Thanks,

    Tim     

  • 0 in reply to Timothy Tran2
    TI__Guru 55815 points
    Hi Tim,

    Thank you for sharing additional details about your application and the issue you are facing. From your description I understand that VDD2 is supplied with one 3.3V supply and the I2C pins are pulled-up with another power supply. When VDD2 = 0V, I am assuming the pull-up 3.3V power supply was still available. This particular operation we have not tested and I am not sure what to expect under this condition.

    Yes, disconnecting all other devices and then measuring impedance between I2C pins & GND2 would be better. Alternatively, if you have a spare sample you can also check the impedance between the above discussed pins. The I2C pins should be in Hi-Z and when you measure impedance you should read a large resistance value.

    Yes, this applies to both the sides of the device. Once you test the devices, please do let us know the result. Thanks.


    Regards,
    Koteshwar Rao