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AD7091R-5 I2C

Hi

I'm a DFAE in Japan. Our customer give us a question. There is a MOSFET inside SDA pin of AD7091R-5. On the other hand, is there NO MOSFET inside SCL pin of AD7091R-5? According to Table 5, we couldn't see Open-drain output about SCL as follows.

Regards,

Hiroyuki



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[edited by: @skowalik at 8:45 PM (GMT 0) on 20 Mar 2020]
  • Hiroyuki,

    I will reach out to the design team to confirm the pad structure of SCL and get back to you with an answer as soon as possible.   Please understand that due current conditions it may take time for me to get an answer to you. 

    I did however want to confirm that you are concerned with SCL and not SDA as highlighted in the table excerpt. 

    Also can you please provide the test conditions for which the customer made the measurement?   

    The datasheet should probably more appropriately call out that SDA is a bidirectional input/output pin and the setup may affect the behavior of the IO pad.    

  • Dear skowalik,

    As a matter of fact, the content about your exchange in the link below is not over and I continue to communicate with customers. 

    https://ez.analog.com/data_converters/precision_adcs/f/q-a/121861/ad7091r-5-and-ad7415-sda-scl

    From your interaction we guess the equivalent circuit as follows: Va is the power supply on the customer's board and Vb is the power supply coming from the end user's board. Both Va and Vb are 3.3V. Even if the power is cut off only for Va, the voltage is visible in SDA and SCL because Vb is supplied. However, at this time, about 1V was seen on SDA and 3.3V was seen on SCL. We guess that the voltage dropped to 1V because current was flowing through the MOSFET inside the SDA, and that SCL remained at 3.3V because there was no MOSFET. So I asked this question again. 

    Regards,

    Hiroyuki

  • Dear skowalik,

    Sorry for my reminding. I consider it a simple question. Is there a MOSFET inside SCL or not?

    Thank you very much in advance.

    Regards,

    Hiroyuki

  • Hiroyuki,

    I do not have access to the design files so I cannot confirm the internal structure presently.   I have requested a visual description of the pad structure so that I can confirm.   I apologize for the delay but please understand the situation with the COVID-19 virus is introducing delays in efficiently retrieving the necessary information.


    Sean

  • HIroyuki,

    I have a discussion planned with the designer tomorrow remotely and will respond once that call is completed.

    Sean

  • Dear Sean,

    Thank you very much for your support. The customer give additional question us below. 

    Assuming that there is an open drain MOSFET inside the SDA pin, is it possible that the MOSFET will turn on when only Vb is supplied?

    My answer is "I don't know." This is because the operation of the internal circuit of the IC is undefined when Va is not supplied. What do you think?
     
    Regards,
    Hiroyuki
  • Hiroyuki,

    So I spoke to the designer and I believe the model you provided in your thread above is accurate for SDA in that there are protection diodes in the SDA pad for when the device is to be a SPI part.   The problem is that POWER must be applied for that selection to be enacted or at least that is my theory.   SCL does not have any protection diode to the supply (VA) and thus why you do not see any loading.  I do not believe the drive MOS is the culprit as internally the gate is tide off when power is removed in both SCL and SDA.

    So if you take the supply you call VA and assume that power removal results in a low impedance path to ground (NOT and OPEN) then you now have a path through a resistor and a diode to ground and I think that explains the issue.   Now I can only state that this is the structure for the AD7091R-5 and can't confirm for the temp sensor.   So the customer may have to try powering up a board with the SDA pin of the 7091 lifted to see if the problem goes away but fundamentally if they can leave the device powered (VIO) and simply not request conversions the issue should be resolved.

    Sean