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ISOW7842: Logic Level across chip (INA to OUTA) not 1 to 1.

William Thompson44
Prodigy 195 points
Part Number: ISOW7842


Hello,

My question is simple, 

I am sending a 3.3V logic signal across the chip at INA, but when I measure at OUTA I am getting 5V. Any idea why this is happening?

My chip is configured for 5V to 5V (SEL pin shorted to Viso).

I am attempting to communicate to a 3.3v logic adc, and I believe this to be the issue why I cannot communicate.

Thanks,

Will  

  • 0
    TI__Expert 6195 points

    Hi Will, 

    Thanks for asking your question on E2E!

    The ISOW784x is a digital isolator that will translate digital signals from one voltage domain to another. 

    Since you have the output configured to 5V, the digital signals will also be 5V signals (regardless of the logic levels of the input signal). 

    If you connect SEL to GND2 you should see 3.3V signals on the output (regardless of the logic levels of the input signal). 

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    TI__Guru 55825 points

    Hi William,

    Dan has already clarified your question and I believe your questions are already answered. I wanted to point out one concern I had in the way customer is using ISOW7842 device.

    When VCC pins (side1) of ISOW7842 is supplied with 5V, the I/O pins on side1 will also operate at 5V logic levels. It is not recommended to connect side1 of ISOW7842 with 5V powered to an ADC that operates at 3.3V I/O levels. This could lead communication under particular test conditions. Since the I/Os of ISOW side1 operate at 5V, the output pins on side1 will output 5V onto the ADC that only sustains 3.3V signals. This could lead to ADC device damage over a period of time.

    Hence, I recommend keeping the VCC levels of ADC and ISOW at the same voltage levels even if they are two different supplies. Please also note that ISOW cannot be supplied with 3.3V on VCC when 5V output is expected at VISO. 3.3V to 5V operation of ISOW integrated DC/DC is not supported. An alternative option approach is to use a level translator between ADC and ISOW which converts signals from 3.3V logic levels to 5V and vice-versa.

    Let me know if you have any questions, thank you.

    Regards,
    Koteshwar Rao