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ISOW7841: Isow7841 Isolator with ADC and reference IC interface issue

Part Number: ISOW7841
Other Parts Discussed in Thread: ADC128S022

Hello Everyone

I have used ISOW7841 inbetween microcontroller and ADC(ADC128s022) with reference Ic(LM4132-4.1V).When i measure the reference voltage it showing 4.35V but i am using 4.1V reference.I have attached the schematic for your reference.Schematic Prints1.pdf

  • Hi Ganesh,

    In the "Recommended Operating Conditions" for ADC128S the range of VD is restricted to the range 2.7V to VA.

    I suspect with VD=5V and VA=4.1V the reference may be being pulled up by currents flowing from VD to VA in the ADC128S.

    Yours,

    Alex.

  • Hello Alex,

    I tested the same circuits without isow7841 it works perfectly.i have given VD=5V and VA=4.1V.when i measure VA it is perfectly 4.!V.
    Shall i make the VD=3.3V and VA=4.1V with isow7841 that solve my issue?

    Best Regards

    Ganesh
  • Hello Ganesh,

    I'm not sure, but it's worth a try if you can regulate VD down to 3.3V for the ADC. I don't know why the ISOW makes a difference except that all the data inputs can be 5V too with your configuration.

    Yours,

    Alex.
  • Hello Alex,

    I tested with VD=3.3V and VA=4.1V reference ic but when i measure at VA=4.26V.I am confused.

    Best Regards

    Ganesh
  • Hello Alex,

    I tested with VD=3.3V and VA=3V reference ic but when i measure at VA=3.98V

    Best Regards

    Ganesh
  • Hello Ganesh,

    That is puzzling, your schematic looks good but your reference voltage is always higher than it should be, no matter what VD or VA you choose.

    I wonder what your SPI interface is connected to. Is the ADC enabled? Are you able to disconnect the reference from the ADC? If you do that is the reference output correct?

    Yours,

    Alex.

  • Hello Alex,

    SPI interface with Microcontroller.The same hardware design i have without ISOW7841 that works perfectly.and i Disconnect the reference from ADC the voltage is 4.25V the same voltage when connected to 4.1V reference IC.

    Should i test the option of VA=5V and VD=3.3V.

    Regards

    Ganesh.S

  • Hello Ganesh,

    Is 4.25V the voltage at the ADC VA pin or at the reference output? If it's the voltage reference output, I would focus attention there.
    Another thing to try is connecting GND to GND(A)2 if you can do that safely. Maybe there is some problem in the ground paths.

    Yours,

    Alex.
  • Hello Alex

    VA is at the Reference output. ADC and Reference are both connected

    I Tested connecting the GND and GND(A)2 but there no effect.

    Regards

    Ganesh

  • Hello Ganesh,

    Can you disconnect the reference from the ADC and see if the reference output is OK with nothing connected to it?
    If it is still not 4.1V then I think maybe the circuit is somehow not as you expect from the schematic, maybe a symbol error or loading error; I know you will have considered these things already...

    Yours,

    Alex.
  • Hello Alex

    I have tested the Reference Ic its Output is 4.09V exact and there is no problem in the Reference Ic.

    Regards

    Ganesh.S

  • Hello Ganesh,

    OK, that is interesting.

    The reference can only source current so it will drift to a higher voltage if a current flows into the output pin. The only place that can happen seems to be the VA pin of the ADC128. You've looked at different VA and VD combinations and you know this problem only occurs when the ISOW7841 is used.

    I think you are looking for an interaction between the ISOW and the ADC128 that causes the VA pin to move towards a higher voltage. I wonder if the SPI interface to the ADC is the source of that interaction.

    The ISOW outputs have logic high voltage values =VISO to will be ~5V in your case. The recommended operating conditions for the ADC128 state that digital inputs must be 0 to VA so in you case the digital inputs exceed VA and the internal circuitry of the of the ADC will probably try to clamp them to VA, maybe with a diode, to prevent damage.

    Maybe current goes from the digital inputs that are high to VA and pulls the reference high.

    I'm not sure how you can test this easily. Maybe operate the ISOW with VISO = 3.3V so the logic inputs to the ADC are limited to 3.3V and use a separate 5V supply for VIN to the reference for now to see if this is the problem.

    That will give you VD=3.3V and VA=4.1V at the ADC which meets the recommended conditions.

    Hope that is it, and you can try it out without too much trouble!

    Good luck,

    Alex.

  • Hello Alex,

    VD=3.3V and VA=4.1V at the ADC.But no effect at the ADC Reference voltage it is measured as 4.36V.

    Thanks

    Ganesh

  • Hi Ganesh,

    Have you made any progress with this? I don't really have anything else to suggest...

    Alex.