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ADC08D500: VCMO loading

Nicolas Schlumpf
Intellectual 340 points
Part Number: ADC08D500

Hello,

The datasheet recommendation for the VCMO loading is no more than 100uA and 80pF.

What should be the value of Riso with CL=100nF?

What could be the consequence if Riso too small? Could the VCMO output oscillate?

In advance thank you for your inputs.

N.

  • 0
    TI__Mastermind 34690 points

    Hi Nicolas,

    Do you plan to hang a Cload on the input of the buffer amplifier?

    I would think giving the boundaries above, 100uA and 80pF, using a spice simulator would help determine the correct Riso value with the specific amplifier in mind.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    TI__Mastermind 34690 points

    To add to this, the main concern would not be that this node will oscillate. Its loading this node down, which can cause issues internally to the ADC. Using the buffer amplifier is definitely the correct way to go in order to use this reference voltage elsewhere on your circuit application.

  • 0 in reply to Rob Reeder
    Intellectual 340 points

    Hello Bob,

    Thank you for your answer.

    My concern is that the VCMO output is used internally as feedback to regulate the VCMO output. In that case, adding a large CLoad with a weak Riso would deteriorate the phase margin and lead to potential VCMO instability. However, from your message, I understand that this is not the case. Can you confirm?

    Thank you and regards,

    Nicolas

  • +1 in reply to Nicolas Schlumpf
    TI__Mastermind 34690 points

    Hi Nicolas,

    I will double check for you to make sure I didn't miss anything.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    TI__Mastermind 34690 points

     Hi Nicolas,

    I checked on this and my confirmation was correct.

    A 100nF on that node will not cause problems. The Vcmo output is actually a buffered copy of what is actually used inside the ADC, so loading it with a cap won’t cause problems. Note that when operating in AC-coupled mode the Vcmo pin is pulled to ground (that is the selection mechanism for AC-coupling). When in DC-coupled mode the Vcmo voltage is used to set the common mode of the driving differential amplifier.

    If putting a 100nF at the input of that amplifier is needed to reduce noise, I would recommend selecting the resistor value to be large enough to give some filtering effect, and small enough so there aren’t significant DC errors due to the input bias current of the amplifier.

    Below is a snapshot of the loading on Vcmo on the old EVM for that part.

    Regards,

    Rob

     

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