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Original question:

UCC28782EVM-030: DIFF OPAMP OUTPUT COMMON MODE CAPABILITY

UCC28782EVM-030: theory behind loop output for freq response

robin gangopadhya18
Mastermind 6055 points
Part Number: UCC28782EVM-030

Hello Mike:

What would be your explanation for the theory of allowing to use of the same nodes to measure loop response as Vout/Va as the nodes where the signal is injected with floating nodes from the transformer?

Therein will be found the reason for using the opamp differential outputs as well with decoupling caps. The theory is the application of the "law of superposition"...right?

see attached.

-pl share your thoughts.

r

loop_output_nodes.pptx

  • 0
    Mastermind 6055 points

    Hello Mike:

    Maybe the attachment to this post will clarify how the opamp output "injects" an ac signal in the loop. If an isolation transformer- with a possible decoupling capacitor can "inject"( under some theory), the low-output impedance of the opamp output as "source" will behave no different from the transformer- under the same theory the transformer "injection" works. Which is "linear Superposition". 

    If we can guarantee that the opamp out o=[put will NEVER EVER jump to any DC value, this circuit is superior to the transformer approach. Since we can never be sure, we will end up adding dc decoupling caps at NA inputs.

    We are trying to make a pcb fast to test out the idea.

    thnx again for the review.

    -r

  • 0 in reply to robin gangopadhya18
    TI__Guru 70115 points

    Hello,

    I generally use an AC transformer and network analyzer to measure loop stability.  This technique that you are trying to do is different and I am not sure how well it will work.

    I do have suggestion for you.  Why don't try to simulate it to see if it does work first?

    Regards,

  • 0 in reply to Mike O'
    Mastermind 6055 points

    Oh, nice idea! Surely I will try. Closing the post now.

  • 0 in reply to robin gangopadhya18
    TI__Guru 70115 points

    Hello,

    Thanks for letting me know.

    I was glad to help.

    Regards,