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UCC28782EVM-030: DIFF OPAMP OUTPUT COMMON MODE CAPABILITY

robin gangopadhya18
Mastermind 6055 points
Part Number: UCC28782EVM-030

Hello:

Wondering if it is possible to inject the small signal while setting up an SMPS for loop gain-phase measurement with a nonisolated opamp for its broadband properties but of course, the limiting factor is the output common mode properties of the opamp.Pl see the circuit attached. Output  DC CMC is never discussed in any Specification/data sheet.

Another hurdle is Network Analyzer such as HP 8751A( only NA we have) to be able to support Vout at Ch1 and Ch2. But that can be passed through a divider. So it is not a show-stopper.

Any review and suggestions for the Diff Opamp will be highly appreciated. It could be a current-feedback amp as well.

-r

DIFF-AMP_LOOP_TEST.pptx

  • 0
    TI__Guru 70115 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 70115 points

    Hello,

    I have not seen a differential amplifier used to inject an AC signal to measure loop stability.  I am not sure how well this would work.

    In general an AC transformer is used to inject the small AC signal into the voltage loop.  The following link will bring you to an application note that describes how to use a network analyzer to measure the voltage loop with a network analyzer and a AC transformer.   This should help you compensate your voltage loop.  https://www.ti.com/lit/an/snva364a/snva364a.pdf

    Regards,

  • 0 in reply to Mike O'
    Mastermind 6055 points

    Mike:

    Thanks a lot for the fast review!

    Have read/studied the app note you reference...There are lots of reasons we are trying to avoid transformer coupling.- DYI type...I used to have from another life one with 50:10 Ohm step down from 100 Hz to 10 Meg, but I do not have access to that anymore. North Hills will cost >$300 & perhaps not different if from Venable or Ridely Engr. 

    The second post proposes a circuit with DC decoupling capacitors. The idea is also used in transformer-coupled setups.

    Now the interesting things is : can we not pick up the outputs from the opamp to feed to the NA since they are already DC decoupled & assuming the capacitors are high in value ( which is the case even in xmfr case) so can be considered " trivial impedance change" from the traditional left side nodes to the capacitor /opamp output side?

    On top, we can add a high qual CMC at the opamp outputs before the NA connection. For any signal level CMC.

    In fact, any diff opamp with wide BW ought to work unless I  am missing something basic.

    Then this setup solves a lot of loop measuring problems!

    Let me know your reviews.

    thnx again.

    r

    DIFF-AMP_LOOP_TEST3.pptx

  • 0 in reply to robin gangopadhya18
    TI__Guru 70115 points

    Hello,

    I don't see how the circuit that you have drawn up will inject an AC signal across the 10 ohm resistor to measure the loop stability.

    It looks like you are using a differential amplifier that is VCC reference.  The signal you inject needs to be +/- Vout.

    If you don't have a network analyzer or an AC transformer.  You could compensate the loop based on theoretical compensation and then test the design with large signal transients such as load, line and startup.  If you there is no ringing during these tests your design most likely has more than 45 degrees of phase margin and is stable.

     

    Regards,