• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 38 subscribers
  • Views 613 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Questions in <<Using op amps to reduce near-field EMI on PCBs>> by Todd Toporski

Xiaobing (Eric)
Intellectual 895 points


Hi All,

  Reference following diagram, as the input signal frequency increases, the reactance of Cdm decreases. Less feedback is delivered to the op amp input, so the gain increases. Eventually, the op amp is operating open loop because the inputs are shorted by the capacitor. Here is a question for R3.

 1. How R3 will prevent the CDM capacitor from reducing the phase margin of the amplifier? any method to improving high frequency gain in this circuit? 

 2. How to select R3 value? 

Regards,

Xiaobing (Eric)

  • 0
    Intellectual 895 points
    Any-body can help me on this questions? TI guy, tks.

    Regards,

    Xiaobing (Eric)
  • 0 in reply to Xiaobing (Eric)
    TI__Guru 55466 points

    Hello Xiaobing,

    I apologize for the delay due to the Holiday. One alternative circuit you could use for the difference amplifier is below, where the input resistor has been split into two resistors R1+R1’.  The DC gain of the difference amplifier is given by the ratio of RF/(R1+R1’); and the corner frequency of the EMI/RFI filter is given using the equations on the article as a function of R1 (excluding R1’).  See example below.

     

    Assuming that the filter is tuned to reduce high frequency RF/EMI interference, the bandwidth of the circuit will be limited by the frequency response of the amplifier at the given gain.

    As you have mentioned, the circuit will need to be verified/designed for stability. You can perform stability analysis performing open loop analysis in TINA SPICE simulator to obtain the rate of closure and phase margin measurement. For more information on stability Analysis see:

    https://training.ti.com/ti-precision-labs-op-amps

    10. TI Precision Labs - Op Amps: Stability

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Intellectual 895 points

    Hi Luis,

     It's fine and so happy got your feedback.

     It's a nice solution to figure out high frequency gain. 

    Just have the questions for original circuit, how the R3 could prevent the C(dm) capacitor from reducing the phase margin of the amplifier? 2. How to select R3 value? 

    Regards,

    Xiaobing (Eric)

  • 0 in reply to Xiaobing (Eric)
    TI__Guru 55466 points
    Hi Xiaobing,

    There are two methods I typically use to compensate the difference amplifier for stability when using an EMI/RFI filter:

    - Split the input resistor into two resistors, as explained on the previous post, and place the CDM/CCM filter capacitors in between them, then verify for phase margin/stability using TINA simulations.

    - Another possibility may be to place a compensation capacitor in parallel with the feedback resistor.

    On the original circuit, resistor R3 could be useful to provide some level of over-voltage protection limiting the current at the inputs during a fault condition, but I don't believe resistor R3 can be used to compensate for stability.

    Thank you,

    Best Regards,

    Luis
  • 0 in reply to Luis Chioye
    Intellectual 895 points

    Hi Luis,

     Tks. Really nice to got your comments and suggestion. I have assume for R3 effect in original note, whether R3 could work with the input stray capacity of amplifier for a R3*Cstray for a filter? How do you think?

    Regards,

    Xiaobing (Eric)