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Why high impedances node are susceptible to pick up nosie？

yongxue he
Prodigy 60 points

HI TI engineers:

      When I read the Document“Non Isolated Power Supply Layout Design for EMI Mitigation”，I confused that high impedances node are susceptible  to  pick up nosie. Many articles also say the same meaning,but none of them explan the reason.So, please tell me  the principle,thanks a lot!

  • 0
    TI__Mastermind 33090 points
    Hello Yongxue,

    The issues with laying out the board with a long feedback trace from feedback resistors to FB pin, is that you make the FB pin prone to capacitive coupling caused by fast DV/DT switching and/or inductive coupling due to fast DI/DTs. You create two nodes with a large distance between the two nodes and one side being high impedence. By placing the FB resistors close to the FB pin the FB trace is kept very short minimizing the noise coupling to the FB pin making it more noise immune.


    If noise is coupled to the FB pin, you could have an error on the output, or in extreme cases, the loop could appear unstable.

    When laying out the feedback resistors, please ensure the FB resistors are kept close to the device and route the Vout sense to the point nearest to your load avoiding any noisy nodes as you route the trace to the said point.

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 60 points

    Hi，David：

    Thanks for your reply！I understand what you say “FB pin prone to capacitive coupling caused by fast DV/DT switching ” .In my opinio，Coupling capacitors and high-impedance nodes form a high-pass filter, resulting in easy coupling of noise.Is my opinion correct?

    I still have the following two questions to discuss with you.

    （1）You mentioned inductive coupling.But I do not quite understand the principle of inductive coupling, please explain further.How does it conduct to high impedance nodes？

    （2）As far as we know, noise includes coupling noise(or conducting  noise )  and radiation noise. But，how high-impedance nodes can easily pick up radiation noise?According to the PCB layout principle, it means that if one of the nodes is low impedance,is  the radiated noise  not easy to pick up ，and why？

    Looking forward to your reply and hoping you understand my meaning. Thank you very much.

  • 0 in reply to yongxue he
    TI__Mastermind 33090 points
    Inductive coupling is two conductors having a change in current creating a magnetic field that couples into the adjacent conductor. If you have a high impedance more noise will couple onto the node, that if it were low impedance.

    See link below.

    en.wikipedia.org/.../Inductive_coupling



    This leads to your second question, yes, the higher the impedance, the more it is susceptible to corruption from noise.

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 60 points

    Hi,David:

    Thank you for your reply.I still have two questions to discuss with you.

    Q1:I understand the mechanism of inductive coupling, and I do not understand how inductive noise affects FB nodes. My point of view is that due to the existence of a trace (both ways), the fast DI / DT produces a changing magnetic field coupled to the feedback trace, resulting in an inductive voltage noise between the output of the power supply and the feedback resistor. Therefore, the purpose of minimizing the distance between the FB node and the error amplifier during layout is only to reduce the noise entering the error amplifier due to the voltage division. Both types of routing will produce inductive coupling noise. Is my understanding correct?

    Q2:

    Regarding radiated noise,in my opinion  radiated noise is what generated by receiving electromagnetic waves in space. Because the power output to the FB node is an antenna, the radiated noise can be absorbed. Since the energy received by the antenna generates current, if both sides are high impedance, the noise is relatively large. If one side is high impedance and the other side is low impedance, the noise will be small. Therefore, wiring requirements try to minimize the distance between high-impedance nodes. Is this correct?

  • 0 in reply to yongxue he
    TI__Mastermind 33090 points

    Hello,

    The way I look at it is as follows.  If you have a resistor which is relatively Low Z and you connect through a long trace length (creating an inductive path) to a high Z node, then you have created a high Z at the switch frequencies under consideration.  As mentioned the high Z node is much more susceptible to noise pick up.

    Hope this helps?

    David.