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Feedthrough capacitor

  1. Aug 9, 2012 #1
    How does a feed thru cap help in reducing common mode and differential mode noise?
    I read a few app notes, but none of them explain them reasonably well.
     
  2. jcsd
  3. Aug 20, 2012 #2
    Feedthrough capacitors provide a low impedance path to ground for high frequency signals. This allows any undesirable high frequency noise to bypass the functional circuit.

    They also act as small power supplies laid out all over a PCB. So, after the initial power up, this allows current to be transferred from the capacitors to the components that need the power, rather then having the current travel all the way from the power supply to the component needing the power. This not only provides cleaner energy for the component that needs the power, but it also reduces the number of large current loops traveling through the whole PCB.
     
  4. Aug 20, 2012 #3
    I know the advantages of feedthru cap. But I can't figure out how it works. An equivalent circuit will help.
     
  5. Aug 20, 2012 #4

    davenn

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    Science Advisor
    Gold Member

    think of it as a low pass filter :)


    D
     
  6. Aug 20, 2012 #5
    Is feed through capacitor a feed through for metal panel where you use to feed the signal through from one side to the other of the panel? If so, then it's a low pass filter. Any power supply line or signal line has certain impedance at higher frequency, it is not the DC resistance that you measure with a meter. At high frequency the line is more like a transmission with impedance of easily above 50 ohm. The capacitance of the feed through shorted the high frequency in the line to the panel which assumed to be grounded. So the high frequency component is filtered out when passing through the panel. I've seen 9 pins D-sub connector with feed through capacitors.
     
    Last edited: Aug 20, 2012
  7. Aug 20, 2012 #6
  8. Aug 20, 2012 #7
    Yes, that's what I thought. It is use on other application than just vacuum feed through. We used the 9 pin D-sub with feed through cap. I just want to confirm we are on the same page. I think it was like 0.01uF from signal to the body of the connector. It would not affect slow signal like RS-232.
     
  9. Aug 21, 2012 #8
    The Feedthrough cap I was talking about is this one -
    http://www.avx.com/docs/Catalogs/w2h-w3h.pdf [Broken]

    I don't understand how to make an equivalent ckt. The grounded pins are like parallel caps to ground and the Signal/Vcc terminals are like a high value cap? That doesn't make sense.

    I think these caps are also called X2Y caps?
     
    Last edited by a moderator: May 6, 2017
  10. Aug 21, 2012 #9

    berkeman

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    Staff: Mentor

    The equivalent circuit is just capacitance to ground. You need to have some series impedance in order to get the lowpass filter (LPF) effect. That series impedance can be parasitic (like the inductance in the DB-9 wires that yungman mentioned), or it can be explicit components like if you used chip ferrite beads in series with the signal connections to the feedthrough caps.
     
    Last edited by a moderator: May 6, 2017
  11. Aug 21, 2012 #10
    likephysics,

    Your google-fu is pretty weak. I got a lot of hits with obvious search strings. This particular example is from this app note:
    http://www.avx.com/docs/Catalogs/ftantc.pdf [Broken]

    qtHpS.png

    EDIT: Notice how the inductance is in series with the signal (which is where you want it) for the feed-through cap, while the inductance to ground is very small. This is what gives it its superior high-frequency characteristic. I'm not sure what they mean by circling that inductor, but functionally you can think of the feed-through cap as one without the parallel inductor, i.e. a low-pass T-filter.
     
    Last edited by a moderator: May 6, 2017
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