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I came across attached Ethernet design for one my application. Since connector used here is with internal MagJack support external transformer is not used, The PHY used here is AR8031 part. In the design 10pF caps is used between P and N line of MDI signals. The Ethernet Phy used here is AR8031 PHY does not recommend this.

Can anyone tell me why these caps are required here.

Thanking you in advance. enter image description here

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1 Answer 1

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It’s likely those capacitors were placed there to shunt high frequency noise across the differential signal. The goal is to couple the noise on both lines so it does not affect the differential signal even though it still exists on each line relative to ground.

EDIT (for clarity)

There are two types of noise that can occur in differential signals

  1. Common mode noise is noise that flows in the same direction on both lines. This is usually not a big concern in differential signals because it does not affect the voltage difference between the two lines and therefore the total differential signal is unaffected.

  2. Differential mode noise is noise that flows in opposite directions on the lines. Differential mode noise does affect the voltage difference between the two lines and therefore the differential signal is affected.

The purpose of the capacitors is to shunt the high frequency differential mode noise between the two lines. This filters out the differential mode noise but does not affect the common mode noise which as stated earlier, is not a big concern.

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  • \$\begingroup\$ Look again (at the circled caps). It looks like they are shunting HF differential mode rather than common mode. \$\endgroup\$
    – Oskar Skog
    Commented Aug 12, 2021 at 7:29
  • \$\begingroup\$ Yup that’s what I was trying to say. I guess I wasn’t clear enough. \$\endgroup\$
    – Ryan
    Commented Aug 12, 2021 at 12:27
  • \$\begingroup\$ Shunting noise means the capacitor should be connected one end to gnd right - or else how the noise gets shunted - Any one can please explain that \$\endgroup\$
    – Puneeth
    Commented Aug 13, 2021 at 7:27
  • \$\begingroup\$ I see now. I just wasn't expecting someone to add capacitance to high speed signals, but it's probably not that much compared to a cable. \$\endgroup\$
    – Oskar Skog
    Commented Aug 13, 2021 at 7:39
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    \$\begingroup\$ This answer is correct, but I think the main point is that the value of the capacitors is such that they filter out high frequency noise, but are small enough so that they dont affect the signal tooo much. This type of filtering is especially important if you're doing something like POE (coilcraft.com/en-us/other/high-power-signal-path-poe-magnetics) \$\endgroup\$
    – BeB00
    Commented Aug 13, 2021 at 7:41

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