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How is common mode noise filtered on differential lines that have a noise source that is driving them relative to chassis GND? I drew a picture of what I am imagining below. Imagine a PCB with differential signals that are in a metal enclosure.

enter image description here

I think the common mode noise will drive the entire circuit with a current shown in purple below:

enter image description here

All the documentation I find show single ended examples like the one in reference image 1 below, where a choke is used between the signal line and system GND to reject the common mode. There are also examples like reference image 2 below where a choke is used to reject common mode between two differential lines (say from a skew mismatch of the transmission lines).

To deal with this do I need to use 3 common mode chokes? One between the two differential signals, and then on each differential signal and ground? Advice is appreciated.

Choke 1: N and P signal

Choke 2: N signal and system GND

Choke 3: P signal and system GND

REFERENCE IMAGE 1: Choke on single-ended signal and system GND. enter image description here Image Source

REFERENCE IMAGE 2: Choke on differential signals.

enter image description here Image Source

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    \$\begingroup\$ What is the reason for wanting to filter the noise? EMC reasons? \$\endgroup\$
    – Kevin White
    May 29, 2019 at 0:22
  • \$\begingroup\$ Yes, EMC reasons. This is not a problem that has occurred but came up in hypothetical discussions with colleagues.@KevinWhite \$\endgroup\$
    – EasyOhm
    May 29, 2019 at 0:30
  • \$\begingroup\$ What is the pink vertical line in the middle? \$\endgroup\$
    – DKNguyen
    May 29, 2019 at 1:17
  • \$\begingroup\$ Wouldn't the CM just short across the circuit through system ground? If it doesn't that seems like it's a really bad system ground which would mean that the first things to do before adding a choke would be to lower the impedance of the system ground and also connecting system ground to chassis ground before a choke is added. \$\endgroup\$
    – DKNguyen
    May 29, 2019 at 1:24
  • \$\begingroup\$ @DKNguyen. Pink vertical line is the CM current. What you are saying would not work. Even if it went only though ground you've created a loop antenna. The current will not go only through system ground but will split between all possible paths based on impedence. \$\endgroup\$
    – EasyOhm
    May 29, 2019 at 20:15

1 Answer 1

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I've never seen multiple common mode chokes used as you describe.

The systems I've been involved with have normally used a double common mode choke with one in each of P and N (none in the ground). This would be used for USB or similar signals (e.g. MIPI-CSI)

In some cases a triple common mode choke with three windings on the same core was used. That was in a cell-phone. I can't currently find a reference to it. I wasn't the EMC engineer so I don't have any details.

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