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I am designing a board with USB 2.0 and Ethernet 10/100Mbps. I found this layout guide with very helpful information.

In the layout guide, at p. 34 (Table 7), it says that the minimum spacing between MDI signals and other high speed signals should be at least 7.5 mm. In my design I will have to route a USB 2.0 differential near the Ethernet RX differential pair at approx. 3.8 mm for a length of around 10 mm.

So, my question: is this issue critical (will it lead to a non-working design)? In this case, what solutions can you recommend me (e.g., how to shield the Ethernet RX pair from the USB 2.0 pair)?

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  • \$\begingroup\$ Usually the rules for parallelism of traces has a length component to it. For example, if the length of the parallel runs is greater than X, then the separation needs to be at least Y, or something like that. \$\endgroup\$
    – SteveSh
    Nov 11, 2020 at 21:04
  • \$\begingroup\$ @SteveSh I understand, but I do not know this information for my case. All I know right now is that the USB and Ethernet RX pairs will be parallel for approx. 10 mm, at a distance of around 3.8mm. \$\endgroup\$
    – Cristian M
    Nov 11, 2020 at 21:06
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    \$\begingroup\$ You can run a GND between them equally spaced from each. That will help reduce crosstalk. \$\endgroup\$
    – user16324
    Nov 11, 2020 at 21:14
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    \$\begingroup\$ @BrianDrummond that ground trace must have a low impedance to the ground plane to be effective. Otherwise it is just a conductor eating up dielectric separation between potential agressor signals. When I say low impedance, I mean at the frequencies of significance in the diff pairs themselves. \$\endgroup\$
    – user57037
    Nov 12, 2020 at 4:31
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    \$\begingroup\$ @mkeith Yes indeed : I'd expect a via to GND plane at either end of the 10mm section in question. Which should be adequate at 100 Mb/s. \$\endgroup\$
    – user16324
    Nov 12, 2020 at 13:30

2 Answers 2

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As far as I am aware there is no specific guideline for the separation between Ethernet and USB. There is however good practices that are employed regarding separating high speed differential traces from any other traces.

Typically the rule would be to keep differential pairs away from other traces by at least 5X the distance to the nearest ground/power plane.

Take for example, 4 layer board that is 1.5mm thick with the following stackup.

LAYER1: Traces/Components
DIELECTRIC: 14 mils
LAYER2: Ground
DIELECTRIC: 14 mils
LAYER2: Power
DIELECTRIC: 14 mils
LAYER4 Traces/Components

A pair of differential traces on layer 1 should be 5 x 14 mils = 70 mils (1.75mm) away from other traces. Having them further is better.

In your case, you say that the traces are 3.8mm away from each-other. This would probably be OK in just about any normal stack-up other than a 2 layer board (in which case a 1.5mm board would need 7.5mm separation).

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I think you are asking about clearance from one pair to another pair. Often you can get away with less clearance for a short distance. Having a 10mm section where the distance is less than 7.5mm will probably be OK. The magnitude of the cross talk from one pair to another is a function of how long they are routed next to each other. So keeping the spacing violation short will definitely make the problem less severe.

Just to clarify (I think you know this, but others may not) the differential impedance is critical when routing a differential pair. So is length matching. The spacing must be such that the differential impedance is correct in your stackup. Typically, the two members of a pair are routed pretty close together (like 0.1 or 0.15mm clearance between them).

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  • \$\begingroup\$ Thank you for your answer. \$\endgroup\$
    – Cristian M
    Nov 11, 2020 at 21:15

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