How to design a CMC (Common Mode Choke) footprint to allow no-pop solution

Question

How would you design in CMC's with a 0504 (1210 metric) 4-lead package to allow for a no populate (do not pop, DNP,DNI) solution? I am thinking 0402 0ohm (zero ohm) resistor but cannot picture the PCB layout?

Specifically, this is for USB data lines and the CMC's may be used to reduce radiated EMI.

Edit 7/9/19 Would this footprint setup work okay? 4-pad CMC with two 0402 resistors in series.

Voltage Spike · Answer 1 · 2019-07-08T17:54:29.730

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In these situations for prototyping I do something like this:

You cut the traces on the PCB if you need to install the choke. That way you don't impedance problems from soldering wires. The problem with this setup is once you cut the PCB traces, you can't go back. Another way to do this would be to put 0 ohm resistors in series with the choke. Or you could use 0 ohm resistors if you select them the same size as the choke gap.

edited Jul 08 '19 at 17:54

answered Jul 08 '19 at 17:23

Voltage Spike

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You can always go back with cut traces. With a suitable ECN and solder bridge – Tony Stewart EE75 Jul 08 '19 at 17:34
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In my case the thermal EMF would not be the same, when I cut, it's one way. – Voltage Spike Jul 08 '19 at 17:38
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In our shops where I worked, it was neat but bridgeable. It is always a better idea to include CM chokes for USB. I have seen too many data errors without this using Towers to Arduino on floating supplies etc. Due to SMPS CM noise. – Tony Stewart EE75 Jul 08 '19 at 17:53
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see edit, how does that look? – RuggedEngie Jul 09 '19 at 18:28
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@RuggedEngie Looks great to me... – Voltage Spike Jul 09 '19 at 18:30

score 3 · Answer 2 · answered Jul 08 '19 at 17:17

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It would be a dual pad layout joined for either SMD CM choke or any SMD 0R parts or fixed R parts depending on design.

The design would be controlled impedance for USB2 (100MHz) or USB 3.

answered Jul 08 '19 at 17:17

Tony Stewart EE75

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see edit. how does that look? – RuggedEngie Jul 09 '19 at 18:28
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Not good for tombstoning without thermally isolated pads – Tony Stewart EE75 Jul 09 '19 at 22:27
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Can you expand on that? I am not sure what you mean. – RuggedEngie Jul 10 '19 at 14:03
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https://www.autodesk.com/products/eagle/blog/what-is-pcb-tombstoning/ – Tony Stewart EE75 Jul 11 '19 at 02:11

How to design a CMC (Common Mode Choke) footprint to allow no-pop solution

2 Answers2