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I am designing a ultralow noise PCB board. Common mode noise has to be minimized, so I have a question

I am planning to use a CMC on power input and to further decrease the common mode noise I also planed to use a x2y cap ( middle going to earth)

Should I place the x2y cap before or after the CMC ? Should I expect a decrease on common mode noise using those 2 filters ?

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That depends on whether you're most concerned with radiation or immunity. If you don't want the noise getting out, put the cap on the line side. If you don't want the noise getting in, put it on the load side. If both are concerns, use a pi configuration.

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  • \$\begingroup\$ Ok got it And a CMC + y2x caps are better at rejecting noise then using only one ? There is no data anywhere showing x2y caps used with CMC \$\endgroup\$ – Johan B. Aug 29 '18 at 13:20
  • \$\begingroup\$ You need caps on the side you want to protect--inside or outside. With caps on both sides, you're protecting both. They don't even need to be Y2C...I prefer feedthroughs for their wide effective bandwidth and the routing discipline that minimizes series L on your bypasses. A Y2C cap is nothing but two caps in one package; a feedthrough is almost the same mechanically, but the non-ground plates are connected on both ends. Standard chip caps will work too, but resonance and layout will be issues. And NEVER use a bunch of different valued caps in parallel, because you'll get antiresonances. \$\endgroup\$ – Cristobol Polychronopolis Aug 31 '18 at 18:18

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