Feedthrough capacitors are for common mode noise?

[cupoftea]

Hi,
The feed-through capacitors like on page 138 of this....

https://www.farnell.com/datasheets/1914683.pdf

Are they much good when you dont have a chassis ground?.....ie, you have one circuit ground and thats it.
They are obviously for common mode noise management. Are they kind of like a Y capacitor....for when you have eg a signal line common mode choke and you want a Y capacitor to go with it?

I appreciate these three term caps are said to be for diff mode, but the frequency they operate at is so high, that you think of common mode for those frequencies.

 

[betwixt]

They are primarily used when high frequencies have to be blocked in DC power feeds. Their construction is that way to eliminate inductance as much as possible. They are not perfect but consider that a normal capacitor is modeled as a perfect capacitor with inductance and resistance in series with it. The 'feed through' construction is minimize the inductive element as much as possible. They only work when there is a low impedance ground at the center connection, typically a ground plane or vias through a PCB to a ground plane underneath. Without a good ground they are no better than any other capacitor.

Brian.