Electronic – Stacked metallized film caps — what’s the “outside” foil


One piece of advice that gets tossed around when low noise analog design is discussed is that the outside foil of a metallized film cap should be connected to AC/RF ground, or to whichever circuit node is closer to AC/RF ground, to basically act as a noise shield of sorts. This is great for wound metallized film capacitors, where there is an "outside foil" to talk about.

However, almost all SMT film caps use a stacked structure similar to a MLCC — see this datasheet for an example. If you were using these capacitors in a low-noise or otherwise sensitive design, does it still matter which end is connected to the node nearer AC/RF ground?

Best Answer

I would think so, particularly if there are traces passing under an SMT capacitor.

The fly in the ointment is that few manufacturers specify which end is which on the datasheet so you may have trouble getting the assembly right. Panasonic 'sort-of' indicates a direction on their datasheet (unless it loses something in the translation), but it's not really unambiguously tied to orientation of the markings or orientation in the tape (via the packaging specifications), that I can see anyway.

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