| Electronics > Beginners |
| Charged capasitive layer inside a pcb - is that really so fantastic? |
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| FriedMule:
I have read a forum post on a HI-FI site, where a person talks about shielding between the power tracks on one side and audio-signal on the other side. He says that the absolute best on earth is using a 4 layer board, where the two inner layer act as a capacitor with constant DC power on. In other word, making the two inner layer work as a charged capacitor that blocks any AC or other signals from going trough. The two layer is charged by two separate wires, going directly from the PSU. Is that real or just ehm, audio-talk? |
| apblog:
there are good reasons to have a ground plane, but capacitance is not one of them. If you enter the plane area and separation into a capacitance calculator, you will see that the capacitance is very small. The highest quality audio recording equipment, stuff that nobody could question, frequently does not have a ground plane on the PCB. |
| AndyC_772:
He's right, but for completely the wrong reasons. A capacitor doesn't block AC, it passes it - but the amount of capacitance formed by the planes in a PCB is so tiny as to be virtually undetectable at audio frequencies. (It's really important in digital circuits operating at hundreds of MHz and above, but that's for different reasons and has no relevance in audio). Charging a good quality capacitor - and a 4 layer board with solid planes meets this definition - makes no difference to its impedance. Xc=1/jwC regardless of dc bias. A solid plane acts as a shield, and is undeniably beneficial. Two solid planes may be very slightly better, but not for reasons that have anything to do with the fact that they form a capacitor. If anything, the capacitor effect allows coupling between them, and moving them further apart (eg. by using a thicker PCB) would help improve the screening effect. Tip: this is often why running a ground trace between a noisy signal and a sensitive one helps. The benefit is nothing to do with the ground trace itself; it's simply that it forces the two other traces to be further apart, so the field strength at the sensitive trace is reduced. |
| filssavi:
As always with audiophools there is a nugget of truth behind a pile of 💩 Interplane what they are referring to is interplane capacitance, and while of very low value (100s of pF at most for large boards, it is one of the best high frequency capacitors available due to the very low inductances and is crucial when the design is going to work in the 100s of MHz or higher region With tat said there are 2 big caveats: -at audio frequencies it is completely useless -you usually need a special “inverted” stackup with prepreg in the middle and core outside that is going to be more expensive |
| FriedMule:
Okay, so we are talking about the same as "the skin effect" of wires do exist but no ware near 20kHz? Do I understand correctly in that a single or double copper layer, connected to ground, is fare better then any 0.5pF of capacity? Should these two inner layer be connected via "vias" or separately connected to ground? |
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