PCB Assembly Services - Screaming Circuits: A question on Schematic Parts Placement


A question on Schematic Parts Placement

Question for the day: Where do the bypass capacitors go?

We all know that, on the finished PC board, bypass capacitors need to be strategically located close to chips and noise sources, but what about on the schematic? In this schematic clip, I've located them all in the same place to allow for a clean and well organized sheet. Sometimes, though, I'll place them in the schematic right up next to the chip, as they would be on the physical PC board layout. FPGA and caps schematicElectrically, both practices are equal, so it comes down to a matter of style.

I kind of go back and forth on it. Sometimes I'll let the schematic symbol take the lead. Here, the IC symbol has pins grouped by functionality rather than in numerical order. If the pins on the symbol were in numerical order, to me, it makes more sense to plop the bypass cap next to the power/ground pins on the IC symbol as it will be on the PCB. However, with this style of symbol, that doesn't seem to be as important.

On the other hand, I'm probably more likely to forget a capacitor or put it in the wrong place on in the PCB layout if I follow this practice. What about you? How do you place your bypass capacitors? Is it a matter of style, or is one way or the other best practice?

Duane Benson
Six of one and 0b00000110 of another

Once you've figured out where to put those capacitors and finished up the layout, visit www.screamingcircuits.com and get a quote on putting them (and your other parts) on your PC board.


"a human see my schematic and understand it completely" - That really is the bottom line.

I am VERY opinionated about that topic.
First, it hurts my eyes when I see a symbol with the pins in nummerical order. It makes absolutelly no sense to me. I allways group them by function, and sometimes I break the symbol in subparts to group them in a more organized way. Sometimes a subpart is in one sheet, and the other is in another sheet. It is all a mather of making the schematic clear and easy to understand, the same way a source code must be organized, indented, etc, to be easy to be understood by a human.
That way, decoupling capacitors must be placed in the schematic in a way to show that it must be placed near a power pin to decouple it. By seeing the component in the schematic, this must be obvious. Sometimes, I create a subpart just for the power pins, sometimes I change the symbol in order to be able to place the capacitors in a more organized way. In the end, what really matters to me is to a human see my schematic and understand it completely.

I've done them all in a row like that when it should be decided in layout where to place them, putting high-frequency decoupling closer and bulk wherever it's convenient.
There are cases where I want the high-frequency decoupling by specific pins. In this case, I put them next to the pin on the schematic.

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