I'm playing with a ATmega32u2 and I'm trying to layout the bypass caps for the supply pins on the board. All three supply pins (VCC, AVCC, UVCC) are within a few millimeters of each other. Can I place a single 300nF bypass cap instead of three individual 100nF ones?
Hope you have an assembly house that can deal with components on both sides of the board. Personally, I avoid that.
Jim
You can do if you want. You will get marginally more noise on AVCC.
You seem to have the capacitors on the underneath of the board. Can you not re-arrange them to fit on the same side as the AVR?
With your two-sided components, you have plenty of board space. Separate capacitors will not cost much money.
I am assuming that you want everything to run at 5V.
David.
David and Jim,
I'm the assembly house and the boards are going to be hand soldered. The connector board at the bottom flips up and solders at right angles to the main board. I've placed components on both sides of the board to try to get it as small as I can and not have to go small than 0805 components if I don't have to.
I'm not concerned about the cost of two extra chip caps, and I could easily add them to the board but their traces would be much longer. I'm not using the ADC so I'm not really concerned about a little extra noise. I guess my biggest question about the capacitor is if I do "combine" them, do I substitute with a larger cap, or keep just one 100nf?
100nF should be fine. 220nF might be marginally better.
David.
It doesn't help when you get differing answers... sorry.
If you are building a hobbyist project, or a prototype, then one, 0.1 uF cap would be fine.
If its a real project, and you want high reliability, I'd go with three, 0.1 uF caps.
Look at your layout and see if you can fit them in.
JC
It's the layout that matters not the actual value (within reason).
Three correctly routed and placed 22n capacitors will perform much better than one 300n badly placed and routed capacitor.
I find 0603 parts pretty easy to deal with. Mistakenly ordered 0402 caps once. Those are insidious.
"We trained hard... but it seemed that every time we were beginning to form up into a team, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing. And a wonderful method it can be of creating the illusion of progress while producing confusion, inefficiency and demoralization." Petronius Arbiter, approx. 2000 years ago.
