Capacitors and 4 layer PC boards - Electronic Circuits Projects Diagrams Free

circuit freak · 3rd November 2009, 01:44 AM

I am designing my first 4 layer PCB, and I am a little confused on how to use the capacitors with it. In the schematic it has capacitors going from the power trace of a component to ground, but on a 4 layer board there isn't a power trace to the component, just an entire plane. how does the component use the right capacitor? Does it matter where I put the cap? Thanks for the help understanding this!

ccurtis · 3rd November 2009, 04:53 AM

You place the capacitor intended for a particular component as physically close to that component as possible. A component will use all the capacitors to some extent, since all the capacitors are connected in parallel, but the capacitor placed closest to a component provides the most effective bypassing for that component while the other capacitors provide the most effective bypassing function for the other components placed closest to each of them.

crutschow · 3rd November 2009, 05:30 AM

Yes, put the capacitor as close as possible to the pin it's bypassing, and use a fat trace between the pin and capacitor to minimize trace inductance. For maximum bypassing effect, flood the top and bottom with ground plane also (fill in all the area between traces) and connect the common end of the capacitor to this flooded plane. Run numerous vias between the flooded plane and the buried ground plane layer.

circuit freak · 3rd November 2009, 06:43 AM

So along with the internal ground plane, use the top and bottom planes as ground planes and connect them all? Also, even though the pad and capacitor are connected by the plane, still connect a trace to them? Do I not connect the positive side of the cap to the power plane, just the trace to the pad? Thanks for all of the good advice!

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3rd November 2009, 01:44 AM	#1
circuit freak	Capacitors and 4 layer PC boards I am designing my first 4 layer PCB, and I am a little confused on how to use the capacitors with it. In the schematic it has capacitors going from the power trace of a component to ground, but on a 4 layer board there isn't a power trace to the component, just an entire plane. how does the component use the right capacitor? Does it matter where I put the cap? Thanks for the help understanding this!

3rd November 2009, 04:53 AM	#2
ccurtis	You place the capacitor intended for a particular component as physically close to that component as possible. A component will use all the capacitors to some extent, since all the capacitors are connected in parallel, but the capacitor placed closest to a component provides the most effective bypassing for that component while the other capacitors provide the most effective bypassing function for the other components placed closest to each of them.

3rd November 2009, 05:30 AM	#3
crutschow	Yes, put the capacitor as close as possible to the pin it's bypassing, and use a fat trace between the pin and capacitor to minimize trace inductance. For maximum bypassing effect, flood the top and bottom with ground plane also (fill in all the area between traces) and connect the common end of the capacitor to this flooded plane. Run numerous vias between the flooded plane and the buried ground plane layer. __________________ Carl Curmudgeon Elektroniker

3rd November 2009, 06:43 AM	#4
circuit freak	So along with the internal ground plane, use the top and bottom planes as ground planes and connect them all? Also, even though the pad and capacitor are connected by the plane, still connect a trace to them? Do I not connect the positive side of the cap to the power plane, just the trace to the pad? Thanks for all of the good advice!