The VCC pins of ICs should be decoupled with a capacitor.
Suppose you have an IC with two VCC pins, say VCC1 & VCC2. You can
connect VCC2 to decoupling capacitor CAP2. Each VCC pins are connected to separate CAP.
Which one of the above choices is better?
Thanks
smdfan
Each supply pin should have a decoupling capacitor.
leon_heller:
Each supply pin should have a decoupling capacitor.
Why?
I’ve seen several schools of thought - the one that I usually subscribe to is that each VCC pin should have 1 [or more] decoupling caps, as close to it as possible, and connected directly to the caps without a via [if possible]. I’ve also seen the other extreme, such as on a few FPGA dev boards I have. They’re laid out with the decoupling caps in 5 or 6 groups, sitting ~4mm from the edge of the BGA to allow route out. That board also works fine - and that FPGA is driving DDR SDRAM, so it’s really going to be making demands on those decoupling caps [fast edge rates w/ lots of signals changing in parallel].
All in all - I’d say the most important bit is to make sure you have lots of caps, and of different values. If I feel particularly motivated; and my PCB supplier hasn’t got my boards to me, I’ll look up the precise recommendations in a few of my high speed design books.
–David Carne
smdFan:
leon_heller:
Each supply pin should have a decoupling capacitor.Why?
It’s quite obvious if you think about it. The chip manufacturer uses more than one supply pin to ensure that different parts of the chip get a clean supply delivering enough current. That can’t be achieved using a single capacitor.
To clarify the current supply statements a bit. The chip pulls current in spikes usually and the capacitor is like a little extra supply for those spikes.
If both VCC lines are used, you should have both bypassed with caps – this will make sure that noise won’t leave the IC through a unbypassed line.
Bypass capacitors provide for when the circuit needs spikes of power fast, but they also “decouple” different circuits from each other so that noise does not transmitt from one circuit to another on the same board. Using several caps around the lines/pins makes for a better decoupling than one large cap. Low “equivalent series resistanse” ESR caps are more effective than say electrolytic caps.