Go Down

Topic: Why so many capacitors between Vcc & GND? (Read 15473 times) previous topic - next topic

GilchristT

Hi

Just trying to educate myself a bit. I'm putting together a project based on a USB hub IC and a ATMEGA32U4.

The reference design for both components has multiple capacitors between the power and gnd nets. I'm not questioning the requirement, I assume they're there for a reason but I was just wondering why so many?

On the assumption that the capacitors are there to provide power smoothing, is the idea of having so many that you dot them around the PCB so they're close to where they might be required i.e. my nice neat PCB design with them all together in a row probably isn't a good thing?

Thanks in advance.

Tommy


mromani

Aren't they used to avoid noise ? If so, then it would make sense to have them spread around the pcb... (mine is just a wild guess, as I'm not into electronics very much :-)

Grumpy_Mike

#2
Feb 14, 2012, 11:21 pm Last Edit: Feb 14, 2012, 11:29 pm by Grumpy_Mike Reason: 1
These are decoupling capacitors, read all about it here:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Quote
my nice neat PCB design with them all together in a row probably isn't a good thing?

Correct.

retrolefty


Aren't they used to avoid noise ? If so, then it would make sense to have them spread around the pcb... (mine is just a wild guess, as I'm not into electronics very much :-)


Noise doesn't 'hurt' the traces or other passive components, only the ICs, so bypass caps should always be mounted as close to the ICs they are 'protecting' and use as short leads as possible.


DVDdoug

Quote
Noise doesn't 'hurt' the traces or other passive components, only the ICs
Just to clarify, it doesn't damage any components, but it can mess-up the signals & data.  Obviously you don't want noise in an audio circuit, and too much noise in a digital circuit can change a '1' to a '0' and that can really mess things up.  (One wrong bit on the data or address bus, and your program will usually crash!)   

When ICs switch (or ouput data) they tend to make noise on the power supply lines, and that can mess-up operation of the IC making the noise, or a different IC.

I work in production, and once in a while we will have a board missing one of the bypass caps.   One missing cap never makes the board fail.   (But, if one is missing, we always replace it.)  In reality, you could probably take half of 'em off and the board would still work.   But, standard practice is to bypass the supply near every IC.

mromani

Thanks Grumpy and retrolefty :)

BTW, Mike, I found that page excellent.

MarkT

One insight that might help appreciate why digital circuitry especially needs thorough decoupling:

3kW mains heater at 240V 50Hz, 12.5A rms, max rate of change of current is 5.5kA/s

1000W audio amplifier into 4 ohm load at 10kHz (16A rms signal ) - max rate of change of current is 1.4 MA/s

20mA LED driven from logic circuit (7ns switching time) - 2.8 MA/s

Lightning is something like 1000MA/s for comparison.


So a little digital circuit driving one LED could easily be causing twice as fast a current change as a large PA amplifier, and far larger than anything on your mains wiring - and this interacts with stray inductance in your circuit to develop unwanted voltage spikes without decoupling real close to the chip.

Incidentally the only output switching time for the Arduino processor I could find on the datasheet was a "typical" value for the SPI SCK output of 3.6ns, so I'm probably being conservative with these switching times.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

GilchristT

Thanks to all of you, lots of useful information.

@Mike, I think you pointed me at that page a while back when I asked something broadly similar ("What are capacitors used for?" if I recall correctly :smiley-red:). It makes a lot more sense to me now. That's no reflection on your writing, just my reading  :)

Tommy

war_spigot

I would think they would need to be close.  Noise can be introduced anywhere, so there's no point in putting them all in one place.

Go Up