AVR Freaks

No problem James. Don't get to stuck on this. Unless you are getting into analog design or RF you really can't go too wrong. Sprinkle some 0.01uF caps around (on every voltage rail). Toss a 1000pF in parallel with the 0.01uF for good measure on your AREF. Put at least 10uF on the input and output of your regulator (unless you get one of the cool ones that say you don't need it.) And finally, if you are doing your own PCB layout, keep analog ground and digital ground separate. There should be plenty of info out there on PCB layout tips. Google is your friend. Have fun!

The trick of big electrolytic with a .01 in parallel uses the small cap to short out the hi freq noise that gets past the higher resistance of the big cap. I've seen some 'careless' designs where they forgot any caps on vcc and the board still worked (somehow?) and of course the other extreme... certainly you need at LEAST one cap on VCC!

Generally, excepting RF, physical size does not matter. Generally, for a given technology (aluminum electrolytic, tantalum electrolytic, film, ceramic, etc), there is a fairly firm proportionality between the volume of the cap and the maximum product C*V (capacitance and voltage rating). This is another way of saying that, for a given technology, energy storage is proportional to volume.

OK, thats the academic stuff. Practically, the small surface-mount Cs are more "ideal" compared to the larger ones with leads. And, as previously suggested, larger capacitance and larger voltage rating generally means bigger cap.

On power supplies for most run-of-the-mill applications (not including motors and other higher current loads), it has been common to put an electrolytic near the regulator (say 10-100uf), a few ceramics (say 0.1uf) near major loads (like micros) and a bunch of smaller ones (say 1nf) as close as possible to each load. More recently with the new high value ceramic caps, a few large ceramics (say 1uf to 4.7uf) are placed near each major load and the 1nf caps are used close to individual loads.

If your voltage regulator is a switcher or LDO, there may be specific capacitance requirements.

All of this does NOT touch on caps used in audio circuits or RF circuits.

Jim

Also, the physical distance from the cap to the IC it is protecting comes into play as well, for pretty much the same reason. In this particular case, inductance is your enemy. There are power supply isolation methods where inductance is purposely introduced to isolate two power supply domains, but we won't get into that here.

As others have said, for simple "low" frequency projects, the 0.01 uF bypass cap will work fine.

Stu (IC designer in another life, hope I remember this correctly :wink: )