Bypass cap's

[Mark Amundson]

Steve Varner wrote:
> Sooo... Mark got me a bit worried when he said:
> 
> Mark Amundson wrote:
> > 3. Use 10uf to 100uf tantalum or electrolytic caps for local supply sag
> > issues. Do not use these caps if you have regulated supplies and good
> > beefy wiring as they may cause more power supply hum than their curative
> > effects.

> Exactly how do these "larger" caps on the power supply inputs to the module
> cause power supply hum? SV

Bill Layer's comments about over-doing the total capacitance should be
heeded. My biggest concern is that return (ground) wire. Since you are
already planning on using the +5 volt supply, all that current returns
on the ground wire (bus) back to the supply. Problems do occur if the
return currents bounce around in value to digital and analog demands.
Any modest wire resistance (more than tens of milli-ohms) will cause
"ground bounce" and milli-amperes of current swing covert to milli-volts
of noise on the ground wire. That is why I like separate digital and
analog ground buses and split supplies (+/- 15v) for analog circuits.

The point to be made is that large caps absorb/provide current changes
to maintain supply voltages, but those currents may also bounce the
ground a bit. That is why I like the larger caps close to the power
supply.

A return wire to ground is not always a good ground wire if there is
significant current flowing through it.

Mark Amundson,