[sdiy] dirty/clean ground again

[harry]

media.nai at rcn.com wrote:

<snipped here and there>

> >So why do we all decouple opamps with dual caps to ground.
>
> Well, why do we choose monolithic caps for decoupling in the first place??
> Isn't it because they are less inductive than other types of caps, and are
> therefore better at high frequency decoupling??  So if the leads to the
> capacitor are longer than a couple of millimeters, or if long board traces
> are used, the inductance increases, and the advantages of using a low
> inductance cap are minimized if not negated.  So one advantage of using
> dual caps is their physical placement.  Being able to place each cap close
> to ground minimizes inductance, as opposed to bridging one cap between both
> power pins -- typically at opposite corners of the op-amp.

I'm gonna jump in here with some philosophy....

If you take a jigger (shotglass) of water, and pour it into the North Atlantic
ocean... does the water level rise ?

Think carefully... Of course it does.  OTOH it is a very small amount... and
probably
if I do it in the USA it will take a long time for the change to be noticed in
Europe.

We "wish" that ground was an infinite sink for current... that we could dump
any
amount of current into it and not have the voltage rise...

and for small currents in short traces at low frequencies...we often get our
wish.

But really the ground has finite resistance, and inductance.  So when we pour
that
"shotglass" of current into it at one point... the voltage rises...and it take
some time
for the current to reach the point where it will get cancelled out (meeting its
equal
but opposite current back at the power supply.)

I prefer to think.... Hot Water Pipes.... Cold Water Pipes.... Drain Pipes.
Now can you see why putting (lets say) separate valves from hot and cold water
to the drain...
makes more sense than valves from hot to cold....

Most load currents end up going from positive supply to ground... or negative
supply
to ground... so thats where the decoupling caps belong.

Some rare cases have the load current going across the rails... I often run
relays
in my audio designs across the rails, rather than from one rail to ground.
Decoupling
these from rail to rail makes sense (and I do).

> >Isn't it more justified to use single cap just across power rails?
>
> I've seen that used in synth circuits, and it is the basis of IC sockets
> with built-in bypass caps.

Au countaire... these sockets have the decoupling cap across pins 7 and 14...
or 8 and 16... which screams that they are for 4000 series CMOS, or HC,
HCT, etc logic families with single supplies...

H^) harry