[sdiy] AGND and DGND etc. When to bring together?

[harrybissell]

LOL

Grounding 101:

I like to think of all electrical currents as water flow in pipes. Easier to visualize.

If you have a sink... and a tub and they both drain into the Atlantic (ok you California
boys...) ... PACIFIC ocean, there is little chance that they will interact

The tub cannot back into the sink, nor the sink into the tub.

If you join them with a 1/4" pipe... it is a sure thing they will interact.

If you run two separate pipes, all the way to the ocean... they may drain slow but
will NOT interact.

If you consider TOILET and DRINKING WATER together, you can see how important
the planning is.

QUESTION :   If you drop a jigger of Vermouth into Gin to fill the North Atlantic Ocean...
what do you get ?

ANSWER : A VERY dry martini   :^P

If you add one ounce of water to the ocean... the level rises. Probably you will say "who the
fvck cares" and you're probably right.  The question becomes... do I notice the change in water
level, and do I care ???

NOW Grounding...  There are several techniques.

1) Make ONE ground, really BIG and hope that all the current you can dump into it does not
cause circuits to interact.

2) Make several grounds and run them separately all the way to the ocean (power supply)

Plan two is MUCH harder to pull off... you need to design that right from the beginning or it
is nearly impossible.  So most people use plan one.  Plan one is CHEAPER, and might be all you
need.

Harry's theorem of circuit equivalents.

Low level audio is like the "Drinking Water"
Most CVs are like the "Tub"
Circuits that drive obnoxious loads like LED displays and Relays are the "Toilet"   :^P

You need to take special precautions for the drinking water. That could be things like
separate power traces ALL the way back to the power supply, or extra decoupling caps
etc.

The same is true for the obnoxious loads... you don't want to pollute the power system.

PARASITIC inductance and capacitance...  friend or foe ???

Every circuit has parasitic elements... they are there without your intentional design.  The power
supply traces have some inductance in series with them and capacitance between them. Inductance
will cause the current to rise slowly when there is a power demand... so there will be a voltage drop
at the load.  Capacitance can cause fast rising signals on one wire to be coupled into another.

In general, for a power supply bus the parasitic capacitance is not a problem and could even help you.
Likely you don't notice... its a few pF in parallel with many uF(s) of filter capacitors.  Inductance DOES
hurt you.  You minimize inductance by running the power feed and return very close together.  Often,
twisted pairs work well for this (in wires) and double sided or multilayer boards work for PCBs.

The idea is to minimize the area of the loop made as the current flows in the circuit.  The smaller the
loop, the lower the parasitic inductance.

HOW to achieve all this is a system design nightmare.   If the maker of the modules did not plan ahead
there is little you can do to improve on their design. The best route would be BIG power supply traces
with lots of decoupling caps to provide local power storage... this will feed the circuit until the current
can reach from the main supply.

You could make a distribution board and run all the wires back to it, using twisted pairs. That would be
good as well.

It is good to have a single point ground (the 'ocean' would be ideal :^).   This is often not possible.
Some chips like A/D converters have both Digital and Analog signals (like DIRTY and CLEAN...) and the
best technique in that case is to run things so that the digital currents have an easier path back to digital
ground, and the analog currents have an easier path to analog ground.  Usually it is nice to tie the
analog and digital grounds together RIGHT AT that chip, so that they are at the same voltage and no
errors are introduced...

But what if there are TWO A/D converters, or a bunch of sensitive preamps etc... then you are back
to the drawing board.

Grounding is as much art as science.  How you do it drastically effects the performance, but there is
no one technique that is perfect for all situations.  I usually (by design) try and run power to my
dirty loads separate from the clean ones... make the grounds BIG and low inductance... use a lot
of decoupling caps (right Mr. Patchell ???)...

...and pray often

H^) harry



Julian wrote:

> Im just working out how to power a system here, and thinking again about the power distribution board.
>
> Most of the 5u modulars have two ground lines, whereas companies like doepfer bond together to one at the modules themselves.
>
> I dont know a lot about this, and im wondering what i should be up to with it all.
>
> I have one powersupply (a power-one one) thats powering both by doepfer system at 12v and some diy modules (ele4music etc which have
> the grounds split).  These will be running off a different bus, but the same psu.
>
> Im wondering the best way to do all this?  And i see designs for busses with caps etc. on them too, whereas the doepfer bus is no
> more than strips. (?)
>
> Basically i need a grounding and power distribution 101... Any volunteers? ; )
>
> Thanks again,
>
> Julain