[sdiy] dirty/clean ground again

René Schmitz uzs159 at uni-bonn.de
Mon Aug 19 11:54:27 CEST 2002


At 21:52 17.08.02 -0400, harry wrote:
>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...

A good design philosophy is to run as little current thru the GND. Using it 
as a mere reference that must not be loaded, then you don't need a big "pipe".
Think of it as voltage reference, and treat it like a Weston Cell.
Ideally circuits should be arranged so that the current "drawn" on the 0V line
is zero. Then naturally it cannot be infested. Of course that means no parts 
can have their power pins connected to GND. No logic, no single supply opamps. 
If you must, you can interpose another opamp which follows ground. Sort of a 78L00 ;-)

>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....

The bipolar supplies would be seen as one feed and _two_ drains, one is GND and 
the other is the negative supply. Then it will make sense to run all current from 
the positive rail back to the negative rail. Then GND is also seeing only small 
currents. IMO then it makes sense to decouple from the pos to the neg rails. 
Because otherwise you will infest your carefully maintained clean GND.

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

IMO the reasoning should be to avoid to do that in the first place.
Or if you must provide a buffered ground, with a follower. Dustbin approach.

>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).

This should keep the GND relatively clean.

Cheers,
 René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159

 




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