[sdiy] fanning grounds vs. dirty/clean ground

media.nai at rcn.com media.nai at rcn.com
Mon Aug 19 19:26:34 CEST 2002


At 11:54 AM +0200 08/19/02, René Schmitz wrote:
>
>A good design philosophy is to run as little current thru the GND.

Taking that a step further, it's a good idea not to run unnecessary current
anywhere.

> 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 ;-)

So are you saying use the inverting input of an op-amp as a virtual
ground??    If ground was drawing so little current so that could work,
would there be a problem in the first place??  Or do you mean using the
output, and if so, how would you reference it??

>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.

Another way of looking at it is by not decoupling to ground, you are
carefully maintaining a clean sewer, at the risk of having a more
contaminated water supply.

I think some of these problems could be reduced by board placement.  If
parts are grouped together onto different "buses", noise in one area is
less likely to effect the other parts.  Place the signal grounds closer to
the supply connection (where ground is closest to zero) or connect them
with a wider, less resitive trace.  Run the op-amps off another "bus", and
all the noisy digital chips near off yet another "bus".

Also, perhaps one should choose parts that create less noise in the first
place.  I'm not an EE, but it's my casual observation that supply noise is
often related to dV/dt -- the faster, the noisier.  The application notes
of slow DC-accurate op-amps barely mention decoupling, yet the application
notes of audio op-amps (with blazing slew rates) go on and on about it like
they were selling capacitors.  Fast logic is very noisy, but the CD4000
series is slow and quiet.  This goes back to what Harry was saying about
the "pipes" not being large enough to handle sudden changes.






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