[sdiy] fanning grounds vs. dirty/clean ground

[Theo]

It seems in general Opamps don't like capacitors on their outputs.
On a resent project using a single supply voltage I had to put 100R on the
output of the opamp buffering the reference voltage to prevent oscillation.
I might miss something here, but it seems to me the same problem could
happen with a opamp buffered "ground".

Theo


From: René Schmitz <uzs159 at uni-bonn.de>

> >>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.
>
> Right!
>
> >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??
>
> I run a "reference" GND lead to the PSU, that is practically without
current.
> (Ideally it would only be loaded by noninverting inputs of opamps...)
> That is used as the voltage for a simple opamp voltage follower. The
output
> now has some current capability (5-10mA, use a power opamp if you need
more).
> Of course that can only be used for ground nodes inside circuits. Never
> attach a jack to this "regulated" ground, external connections are tied to
> the direct unbuffered ground. I did it that way for some of my modules
now,
> and this works quite well.  (A couple VCO3s that now also have a seperate
> regulator for each module, so there is even no problem from the now
noisier
> supply rails.) Its of course best suited if one designs with that in mind,
> so that one returns as little current to GND as possible.
>
> >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.
>
> Right. My reasoning is that most opamps have a some PSRR, but a load
> referenced to a dirty ground will have _none_. Thus the somewhat noisier
> supply is the smaller nuisance.
>
> >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".
>
> Thats also a good idea.
>
> >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.
>
> Thats right. At high frequencies (i.e. high dI/dt) the inductance is your
> enemy, since U = L * dI/dt. Thus makeing things not as fast as possible,
but
> only as fast as necessary makes sense.
>
> Cheers,
>  René
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>
>