[sdiy] More ground questions

Mon Aug 5 18:42:32 CEST 2002

Hi (again) Ingo.

There 'might' still be some benefit from running two grounds...

Most Opamps have very good power supply rejection ratio... so some
spikes on the supply could be tolerated (by being rejected..) 

the sensitive spots are where the opamp gets the ground reference
(like the non-inverting input) and a change in voltage here is sure
to be coupled, maybe amplified as well.

Keeping nasty power hungry loads (LED etc...)away from these points 
could still give a benefit...

...if the rest of the design and grounding is done correctly as well.
In most cases the builder will blow the concept somewhere and get no
benefit.

I go so far as to isolate all audio jacks from the panel with shoulder
washers so I can control every ground point to where I want it. I get
very low noise this way... but it takes a lot of dicipline to use this
method.

The analogy of water pipes carrying 'current' flow can help... that's if you understand how water flows in a pipe. Many don't  ;^P

I'd agree... probably little benefit from the separate ground in this
case unless the board is SO carefully designed.

H^) harry

>harry wrote:
> > Hi Ingo...
> >
> > I either don't understand (or don't agree with..) you.
> >
> > If the grounds are separate... AC currents from dirty signals bypassed
> > to the dirty ground... MUST flow in the dirty ground all the way 
>back to
> > the common ground point.
> >
> > This is presumably the lowest impedance point of the power supply 
>system...
> >
> > These currents do not flow in the clean ground, ever... so they 
>cannot cause
> > voltage drops and therfore noise in the clean ground.
> >
> > I'm also presuming that the positive and negative power supply 
>leads are separate
> > as well.
>Yes, if there are separate supply lines for dirty and clean circuitry,
>then separate ground lines make sense.
>But in the original mail "four pin connector cables" were mentioned,
>as I understand it, one for clean ground, one for dirty ground, one
>for positive and one for negative supply. No separate dirty and clean
>supply lines.
>If there are bypass caps across both the dirty and the clean loads
>these two caps (in series, with the supply line in the middle) will
>short both ground lines together.
>Ingo