"Peter L. Peres" wrote:
>
> The problem is, I often have to make things that are modifications of
> existing equipment, and I have to move like an elephant in a china store.
> Also, most stuff I am modifying is irreplaceable. So I do what I can.



> My usual problem is, that the only place where I can attach my attachement
> has about 0.5 V potential over the 'clean' ground with some motor or other
> supplying the current to do that. So, the only way I can use a reference
> is to use one that is NOT gnd. The next best thing is my inside Vcc or a
> reference voltage. So I use that, and plenty of symmetrical input
> instrumentation amps or current mirror/reflector inputs for un-important
> things. It works.

Still, your system should be designed and laid out properly within
itself.. The noise on the external ground is something to deal with, and
your internal ground may attach to some point on their system that isn't
ground to them. I still see no reason to compromise the osc design like
that, since it's only possible reference point is the uP's ground pin.
Is it possible that you've got multiple inputs, and current flowing
across your system through the "ground" connections?  That could cause
all sorts of nasties!


Try those Murata EMI filters on inputs. They really do give 50-60dB
supression according to the curves, and I've yet to see one cause any
unforseen effects.