[sdiy] Mysterious ground noise

[harry]

Hi Antti:

In the USA this noise could be from having safety ground connected in
two places.  One on the circuit, from some point... the other from the
scope case.

The answer here is to run the scope from an isolated power supply (isolation
transformer). The other way is to temporarily defeat the safety ground... 3 to
2
wire adapter.

THIS is potentially dangerous, as the scope case will float at the circuit
potential...
possibly line voltage.  Don't do this unless you are very sure of what you are
doing
and that the voltages are non-hazardous.   In high voltage use I NEVER do
this...
I use a differential probe.

The other idea is to shorten the loop created by the ground lead and the probe
tip.
Use the shortest lead possible... of use the little wire spring clip that
slips over the end
of the probe (do you have that or did you toss it... saying what the $@&% is
this for ?).

A lot of circuits can throw quite a bit of noise into thin air !!! I see this
problem all
the time. It can be hard to eliminate....

hope this helps (well, it would on USA 120V...)

H^) harry

Antti Huovilainen wrote:

> Hi
>
> I fired up my oscilloscope yesterday again after a long pause and
> ocntinued debugging the mysterious ground noise problem with my Midi2CV
> unit (on protoboard). Turned out, that the scope shows the same noise
> (about 4mV p-p) even if the probe tip is connected to probe ground lead
> and I touch the tip/ground (doesn't matter which one, since they're
> connected) to circuit ground. The noise goes away if I remove power from
> the digital section (PIC, UART & 3 latches). It can't be just the case of
> grounding the circuit (which uses wall-mart supply and is thus floating
> with respect to mains ground) to the scope as I can ground it with other
> channel and the noise appears only on the channel that is connected to the
> circuit ground (It's not a defective channel on the scope. I've tried
> switching them). The noise also goes away if I have a resistor (x kohm,
> didn't measure. 1k < x < 50k) between the circuit ground and the
> probe-tip/probe-ground.
> Does anyone have any ideas how this can happen? A defective scope?
>
>   /----\
> --+    +-+15V ----+-----[100ohm]----78L05---digital +5V
>   |7815|          |
>   \-+--/          \-------- Analogue +15V
>     |
> ----+- gnd --+--------- Digital ground
>              |
>              \--------- Analogue ground
>
> Antti
>
> "If you don't eat yer meat, you can't have any pudding!
>  How can you have any pudding if you don't eat yer meat!?"
>    - Roger Waters

--
Jihad Terrorism Conspiracy New World Order
Revolution Black Helicopters Freedom of
Speech First Amendment Rights: Carnivore Bait
go ahead and READ my e-mail I have nothing to
hide... how about YOU ???