[sdiy] Mysterious ground noise

[Antti Huovilainen]

On Thu, 22 Nov 2001, harry wrote:

> 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 circuit is floating (since it uses wallwart transformer), so multiple
grounds shouldn't be an issue.

> 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....

It throws some radio interference, BUT this only appears when I hang the
probe tip/probe ground wire over the digital section. However, no matter
how far I am from the circuit (say, attach a 1m long wire to circuit gnd
and touch probe tip/ground to that) the mysterious noise appears whenever
probe gnd&tip touches the circuit gnd. I tested it with a 500ohm resistor
between circuit gnd and probe gnd&tip and this mostly eliminated the
noise show on scope (1/10 the amount of noise). However, I obviously can't
do this since 50mA*500ohm = 25V, so adding 500ohm resistor between digital
gnd and main gnd is out of the question.

Antti

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