>BUT, i have a 50Hz superimposed over my signal,
>and showme not really what i expect

This sounds as though the power supply to the circuit you were looking at is
floating and not earthed. If you connected the scope probe ground lead to a
ground terminal on the power supply (which seems likely from the way you
write) then the power supply output probably is floating with respect to
ground. Join the black and green terminal on the power supply together, and
the hum signal should go away.

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