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how to remove the 50HZ/220V noise?

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kimjin

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i used the switch power supply,but the power source introduced the 50HZ/220V noise onto my circuit board,the signal is overide by the noise when measuring the signal using the scope,how can i remove the 50HZ/220V noise?
i have used the 100uf decouple capacitor,but it doesn't work.
 

You measure with the ground of the scope probe connected to your circuit right?

You need to start by figuring out if the 50Hz signal is between the 0V and +v on the power supply or if it is common mode between the supply rails and mains earth. There can be significant capacitive leakage through the transformer in the SMPSU.

It is most likly to be a common mode problem. Connecting 0V to mains earth at the psu may help.
You also need to consider earth loops through connections to other equipment.
 

use a dc voltage.
all the device put into a metal box and try
 

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