2)spike gaurd with faulty power line detector.
How it is done ?
First we look out for proper earthing plug with the spike guards faulty line detector.
On the spike guard with power line fault detector you can see that the socket has proper earthing as it is indicated by “ALL OK” led signal. The purpose of having a proper earthing is that the oscilloscope has a built in EMI filter at the primary side of SMPS which needs to filter common mode noise to the earth using y2 capacitor. If it doesn’t filters the conduction EMI noise, it will be displayed on the oscilloscope which will create problem in debugging the appropriate signal. because it will be superimposed on the original signal.
Second, we connect the oscilloscope to the spike guard with proper earthing.
switch on the oscilloscope, connect all the input leads of the scope to ground and than we set both the oscilloscopes channel to AC coupling mode. We measure the noise in the oscilloscope. Because of proper EMI filtering the EMI noise is canceled at the primary side of SMPS of the scope. The noise you are seeing in the scope is the permissible DC noise supplied to the power input or to the VCC of the ic, microcontroler, adc present on the scope. you can see it is measured as 20 milivolt max.