Sensing mains input current in offline SMPS

[ocset]

Hello,
We sometimes monitor the input current to our  offline SMPS’s by using a diff probe and scope to observe the voltage across a sense resistor at the input to the SMPS (as in the attached diagram).
Why is it that the reading is always more noisy (and this is seen as ‘more ‘fuzz’ on the scoped waveform) when the sense resistor is placed in the Live line as opposed to the Neutral line?

[Gyro]

Neutral, being near earth, making less demands on the probe's CMRR?

Either that or the parasitic capacitance between your circuit and ground (bench?) is providing some HF filtering.

[dmills]

Seems likely, UK practice has the neutral bonded to earth (In most installations) at the supply company terminal, so while this is not to be relied upon there will usually be less common mode voltage measuring in the neutral.

Regards, Dan.

[ocset]

Thanks,
If the sense resistor is placed  just downstream of the mains rectifier  bridge, (as in the attached schamtic)  then it shouldn’t matter whether the sense resistor is placed in the [DC Bus –ve]  or the [DC Bus +ve]. Either place should be equal from a noise point of view, since each node alternately connects to mains live then mains neutral.
However, even in this case, I always  seem to see less noise when the sense resistor is connected into the [DC Bus –ve]. Do you know why?