Well, first of all, what I'm interested in right now, is aliasing. I want to know more about it and this device could be a good place to look at it. That's interesting in itself. So far I readed this:
https://www.asdlib.org/onlineArticles/elabware/Scheeline_ADC/ADC_NumRep_Nyquist.htmlThen we have the first derivative. If aliasing makes DSO2512 untrustworthy, even for looking at PSUs, then I wouldn't recommend it again. Analog 20 MHz scopes would still be the way to go for me.
But I have to put my finger into that wound first.
It's said ripple/noise are usually well under 2 MHz and indeed the three /four SMPSs I have look into, showed it under 200 KHz. Of course, DC is 0 Hz. So, to me, its really difficult to believe aliasing could be making this waveforms false. However switching noise has higher frequency. I'm willing to learn and ready to be proven wrong.
See attachment. That comes from a cheap 12 V chinese SMPS if memory serves me well. This was done with my benchtop scope, which is unlocked to 100 MHz. You can see the voltage ripple as the caps charge/discharge and the switching noise. Even Zeeweii DSO154 showed remercably similar things, but it's clearly not a 20 MHz BW device.
PSUs can pick a lot of noise almost from everywhere. So it's a tricky task. One advice is to make the first measurement with the PSU turned off so you can then make a second measurement with the PSU turned on. Then you have an idea of what really comes from the PSU.
Since you have, say 150 mV riding of top of 12V, you would use AC coupling and 1x attenuation. Some sort of coax cable with BNC on one side and attached to PSU's output on the other side. You also limit the bandwidth to 20 MHz. PSU output impedance is low so you want also a 50 ohm terminator. You have to be careful about terminator's power rating. W=V squared/ohms. To look at 12V and above you would use a voltage divider and take that into account. Dave has a video on it. I used a 10W RF dummy load with a T adapter instead.
Of course it's also said it's better done by using some sort of differential means of measurement to get rid of that noise. However differential probes are usually HV while ripple/noise is on the mV range. So one would like to use some sort of LV differential amplifier, which is far from cheap.
You would want also check if PSU overshoot at turning on/off, etc. DC coupling for that, of course.
Generally speaking, you want to make sure that no components will be killed by any overvoltage and that things like ADCs aren't disturbed by the noise.