All of which implies there's additional input power supply conditioning when externally powered.....wouldn't you think ?
Yes, but having dealt with more than one LDO which in itself has horrible PSRR at high frequencies I figured I'd ask just in case...
External power for differential probes is common, most of the Pintek range are supplied with wallwarts.
Try and see if you can find additional noise from the probe when it's externally powered, if there's any discernible I very much doubt it will impact on measurements.
So I ended up being done with one project on my bench and could test this out between projects...
Oddly, the amount of noise in my testing seems to be *better* with the USB supply than with fresh Alkalines, although it is a bit more spiky. The DC offset seems to be different by about 2mV, regardless of the scale, but the noise is definitely higher amplitude with the batteries, not a lot, but definitely visibly more. A FFT seems to indicate this as well.. noise level on the batteries are higher across the spectrum, again not by much, but some. Not the result I was expecting.
I also tested with the linear supplies on my bench, they both seem to be between the batteries and the USB supply.
In reality the difference is so minor, I don't think it's going to matter to me what I use.