For accurate ripple measurements, you need an AC coupled probe and this is not standard issue with a Nano. AC coupling acts like a high pass filter where the DC component gets eliminated from the input so that the full DSO range can be used for the ripple itself. This is typically handled by circuitry internal to the DSO, but the Nano is hardware limited to DC coupling.
Due to its relatively high vertical sampling resolution, Nano users may still get a decent measure of ripple for low voltage circuits, and some provisions were made in firmware to facilitate this. Forum user lygra prepared a video that explains this in some detail.
http://www.youtube.com/watch?v=NUx9ulEjvLA
For 24V circuits and beyond however, this approach may not be practical.