analog GND

http://forum.banggood.com/forum-topic-61279.html?page=3

As you can see, I lifted C26 and the ground pin of C15, and connected them to analog GND at c8, and the positive of C26 to the output of L4.

I have also connected the VSSA pin to the "new" AGND of the C15 and attached an extra capacitor (tantalum 4.7 uF) to the output of the U4. These two mods have slightly improved the situation in my case.

https://www.youtube.com/watch?v=aIb3azRrJbc

http://www.jyetech.com/forum/viewtopic.php?f=18&t=542

I would estimate that this mod has reduced the noise evident in the display by 60-70% which is a significant reduction. For anyone who has built a DSO138 kit prior to the new release firmware, I would highly recommend this easy mod.

To recap on MorganFlint's work, it simply involves removing C26 from the board and laying it flat with its +ve lead soldered directly to the end of L4. The earthy end of C15 is lifted and a short cutoff component lead joins the negative of C26 to the lifted lead of C15. Then a short cutoff component lead joins the end of C15 to the earthy end of C8.

JP3

Using some ferrite bead or low inuctance instead of soldered jumper JP3 can partialy remove spikes.
I've used 4mm ferite ring with 3wounds of 0.4mm wire.