I would have to touch the oscope to confirm something.
This amount of noise is common. There are many sources of noise, but with a noise magnitude that small, it is probably not a concern.
Let me list off a couple things.
Sources of Noise
The loop of your ground clip creates has an inductance with everything on your board. any electrical signal nearby can induce a wave on your ground loop. This ground loop also acts as a low pass filter, albeit the frequency that it cuts off is normally in the high MHz range, not normally any issues for a 40Mhz scope. I have drawn a red line around what I am referring to in the following image.
The fact that you have a ground reference in one circuit, and another ground reference in the other circuit plus your ground line connecting them can create ground loops(not to be confused with the ground clip issue I mentioned above, have to love similar terminology). This can actually create quite a problem, and is also no fun to fix. One method is to wrap the probe almost the the extreme tip with aluminum foil and ground it at the o'scope. This shields the interior ground connection and greatly decreases ground loop problems. It also looks awesome(not at all).
Testing for issues
To see if it is just a 1bit bounce,
try changing your resolution and
watching if the noise changes in the
amount it is jumping by.
To check if the ground loop is your
issue, try using an sharp blade and
touching between the ground at the
very tip of your probe to the ground
on your board. This gives more than
an order of magnitude drop in
To check for ground loops, you get
to wrap your probe in foil. Do not
enjoy this too much.
- If your noise disappears just from connecting to the waveform generator on your scope then your circuit is probably the cause.