groundhigh frequencypcb-designraspberry pitrace
I have seen several designs where there is extra spacing between some traces and the ground plane. For example, see the traces between the points A and B or C and D in the following picture (this is a Raspberry Pi 2, but I have seen it on other boards too):
What is the purpose of this extra spacing?
My guesses:
With proper decoupling Vcc is as good as ground as far as EMI is concerned. So an extra plane is a good thing. However, if you're working on only two layers the plane may be cut into several islands. Try to keep connections as short as possible.
Using isolators is the most straight forward way to couple boards like this while avoiding grounding and noise headaches.
For an isolated design to function, you need isolating buffers for all the digital input and outputs as well as an isolated power supply, although the later can be ignored if the power ground acts as the "star point" for your daughter board and you do not require isolation on the sensor part (e.g. a 'floating adc').
Conceptually this can be imagined by the following diagram
simulate this circuit – Schematic created using CircuitLab
Best Answer
I would put money on #2.
That is clearly a differential pair between A and B, referenced, no doubt, to a plane on the next layer. Between C and D appears to be a single ended signal.
That is a microstrip configuration, and if the surface plane gets too close it would change it to a coplanar waveguide with ground, which apart from changing track and gap for a given impedance, also induces (if you are not careful) propagation mode differences.
No hyperlinks - on mobile. Maybe tomorrow.
Edit: updated with simple picture.
Looking at the top picture, we have the classic differential microstrip configuration. The required clearance is beyond the scope of this post, but we do not want to couple the fields to anything other than the pair and the reference below.
If we allow the same reference to approach from the sides, we get a coplanar waveguide with ground, a very different beast indeed. I am not going to post the equations, but they are easily available with a search.