To achieve a proper ground routing in sophisticated applications can be more an art than a science. You must always be clear, what kind of currents are flowing across your ground star point(s) and must always take into consideration, that there's no ONE single ground in any application. There are as many ground points as components and cable terminals exist. The interesting question then is, how to group these many ground points together, or better, how to connect these individual ground points together and where.

Let's have a look at three simple examples:


The picture above shows, that it's distinguished between two grounds, an analog one and a digital one. Each ground should be made of a solid ground plane. Both grounds are joined together at only one single point, namely directly at Vss pin of MCP3201.

In the second example




and




you can see, how two ground star points are choosen, one at the micro board and one at the driver board. (Take care, there are two different akkus in this application!) All the cables running from the micro board to driver board are filtered, with the according caps referenced to the central ground star point at the micro board. This keeps the digital noise minimal across the connection between both ground star points. Any digital ground return currents flowing back from driver board to micro board are current limited by the help of 10k resistors at the outputs of 4025 and at the bases of BC550B.
Minimizing ground noise between several local ground star points, like in the example above is always essential! It's always of benefit to keep digital ground noise as low as possible. For this have a look what currents are flowing there, and how they can be minimized. If resistors cannot be used, then soft ferrite chokes (common mode types preferred) shoud be inserted.

The last example shall focuse on video drivers and recievers. Here the concept of one common and central ground star point will not work, especially if we have a look how the cables shields are treated. They MUST be conncted to signal ground at both ends, even if they are lumped together elsewhere in the application! The reason is, that only the cable shield is the correct ground connection between video drivers and video recievers. Any other ground connection instead would violate the signal transmission physics in the video cable, where electromagnetic waves are running in a transmission line.
If troubles occur due to multi grounding (ground loops), then this must be solved by different techniques, like insertion of video transformers or the use of symmetrical signal routing.

Kai