Ground Plane Net Tie - Question about current design

Question

I am doing net-tie method to separate two different Ground Planes (for noise mitigation purpose), one ground plane is for RF circuitry and MCU, the other ground plane is for power and motor switching. I wonder if this is a good way of laying out net-tie

Here is the preview of the circuit, the net tie is laid at the top right hand corner, far away from power source. It stay on the top layer and I use via to tie to the ground planes that are mostly at the bottom layer.

Here is the closer look at the net

Please let me know if this method works.

Thanks.

Answer 1

Yes that works if all you are doing is tieing the ground planes together. You are also creating an inductor with about 0.1nH and 7mΩ for the copper trace and roughly 0.3nH and 0.1mΩ for the vias on one side (for only three vias).

What doesn't make sens to me is why the vias are paralleled and then the trace is necked down to what I'd estimate somewhere in the range of 10 to 14mils. The resistance of the trace is much higher than the paralleled vias.

The other thing is your creating a long path for any return currents to get back to the processor by placing the net tie so far away from the other traces. The ground planes are separated with inductance and it doesn't look like you have a lot traces that crossover. If any of these are high speed (more than 20Mhz) the net tie will create problems.

Separating the ground planes with a net tie such as this creates a dipole antenna, which could create problems if you need to pass the limits of an FCC unintentional radiator (or equivalent) test.

If adding additional inductance and resistance between ground planes is what you intended then it looks good.

One recommended strategy (by Henery Ott) is to place zero ohm stitching resistors (maybe about every 2-3cm along the ground plane separation and see if removing them helps or hurts the situation, usually separated ground planes creates more EMI EMC problems then separating them solves. If your seeing your ground plane bounce from a high load, this can usually be solved by directing the return currents than separating ground planes. If you are moving power from one side of the ground plane to another, the net tie will create common mode noise problems, and you need a better way to stitch the grounds together.

Answer 2

The construction of tie doesn't matter much. What does matter is the location of this bridge. Separate grounds should be tied at the main power supply capacitor, as schematically illustrated below:

You need to determine where this point is in your design, but it doesn't look like it is at the corner where you placed the bridge.