What are the special cases when it might be a good idea to split the GND plane?

Question

Splitting the GND plane is a terrible idea for nearly any circuit, but there are exceptions. Let me come up with one example I think that is an exception, a motor controller.

In a motor control application there are huge currents with high slew rates and analog and digital signals are involved. From my experience it makes sense to split the GND return currents from the motor controller and connect them somewhere to the GND plane where the analog and digital parts are embedded. The most sensitive part of such a motor controller is the current measurement, where the noise needs to be suppressed as good as possible. What I have done in many inverter designs is something like this (here the shunts are in the motor phases):

As long as no signal goes over a split in the GND plane (and also in the power plane), there should be no problem. I never could really compare a design with and without a GND split for exactly the same setup, that would be really interesting.

Theoretically, the HF currents will not flow into the ADC section also when there is a solid GND plane (including the motor control return currents), but the LF currents might. I'm unsure about the influence of the LF currents.

So what is your opinion about the GND split in a motor control application?

Answer 1

Splitting the GND plane is a terrible idea for nearly any circuit

I would say that splitting the ground plane is a terrible idea for some circuits.

There are two things at play here that dictate how you "wire-up" components on a circuit board.

• The first is the need (for performance reasons) to "star-point" certain power and ground connections.
• The second is the need (for EMC reasons) to provide full ground planes

A mixture of the two is usually the most beneficial solution in nearly all cases. There are some exceptions; RF circuits with convoluted ground planes may form a resonant antenna on a section of the convoluted ground plane.

So what is your opinion about the GND split in a motor control application?

Asking for opinions are a definite reason to close a question on this site so, I shan't give one other than to say that a convoluted ground plane is an extension of star-pointing techniques and this is a perfectly valid approach but, watch out in case anything turns into an antenna.