XrossTalk Magazine

Monday afternoon I attended the Tec Panel titled “Power Distribution Planes: To Split Or Not To Split” which talked about splitting the power plane in a PCB design to accomodate different power supplies. This is very common in the industry because you can reduce cost by using less layers in your design if you put multiple power supplies together on one plane.

The panel was going to be chaired by Istvan Novak but unfortunately he had to have minor surgery and couldn’t make it (get well Istvan!) so Bruce Archambeault took over as the chair and did a good job.

So the panel tried to address the issue of what happens when you do have a split power plane and than need to route signals over the plane. Of course everyone agreed to try and avoid this if possible but sometimes you can’t avoid it.

Here are some of the notes I jotted down from the various panelists:

• You need to try and quantify the EMI and SI impact of routing over a power split. You can do this by looking at the transfer impedance.
• Adding a solid plane below the split power plane can help from a little to a lot
• Another knob you can turn is trying to control the current di/dt to reduce the impact of routing over a power split
• Need to think about the return current and where it is going. You can measure/estimate the return current in the top plane to get an idea of the impact
• Using differential signals helps alleviate SI issues with routing over split planes
• Voltage islands will couple together with varying intensity based upon resonant frequencies
• Can change spacing of voltage island separation to reduce impact
• EBG structures can be used to create narrow band bandgap isolation

I think Prof. Swaminathan had an interesting presentation describing EBG structures and how they can be used to help create isolation at certain frequency ranges which can be useful for PCB designs with sensitive analog components like ADC’s that also have digital components.