The very small inflection upward in the middle of the waveform is the location of the plane split. The blue waveform is the signal crossing the plane split. The return current has an AC path around the plane cut and is not visible to the signal.įigure 3 is a TDR waveform showing that there is no significant degradation as a result of crossing the split. This effectively “shorts” the power planes to the underlying plane at all frequencies of interest. When power delivery systems are engineered correctly, a combination of the discrete capacitors and plane capacitance will be used, which results in the impedance at or below 10 milliohms between Vdd and ground from DC to a gigahertz or more. The 1 nF and 10 nF capacitors commonly used for discrete decoupling capacitors, even one at a time, produce a relatively low impedance. Note: In the upper left-hand corner of this diagram is a table showing the capacitive reactance of three different-size capacitors as a function of frequency. Side View of Trace Crossing a Split Plane in PCB with Arrows to Show Current Both the outbound and return currents are shown with arrows.įigure 2. Test PCB With Traces Crossing Plane Splitsįigure 2 is the cross-section of the split power plane underneath a trace. To illustrate the foregoing, Figure 1 is a test PCB with traces in the buried microstrip layer (layer 2) that cross the plane in layer 3.įigure 1. Also, it should be noted that this gap never needs to be wider than 10 mils (0.254mm). The low impedance of each of the Vdds, between each Vdd and the ground plane, is the AC path across the gap. First, you have to be sure that the impedance of the Vdds that are on opposite sides of the split is very low (milliohms), so that the power delivery integrity is good for all voltages. In truth, modern electronics would not exist without this capability. There is only one reason ever for splitting a plane, and that is done in a power plane when you have two or more Vdds in the same plane. Distribution of Two Power Supply Design Plane Voltages in the Same PCB Layer Those reasons and the ways to implement them are detailed below. So, once we have eliminated all the bad data which continues to circulate about ground splitting, the discussion moves to power planes, and there are legitimate reasons for splitting them. In actuality, every time I have seen someone cut a ground plane they have created an EMI problem.” In almost every case I have seen, someone is assuming some sort of magic problem exists because the currents are circulating in the ground plane. The idea here is that you turn a patch of the ground plane into a little island and attach it to one place. Lee Ritchey, Founder, and President of Speeding Edge notes, “There are self-proclaimed EMI gurus who advocate the cutting of the ground plane because there is a current circulating in the ground plane that will upset some analog signal somewhere. If you do, you will destroy the integrity of your PDS. But, perhaps one of the most important takeaways is that you should NEVER, EVER split ground planes. To make things easier, we can readily debunk all of the foregoing and say they are not true. “Crossing a split power plane should never be done because of the increased risk of crosstalk and not meeting EMC requirements.” “If you isolate your analog sections, you need split planes.” “Position your board into separate analog and digital sections.” “You split ground planes to reduce common mode noise on its analog side “ “Crossing a split plane with a trace is bad because it increases inductance and complicates the path for the return current.” The higher the switching rate, the worse the effects will be.” They include:Īny signal that crosses in the split ground plane or power plane is undesirable. It should be noted that the “anti-split” warnings are somewhat randomized in terms of where they should be placed, why they should be done, and what harm they can do. Following are some of the more frequently made comments, which serve to confuse the topic as a whole, and do a disservice to product developers. Truths, rumors and misconceptionsĪs noted above, splitting planes or making plane cuts is one of those topic areas that is plagued with a lot of misinformation and confusion. This article will debunk the myths surrounding split planes, provide evidence as to when they are useful, and describe when they should not be made. Some say it’s a good thing to split power planes others say you can split ground planes and power planes both, some say you make cuts only in power planes, and others say to avoid plane cuts altogether. Splitting planes or making plane cuts is another one of those technical issues wherein there is a lot of conflicting information.
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