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58 DESIGN007 MAGAZINE I JUNE 2020 skin effect forces current to flow in the outer pe- riphery of the plane at high frequencies, it vir- tually creates two independent ground planes on the top and bottom surfaces with no return path crosstalk, although it is actually one physi- cal sheet of copper. However, this methodology is also frequen- cy dependant and determined by geometry. As the frequency increases, more power is inject- ed into the plane cavity propagating along the slot. Above 10 GHz, the slot mode radiation begins to couple excessive electromagnetic en- ergy into the adjacent traces. Properly designed power distribution net- works have the power planes tightly coupled to their respective ground planes, as in Figure 5, to provide planar capacitance. Of course, none of this is valid if the two planes are far apart. Stackup data can be transferred to the iCD PDN Planner to demonstrate the impact of planar capacitance on plane resonance for dif- ferent configurations. If very thin dielectrics (2–3 mils) are used between the power and ground planes, split planes can have minimal impact on the qual- ity of signal transmission if designed correctly. If the split plane is covered with solid ground planes both above and below in a stripline configuration, radiation will not be an issue. And keep in mind that this also applies to dif- ferential pairs traversing a split plane. The re- turn current flows in the reference planes(s)— not the opposite signal trace, particularly if the pair is not very tightly coupled. One would typically use a minimum gap of 20 mils to isolate planes of dissimilar poten- tial. However, with today's modern fabrica- tion etching techniques, a 10-mil gap is quite acceptable for low voltage applications (but please check with your fab shop first). Ground planes should never be split with the exception of audio frequency circuits, special RF/microwave applications, and for high-volt- age isolation. Functional partitioning and con- trolled routing is the key to a successful mixed- signal design, while utilizing planar capaci- tance is an excellent solution for high-speed digital designs up to 10 GHz. Key Points • A split in a ground plane creates an imped- ance discontinuity in the return signal path. • Low-frequency circuits in the audio range can often benefit from splitting the ground planes. • The key to a successful mixed analog/ digital design is functional partitioning, understanding the current return path, and routing control and management. • Route fences are a useful tool to control the routing. • The ground plane should not be split; rather, a pass-through gap is left in the fences so that data and control signals can enter and leave that area. • Dual-stripline layers can provide mixed- signal/power pours and reduce the layer count. Figure 5: Thin dielectric creates more planar capacitance (iCD Stackup Planner).