Issue link: https://iconnect007.uberflip.com/i/1093246
62 DESIGN007 MAGAZINE I MARCH 2019 It is important to have a clearly defined re- turn current path and to know exactly where the return current will flow. This is particular- ly critical with asymmetric stripline configura- tions where one or two signal layers are sand- wiched non-uniformly between two planes. The question is not which plane does the re- turn current flow on but rather how is the cur- rent distributed on each plane? Also, if a return path discontinuity (RPD) exists, then the cur- rent tends to divert increasing the loop area, inductance, and delay. A via that provides the connection between signal traces referenced to planes of different DC potential creates RPDs. In other words, the return current has to jump between the planes to close the current loop, which increases the inductance and affects the signal quality. This return current can also excite the parallel plate resonance mode, causing significant electro - magnetic radiation from the fringing fields. If the reference planes are at the same DC potential, then they can be directly connect- ed by stitching vias near the signal via tran- sition to provide shorter paths for the return current. However, if the planes are at differ- ent DC potential, then decoupling capacitors must be connected across the planes at these points. Unfortunately, this can pass AC noise between power supplies. Two decoupling ca- pacitors configured as the right example in Fig- ure 4 is a much better solution because it elim- inates the transfer of power supply noise from one supply to another. Although this does add a little loop area, it also provides additional de- coupling to the planes, reducing power distri- bution network impedance. In addition, some of the return current flows through the inter- plane capacitance to close the loop. Equations The distribution of current for the three basic configurations depicted in Figure 5 is: (a) The distribution of current J(D) on a sol- id microstrip plane is given by: h = height of trace above/below the plane (mil)* d = horizontal distance away from the center of trace (mil)* * um or mm can be substituted for mil pro- viding the same units are used throughout Figure 3: Return path current density for dual asymmetric stripline. Figure 4: Eliminating the transfer of noise in the return path of split power planes (right).