Issue link: https://iconnect007.uberflip.com/i/1481368
60 DESIGN007 MAGAZINE I OCTOBER 2022 ing the proper termination resistance to both ends. Since planes do not have well-defined "ends," as traces do, we need to connect ter- mination components along their periphery. Power-plane pairs, except for a few special shapes, do not have a specific characteristic impedance and therefore we need to rely on approximations, such as this approximation of a rectangular plane pair with X and Y horizon- tal dimensions: Where Z p is the approximate characteristic impedance of the plane pair in ohms e r is the relative dielectric constant of the laminate h and P are the laminate thickness and periphery in arbitrary, but identical units With typical plane sizes and laminate thick- ness values we use today, the impedance comes out in the tens to hundreds of milliohms range. We need to match this impedance with a number of termination elements, placed around the plane periphery. e number of elements depends on our fre- quency of interest. We need to make sure that up to the highest fre- quency of interest, oen chosen as the tenth har- monic in the modal reso- nance series, the phase difference between adja- cent termination com- ponents is much less than 90 degrees. As a result, we typically end up with a centimeter or so spacing. We then take the P periphery of plane shape and divide by the spacing between adjacent elements and it gives us the N number of terminations. Each termination resistor has to have an N*Z p value; many times it comes out as a few ohms. We also add a small series capacitor in series to each termination resistor to avoid shorting the power-ground plane pair with the termination resistance. Terminating power planes was an attrac- tive and viable solution a couple of decades ago when computer systems still had a lot of single-ended signaling and fewer supply rails with larger planes. In these days it still could be a viable alternative if system constraints pre- vent us from placing bypass capacitors to their optimum location. An example of plane termi- nation on a recent computer board in volume production was described in a paper presented at DesignCon in 2021 8 . In this case, the very high density of memory sockets ruled out the placement of bypass capacitors next to the power pins of memory sockets. e simulated and measured impedance of that supply rail is reproduced in Figure 5. Note the logarithmic Figure 5: Simulated and measured power rail impedance on a production board with and without resistive edge termination 8 .