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JUNE 2021 I DESIGN007 MAGAZINE 49 reflections. e most important parameter of the AC coupling capacitor is its relative geom- etry with respect to the substrate. e capaci- tors are placed in series with high-speed traces and, as such, the capacitor body becomes a section of the transmission line. ere are two approaches to placement and routing of the AC coupling capacitors: 1. To eliminate the excess parasitic capaci- tance associated with surface mount lands, a portion of the reference plane that is directly beneath the component can be removed. is allows the signal that tra- verses through the capacitor to reference a lower plane (further away) and reduces the parasitic capacitance, thereby minimizing the impedance mismatch (Figure 2). 2. A more elegant alternative is to use a smaller capacitor such as a 01005. e explicit assumption (above) is that capacitor land width exceeds the trace width and hence represents a short sec- tion of transmission line that requires a cutout to maintain impedance. However, a 200 µm trace can be used in conjunction with a 01005 package of 200 mm land width to avoid the impedance mismatch altogether. In this case, the cutout is not required (Figure 3). Note that the required wider trace will reduce the impedance, so a thicker dielectric will be necessary to control impedance. ere are two compelling arguments for the placement of AC coupling capacitors. e first Figure 2: Plane cut-outs. Figure 3: 200 µm trace with 01005 capacitors.