Take steps to reduce antiresonance in decoupling
Edited by Bill Travis
Dale Sanders, X2Y Attenuators, LLC, Farmington Hills, MI -- EDN, April 15, 2004
To maintain power integrity on pc boards, you need multiple capacitors to decouple the power-distribution system. A typical configuration might comprise five capacitors connected in parallel between the power and the ground traces or planes. To provide broadband decoupling performance, assume the individual values of the capacitors are 470, 1, 10, 100, and 220 nF (Figure 1). This parallel network provides 801-nF total capacitance to the power-distribution system. If you measure each capacitor with a vector-network analyzer, you can identify each capacitor's SRF (self-resonant frequency). Figure 2 is a plot of each capacitor's SRF, as well as the SRF of the overall parallel connection. Each SRF can cause antiresonance in the parallel decoupling configuration. The antiresonance occurs when one capacitor is still capacitive, while another has become inductive.
A way to considerably reduce the antiresonance effects is to use a single 400-nF X2Y capacitor for decoupling. (Capacitors using X2Y technology are available, for example, from Johanson Dielectrics (www.johansondielectrics.com). You measure the capacitance rating for an X2Y component from line to ground; in other words, from an A or a B terminal to either of the G1 or G2 terminals in Figure 3. So, the total capacitance a 400-nF X2Y component supplies, connected as in Figure 3 would be double the capacitance rating, or 800 nF. Figure 4 shows that a single X2Y capacitor with the same total capacitance as in Figure 1 provides the same broadband decoupling as the standard decoupling configuration but without the antiresonance effects. In addition, because X2Y components come in the same package sizes as standard capacitors (1812, 1210, 1206, 0805, and 0603), the use of X2Y components saves pc-board space and reduces layout complexity.
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