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Signal Integrity and High-Speed Interconnects January-May 2006 Return Paths and Power Supply Decoupling Dr. José Ernesto Rayas Sánchez 1 Outline Ideal return paths Microstrip return paths Stripline return paths Modeling metallic reference planes Power supply bypassing Ground bounce and SSN Dr. J.E. Rayas Sánchez 2 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] Signal Integrity and High-Speed Interconnects January-May 2006 Ideal Return Path Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 3 Return Path with a Discontinuity Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] 4 Signal Integrity and High-Speed Interconnects January-May 2006 Discontinuity in the Signal Path Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 5 Microstrip Return Path Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] 6 Signal Integrity and High-Speed Interconnects January-May 2006 Stripline Return Path (Off – Steady State) Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 7 Stripline Return Path (Turn On) Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] 8 Signal Integrity and High-Speed Interconnects January-May 2006 Stripline Return Path (Turn Off) Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 9 Stripline with Unrelated Power Plane (Turn On) Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 10 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] Signal Integrity and High-Speed Interconnects January-May 2006 Return Paths The return paths in microstrips are very different to those in striplines For striplines, the current distribution in the two reference metallic planes can be very different Usually, the return path is considered as an ideal short circuit. However, any parasitic inductance in the return paths can significantly affect the signal integrity in both the return paths (metallic planes) and the signal trace Dr. J.E. Rayas Sánchez 11 Modeling the Metallic Reference Planes Cplane w Lplane l Rplane R plane ≈ ρ εr metallic planes h C plane ≈ ε r ε o lw h L plane ≈ µ r µo hl w Dr. J.E. Rayas Sánchez l tw 12 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] Signal Integrity and High-Speed Interconnects January-May 2006 Using Decoupling Capacitors Ideally, the metallic reference planes should be pure short circuits at the signal frequency range The metallic reference planes essentially behave as capacitors Decoupling capacitors are used to lower the metallic planes impedance at the frequency of interest Dr. J.E. Rayas Sánchez 13 Using Decoupling Capacitors – Example Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 14 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] Signal Integrity and High-Speed Interconnects January-May 2006 Using Decoupling Capacitors – Example (cont) Dr. J.E. Rayas Sánchez 15 Power Supply Inductance Dr. J.E. Rayas Sánchez (S. C. Thierauf, High-Speed Circuit Board Signal Integrity, Artech, 2004) 16 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected] Signal Integrity and High-Speed Interconnects January-May 2006 Ground Bounce Ground bounce refers to a change in the reference voltage caused by switching currents in the return paths It is also called “Simultaneous Switching Noise (SSN)” or “Delta-I Noise” It is mainly produced by the parasitic inductance in the return path The problem gets worse in digital systems when many drivers are switching simultaneously Dr. J.E. Rayas Sánchez 17 Dr. J. E. Rayas Sánchez http://iteso.mx/~erayas [email protected]
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