AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.1.5.6.2. Capacitive Discontinuity

This section discusses the effects of capacitive discontinuity, which usually occurs when components are introduced on the transmission path.

The two connector plots in Figure 56 show capacitive loads, one acting as a lower capacitive discontinuity and the other as higher capacitive discontinuity. The capacitance (C) for the load can be calculated with the equation:

A 3.125-Gbps signal (a pseudo random binary sequence (PRBS) pattern) is sent through the first connector that looks like a lower-capacitive connector (1.2 pF); the eye opening and jitter are observed on the other end.

Figure 56. Lower- and Higher-Capacitive Load Connectors Illustrating the Effects of Capacitive Discontinuity

Figure 57 shows the eye opening with the connector that induces a discontinuity of 1.2 pF. The eye opening is a 330-mV differential. The expanded view of the eye shows the peak-to-peak jitter as approximately 27 ps.

Figure 57. Lower-Capacitance Connector Eye Opening and Expanded View of the Eye Opening

Figure 57 shows a 3.125-Gbps PRBS pattern sent through the second connector that looks like a higher-capacitive connector; the eye opening and jitter are observed on the other end.

Figure 58 shows the eye opening for the same signal passing through an SMA connector with a capacitance of 2.9 pF. The eye opening is approximately 280 mV, differential. The expanded view of the eye opening shows that the peak-to-peak jitter is 43 ps.

Figure 58. Higher-Capacitance Connector Eye Opening and Expanded View of the Eye Opening
You should avoid adding connectors and components on the transmission path whenever possible. However, if connectors are required, select ones that creates the least amount of inductive and/or capacitive discontinuity on the transmission path. The jitter and amplitude impact on a 3.125-Gbps signal when transmitting through a 2.9 and 1.2 pF capacitor is very significant. The eye opening shows an amplitude difference of 50 mV, and the expanded view shows a peak-to-peak jitter difference of 16 ps.
  • ρ = RC = (Z0C/2)

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