AN 958: Board Design Guidelines

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ID 683073
Date 1/28/2022
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5.1.5.4.4. Series-RC Parallel Termination

A series-RC parallel termination scheme uses a resistor and capacitor (i.e., series-RC) network as the terminating impedance. The terminating resistor (RT) is equal to Z0. The capacitor must be large enough to filter the constant flow of DC current. However, if the capacitor is too large, it delays the signal beyond the design threshold.

Capacitors smaller than 100 pF diminish the effectiveness of termination. The capacitor blocks low-frequency signals while passing high-frequency signals. Therefore, the DC loading effect of RT does not have an impact on the driver, as there is no DC path to ground. The series-RC termination scheme requires balanced DC signaling (i.e., the signals spend half the time on and half the time off). AC termination is typically used if there is more than one load. Refer to Figure 35.

Figure 35. Series-RC Parallel Termination

Figure 36 shows series-RC parallel fly-by termination.

Figure 36. Series-RC Parallel Fly-By Termination

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