D.2.3. Rules for series terminators

Series (source) termination is the most commonly used method. The basic rules are:

  1. The series resistor must be placed as close as possible to the ASIC pin (less than 0.5 inches)

  2. The value of the resistor must equal the impedance of the track minus the output impedance of the output driver. So for example, a 50Ω PCB track driven by an output with a 17Ω impedance, requires a resistor value of 33Ω.

    Note

    It is recommended that the overall source impedance be as close as possible to 50Ω.

  3. A source terminated signal is only valid at the end of the signal path. At any point between the source and the end of the track, the signal appears distorted because of reflections. Any device connected between the source and the end of the signal path therefore sees the distorted signal and might not operate correctly. Care must be taken not to connect devices in this way, unless the distortion does not affect device operation.

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