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Five Things Every Engineer Should Know About PDN

Keep it flat

Most PDN books tell you that the best performing PDN impedance looks flat over frequency.  That is because noise signals are generated as a result of discontinuities or impedance peaks in the PDN.  The PDN is comprised of resistance, inductance and capacitance associated with the PCB traces and planes, the decoupling capacitors and their parasitics and the package parasitics including the bond wires and die capacitance of the high speed devices.  Minimizing the Q of these resonant circuits is the key to obtaining a flat impedance.  One of the fundamental PDN management tools is target impedance. This is the impedance below which all peaks should be maintained for good performance.  The target impedance concept may have significant flaws [1], but it is still the most common PDN design technique in use today.  However, the more basic goal should be to maintain the impedance as flat as possible up to a bandwidth that is dependent on the edge speed of the load signals, the amplitude of the dynamic current change and the impedance of the PDN.  Note that this is the edge speed and not the pulse repetition frequency dependent.

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