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DP83TG721S-Q1: Required Ferrite Beads

Katharina Ehmke
Intellectual 970 points

Part Number: DP83TG721S-Q1

Hi team,

In the data sheet of the DP83TG721S Q1, different ferrite beads are recommended for the three supply rails VDDIO, VDDA and VDD1P1. Questions:
1.Can you please explain  what criteria are used to select these ferrite beads?
A) Which parameters of the ferrite beads are decisive?
B) For example, at which frequency must the impedance be ensured?
2.What risk do we see in this case if the supply rails VDDIO, VDDA and VDD1P1 are not filtered via a separate ferrite bead?

 

Appreciate your feedback!

Katharina

  • 0
    TI__Mastermind 42038 points

    Hi Katharina,

    1

    A) The Impedance at 100 MHz, DC resistance, and rated current are all parameters to consider when selecting a ferrite bead.

    B) Typically, ferrite bead impedance is specified at 100 MHz.

    2

    There is a risk of negatively impacting EMC performance if separate ferrite beads are not used.

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    Prodigy 30 points

    1) Why is 100MHz the decisive frequency for the Ethernet transceiver DP83TG721S Q1? It is the 4th overtone of the quartz frequency. Why is the fundamental frequency of 25MHz not decisive in that case? Why are there two different ferrite beads chosen? The given answer does not explain all these questions
    2) Does it mean in case EMC performance can be ensured without ferrite beads that TI does not see a necessity to implement the ferrite beads?