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LMK04828BEVM: Why ferrite bead in ground?

Dave Lewis93983
Prodigy 100 points
Part Number: LMK04828BEVM

The VCXO (U2 CVHD-950-122.88) uses a local ground (GND_VCXO) which is connected to the board ground through a ferrite bead (R374).

This is very unusual. Why a ferrite bead in ground?

What I am seeing on the support blogs for other chips ….

 

“The ferrite beads on PLL GND/supply was recommended by the PLL design team (the folks from this design team are no longer at TI, so no further follow up or clarification is possible)”

 

“The PLL was characterized with 50mV noise on the supply” with a mention of preventing RF ground loops  supports perhaps the ferrite beads may be used on the evaluation boards to isolate the VCXO from this inserted noise for  PLL test"

 

Thanks

Dave

  • 0
    TI__Mastermind 28560 points

    Hello Dave,

    The purpose for the ferrite bead in ground was to attempt to isolate noise caused by the CMOS output switching from the rest of the PCB.  Granted the return current would have to pass through this ground ferrite bead... although in typical CMOS application the load is high impedance capacitive.  This strategy may be more effective with a differential VCXO.

    73,
    Timothy