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LMX2581EVM decoupling VCCBUF

cosmo little
Prodigy 170 points

Dear Dean,

Another small point about the evaluation board- the decoupling of pin 16 (VCCBUF). This pin has an inductor or ferrite bead in series, but without a decoupling capacitor from the pin to ground. This is quite unusual, and I am asking if there was a specific reason for this arrangement. Pin 16 will see a high impedance due to the ferrite bead. A standard method of decoupling is the PI network, which in this case will have a capacitor from the pin to ground. 

regards

Cosmo Little

  • 0
    TI__Mastermind 27581 points

    Cosmo,

    With the supply decoupling, there are two considerations.  One is that you want to filter noise coming to a pin.  However, if there is a capacitor to ground, it is possible that the pin can generate spur energy that corrupts the ground plaine, or a ground plane corrupted with spur energy can couple noise up through this pin.  In the case of this pin, we found that putting a ferrite bead as opposed to a 0 ohm resistor imprvoved the phase detector spur about 4 dB, dependng on conditions.  It had no impact on the integer boundary spur.

    Regads,
    Dean