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LMX2492 Fin* Pin

Alber, Moon
Intellectual 720 points
Other Parts Discussed in Thread: LMX2492

Hello

I'm Alber as a FAE in Arrow Korea.

My customer asked about Fin* Pin of LMX2492.

They can't add a capacitor(10pF) and resister(51ohm) at Fin* pin due to PCB Size.

If they do not add C and R at Fin* Pin, is there any problem in performance?

If possible, they will add only a C(10pF) and will not add R(51 ohm) due to PCB Size.

  • 0
    TI__Expert 4085 points
    Hi Alber,

    For optimal performance Fin impedance must be matched with Fin* as shown on page 34 of the datasheet (layout guidelines and layout example).

    Thanks,
    Julian
  • 0
    TI__Mastermind 28001 points
    Alber,

    In general, Julian is right.

    The series resistor is not critical for operation, but is for better spurs and phase noise performance. Specifically, on these differential inputs, you want the load as seen looking out from both of the pins to be the same. We found that the 51 ohms on the Fin* pin helped the integer boundary spur a few dB. But we found that without this, we could sometimes get up to a 5 dB degradation in the phase noise for some frequencies. If we put our finger on the board, the phase noise would improve back to normal. So this 51 ohms made things much more stable.

    The results you see will vary drastically on what matching network you choose at the Fin pin and I'm sure what we saw was very related to our board, but just be cautious of this.

    Regards,
    Dean