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Original question:

LMX2491: LMX2491 use questions

LMX2491: LMX2491 use questions

Doug Padrick
Intellectual 350 points
Part Number: LMX2491
Other Parts Discussed in Thread: LMX2492

The application schematics do not show an external 50 ohm termination to ground on the Fin port.

That is, on the side away from the Fin port on the other side of the DC blocking cap.

Is this port low impedance (~50 ohms) by design, or is it ~high impedance, at least at lower frequencies?

  • 0
    TI__Guru** 107780 points
    Hi Doug,

    Are you referring to Figure 14 in the datasheet?
    There is a power splitter on the left hand side of the 10pF DC-blocking cap at Fin pin. The equivalent impedance of the splitter is approx 50ohms.
    The input impedance of Fin or Fin* is not 50ohms, so an external 50ohms to ground on each pin is recommended for optimum performance.
  • 0 in reply to Noel Fung
    Intellectual 350 points

    Hi Noel,

    Fig 14 is a good example. although the resistive splitter as shown is a little odd.

    Normally a 2 way splitter uses 3x ~18 ohm resistors. Fig 14 shows one 68 ohm (18+50?) resistor to ground, along with the 18 ohms to the "to circuit" line.

    What is the purpose of the series 36 ohms into the Fin port? There is no termination or match shown on the Fin port.

    Note too that the resistor splitter scheme shown on page 1 at the bottom is a little different than fig 14.

    The schematic for the LMX2492 eval board agrees with the page 1 scheme, which essentially uses a ~6 dB tee pad but still no termination to the Fin port.

    You suggest using 50 ohms to ground on the Fin and Fin* ports which is pretty common, but none is shown on the Fin port.

    Can you supply an equivalent circuit for the Fin port? Thank you.

  • 0 in reply to Doug Padrick
    TI__Mastermind 28001 points

    Doug,

    The impedance of the Fin pin (at least at lower frequencies) is high impedance, but at higher frequencies, it can be lower, even compared to 50 ohms.  Therefore, it is best to use a resistive pad to ensure better matching that is not so dependent on the impedance of the Fin pin.  

    I have no models for the Fin port.

    Regards,

    Dean