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LMH5401: Layout techniques for sampling the output for gain control

Robert Abel1
Prodigy 80 points
Part Number: LMH5401
Other Parts Discussed in Thread: LMH6559

I want to use the + output of the LMH5401 as feedback to an AGC circuit. It will buffered by the LMH6559 for subsequent processing. The application frequency band is 1MHz to 500 MHz; +/-1.0V Peak max; VCM = 0V;+/-2.5VDC sup.. I am concerned about the imbalance in the differential output signal paths (impedance mismatch) caused by the single ended routing of the LMH5401+ output to the LMH6559 input. The application of the LMH5401 is a differential line driver for an Ethernet transformer (100 ohm differential), Ideally, I would like the Independences to be equal to match ubiquitous recommendations for differential lines, but I am not sure a phantom trace to match the feedback trace is necessary or desired. Are my concerns valid at these frequencies? Any suggestions?

  • 0
    TI__Genius 15555 points

    Hello Robert,

    My understanding from your description is that shown in the schematic below:

    Is this correct? If so, I would recommend trying to keep similar traces similar and adding a capacitance at OUTN equivalent to the input capacitance of the LMH6559. It is also worth noting the the noise and linearity at the buffered OUTP signal will not be equivalent to the specs shown in the datasheet of the LMH6559. This is because you are also seeing your common-mode noise and distortion at the single-ended output.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 80 points

    Yes, that is correct. What do you mean by keep similar traces similar. One trace goes to the buffer and one does not. How do I route them similar?

    Thanks..

  • 0 in reply to Robert Abel1
    TI__Genius 15555 points

    Hey Robert,

    In terms of lengths/thickness etc. This was in regard to your comment on adding a phantom trace.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 80 points

    So you are saying I should add a phantom trace so they are similar in geometry even though the inverting output trace would be terminated with only an RC and not the buffer? Two differential lines going to the transformer, and two parallel lines going to the buffer and an equal length phantom terminated with the RC equivalent of the buffer? Sorry if I'm a little repetitive, but I just want to make sure we are on the same page. I will send a sketch of a board layout tomorrow. Thanks..

  • 0 in reply to Robert Abel1
    TI__Genius 15555 points

    Hey Robert,

    To clarify, I recommend using a similar geometry at both outputs. In this case it would be a capacitor added on the OUT- line at the equivalent position to where the buffer at OUT+ is placed.

    Best,

    Hasan Babiker

  • 0
    Guru 140200 points

    Hi Robert,

    I wouldn't do that. The LMH5401 is not designed to do see uneven loads at the outputs. If you need an unsymmtrical output signal, use the circuit shown in figure 46.

    Kai