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LMH5401: Differential drive vs single-ended drive

Nicolas Schlumpf
Intellectual 320 points
Part Number: LMH5401
Other Parts Discussed in Thread: LMH3401

Hello,

I am wondering if a 50Ohm 1:1 transmission balun in front of the LMH5401 would improve its distorsion performance? 

In that case the LMH5401 is driven differentially with an equivalent 25 Ohm for each single-ended source.

Almost all the datasheet covers the single ended to differential configuration. 

Any input concerning the benefit of the LMH5401 differential drive would be appreciated.

Thanks,

Nicolas

  • 0
    Guru 48395 points

    There can be an improvement in HD2 going that direction, but then you lose your DC coupling. what is your frequency span of interest? 

  • 0 in reply to Michael Steffes
    Intellectual 320 points

    Hi Michael,

    Thank you for your feedback. 

    Assuming I don't need the DC component, what would be the HD2 improvement?

    Thanks

    N.

  • 0 in reply to Nicolas Schlumpf
    Guru 48395 points

    Hard to say without bench data and balun characteristics. I had done quite a lot of work on this topic some years back in the context of the ISL55210 where I was only taking balun input HD data up through about 100Mhz. For HD2 everything matters, board layout, supply decoupling etc etc etc. Doesn't simulate well so you need bench data to ascertain. Back in those days, one of the BAE sr. scientists suggested I try a 2nd common mode choke type balun after my input step up, and indeed that did buy a few more dB of HD2 suppression, never did come up with plausible (for public consumption) rationale for that. 

  • 0 in reply to Michael Steffes
    Intellectual 320 points

    Ok I understand that without measurement it is difficult to precisely predict what the improvement could be.

    The AC differential simulation shows that each SE 25 Ohm input of the LMH5401 is only valid at low frequency. The vswr increases with frequency and that is not good for the balun response. 

    Roughly, would you say the HD2 improvement counts in dB or or more than 10dB?

    I guess, the LMH5401 spice model won't show any true HD behavior. What do you think?

    Thanks

  • 0 in reply to Nicolas Schlumpf
    Guru 48395 points

    The public models do not show HD very well, 

    I did vaguely recall doing this work for the LMH3401 or LMH5401 comparing IMD3 and IMD2 for active match single to diff input vs balun input, I found the attached file but I know this later became a full presentation I did inside a few mil companies, Have not found that yet, but yes the balun input was good for about 5 to 8dB better IMD2 and a little better IMD3 - about 3dB. But there are many other variables here. 

    Comparison of LMH3401 for active balun vs input balun for IM.docx