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Original question:

DP83848I: Hardware schematic design problems

LMH5401: Bandwidth difference compared with datasheet

Xenon Li
Intellectual 1505 points
Part Number: LMH5401

Dear team,

Hope you are doing well!

Recently, one of our customers is using LMH5401 in their products but find some issue that need your support.

The datasheet mentioned that the LSBW is 4.4GHz (Typical), shown like below:

However, the customer's test result is about 3GHz, following the reference design.

The figure below is the product's schematic:

Could you please help me to analyze the reason please?

Thanks a lot!

Best regards,

Xenon Li

  • 0
    Guru 140200 points

    Hi Xenon,

    in this frequency range it is very difficult to perform an accurate frequency response measurement. Even the least deviation from the ideal setup can cause a considerable degradation of bandwidth.

    In the simulation below I have merely assumed 3pF stray capacitances at the output of circuit and a 7nH inductance in series to the 100nF AC coupling cap. 7nH is the sum of 2nH equivalent series inductance of cap and a 5mm long copper trace from the 50R socket to the 100nF cap which -as a rule of thumb- equals about 1nH/mm:

    xenon_lmh5401.TSC

    Additional errors can be caused by non-ideal cables, T-boxes, the signal generator, the connected ADC and the printed circuit board layout itself. Because of this it is strongly recommended to exactly follow the layout example of datasheet.

    Kai

  • 0 in reply to kai klaas69
    Guru 47485 points

    The datasheet was done with a 4 port network analyzer with its auto calibration fixture. Yes, these are very difficult measurements to exactly duplicate and there is going to be some part to part variation. 

  • 0
    TI__Intellectual 2310 points

    Hi Xenon,

           At 3GHz operation, board parasitics are critical. Make sure the layout recommendation in the datasheet is followed. 

    Regards,

    Hruday

  • 0 in reply to kai klaas69
    TI__Intellectual 1505 points

    Dear Kai,

    Thanks a lot for your support!

    By the way, can I say that the 4.4GHz bandwidth is under ideal model and in reality, it is hard to achieve such accurate bandwidth?

    Best regards,

    Xenon Li

  • 0 in reply to Hruday Kumar Mudireddy
    TI__Intellectual 1505 points

    Dear Hruday,

    Thanks a lot for your reply!

    Best regards,

    Xenon Li

  • 0 in reply to Xenon Li
    Guru 140200 points

    Hi Xenon,

    I am convinced that you can get the 4.4GHz bandwidth specified in the datasheet even in reality. But you need an optimum layout and optimum measuring conditions.

    Due to unwanted layout parasitics which may result in an unwanted decompensation of OPAmp, sometimes even a higher bandwidth can be observed than being specified in the datasheet.

    Kai