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LMH0302: LMH0302 Input Cap and Output Filter

Chuchen Wang
Expert 3440 points
Part Number: LMH0302

HI,

My customer has the following questions:

The input capacitors on the LMH0302SQE/NOPB device and output filtering is different than the data sheet recommendations.

The ac coupling input capacitors are 0.47 uF rather than the suggested 0.1 uF.
In addition, the output inductors are 5.6 nH rather than the suggested 6.8 nH.

The circuit that we're working with does operate as expected. This design has also been through extensive qualification testing (temp, vibe, voltage input, etc) without any issues.

1) Do you recommend we maintain these values or changing them to the suggested ones from the data sheet?
2) What type of impact would these different values have on the signal quality?

Thanks,

Chuchen

  • 0
    TI__Guru* 88905 points

    Hi Chuchen,

    1). Please hold on to the values that you've been using.

    2a). I believe you are referring to power supply coupling capacitor. Based on your power supply noise, the lower value capacitance attenuates higher frequency noise. Given the fact this has been working for you, it seems you may have high noise on your power supply. So using 0.47uF instead of 0.1uF should be fine.

    2b). 6.8nH provides more margin on return loss measurement and this is pcb layout dependent. Again, if 5.6nH has worked for you, I would continue using it.

    Regards,,nasser 

  • 0 in reply to Nasser Mohammadi
    TI__Expert 3440 points

    Hi Nasser,

    There was a typo in the original request - can you confirm if 4.7uF in series with SDI/SDIn are acceptable as opposed to the DS suggested 1uF?

    Thanks,

    Chuchen

  • 0 in reply to Chuchen Wang
    TI__Guru* 88905 points

    Hi Chuchen,

    Using 4.7uF instead of 1.0uF on SDI/SDIn is fine and I don't expect a negative side effect. Actually 4.7uF could be a better choice since it provides a lower DC wander than 1.0uF. When we productized LMH0302 device there was not a 0402 4.7uF capacitor. Device was characterized using 0402 1.0uF capacitor and 1.0uF AC coupling capacitor is fine as well.

    Regards,,nasser