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LMH6521 lowest possible frequency.

john greene
Intellectual 435 points
Other Parts Discussed in Thread: LMH6521, LMH6517

I am in need of a dual programmable gain fully differential amplifier for using on an I/Q demodulator to provide some additional dynamic range but I need the frequency response to go down to at least 1 kHz. Can the LMH6521 operate this low? The data sheet seems to imply it is intended for frequencies above 1 MHz. 

  • 0
    TI__Genius 13935 points
    Hello John,

    What is the upper frequency of your signal? The LMH6521 has a class A output stage. In normal operation it is biased with a 1uH inductor. This sets the lower limit on frequency response.

    If you use a larger inductor (10uH or 100uH) you can reduce the lower effective frequency. This will come at a cost of reduced high frequency operation. I have not tried it , but in theory using two (or more) series inductors would extend the frequency range. For example, if you used a 1uH inductor in series with a 100uH inductor you should be able to get more bandwidth than with a single inductor. What you are looking for is a large value inductor with a very high self resonant frequency. Typically a 100uH inductor would have so much inter-winding capacitance that it will short out at high frequency.


    For DC coupled operation I have written an app note/design guide. This is another option, but it's more difficult.


    www.ti.com/.../tidu585.pdf

    Does this answer your question?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Intellectual 435 points
    I think your application note is just what I needed. This is basically a zero IF receiver. What is nice is the common mode voltage of both the I/Q demod and the A/D is 2.5V. The upper end of the frequency range at this point is only 100kHz. We are not sure yet how many of the harmonics of the modulating signal we need to capture to get enough resolution for what we are doing.
    Thank you so much for the timely response!
  • 0 in reply to john greene
    TI__Genius 13935 points

    Hi John,

    I would recommend the LMH6517.  It has a 2.5V input and output common mode voltage and is inherently DC coupled, so you won't need the external bias resistors. 

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Intellectual 435 points

    Well that looks perfect for what I need! How did I miss it? 

    Thanks again!