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LMH5401: LMH5401 Unity Gain

Nhan Nguyen98
Prodigy 40 points
Part Number: LMH5401
Other Parts Discussed in Thread: TINA-TI

I am trying to get all the gain Av configurations, using table 1 and table  2, also using equation 3, 4, 5, and 6 from the datasheet; however, I could not come up with the unity gain (Av =1) or half gain (Av=0.5). All the math and calculations did not give me Av=1 or Av=0.5

Does this part LMH5401 even support unity gain? or half gain?

Any help please?

Thank you.

Sincerely,

Nhan Nguyen

  • 0
    TI__Genius 13935 points

    Hello Nhan,

    The LMH5401 does support unity gain however, it requires some extra care. There is a reference design that illustrates this.

    For gain less than 1 you need to be aware that the amplifier feedback resistors create a bypass path around the amplifier, so gain below 0.5 is not practical.

    www.ti.com/.../TIDA-00522

    Regards,
    Loren

  • 0 in reply to Loren Siebert 1
    Prodigy 40 points

    Loren,

    Thank you so much for your feedback. I am also looking at this post https://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/t/391312 but I don't see any configuration for single ended input, and that is what I am interested in. So I am investigating more on the single ended input and single output. I may have more questions later.

    Again, thank you for your help.

    Sincerely,

    Nhan Nguyen

  • 0 in reply to Nhan Nguyen98
    TI__Genius 13935 points

    Hello Nhan,

    The LMH5401 datasheet has several examples of single ended configurations. Figure 56, 59 and 60 all show the LMH5401 with single ended inputs.

    I would also suggest you use our free simulation tool TINA-TI to simulate your circuits so you can see how they work.

    www.ti.com/.../toolssoftware

    Regards,
    Loren

  • 0 in reply to Loren Siebert 1
    Prodigy 40 points
    Loren,
    To sum it up, I am interested in single ended input, single output (either P or N, the other will be terminated) with unity gain. And yes, I have TINA installed and working on a similar circuit as one of those configurations that you mentioned but with unity gain. Thank you.
    Sincerely,
    Nhan Nguyen
  • 0 in reply to Nhan Nguyen98
    TI__Genius 13935 points

    Hello Nhan,

    While it is technically possible to use the LMH5401 as a single ended output amplifier there are several drawbacks.   We did not characterize the device with single ended output, so there are no performance curves that you can use to predict performance.  See my blog post for more details:

    Differential to single ended: What happens when you use only one differential amplifier output

    TI E2E support forums
    Many applications require the conversion of a differential signal to single ended. Some common examples are an RF DAC buffer or a coaxial cable driver. Most of the time you can accomplish this with a magnetic transformer, but sometimes a transformer...

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points

    Hello Nhan,

    The blog post seems to have suffered some formatting problem.  I am attaching the article in PDF format so that you can see the inline equations. 

    {EDIT - Changed attachment to PDF for more universal access}

    Regards,

    Loren

    Differential to Single ended-v4_lds.pdf

  • 0 in reply to Loren Siebert 1
    Prodigy 40 points

    Hi Loren,

    For the LMH5401, if I was using +/- 2.5V supply with SE input, could I set the VCM to 1V (instead of 0V) to offset the outputs? It's kind of shifting the output without actually using a level shifter. I have the circuit in TINA and was simulating it. It took a long time, but after a while, it actually completed and the result is exactly what I was expecting (the outputs are 1V offset). I just wonder if setting VCM to a different value other than 0V (given +/- 2.5V supply) is OK or not.

    Another question is that I have the output of the LMH5401 going into a LT part, which is a comparator LTC6754. How would you go about simulating these two chips using the same SPICE simulator (either LTSpice or TINA)? If using TINA, is there a way to import the LTC6754 spice model into TINA? or vice versa?

    Thank you very much.

    Sincerely,

    Nhan Nguyen

  • 0 in reply to Nhan Nguyen98
    TI__Genius 13935 points

    Hello Nhan,

    There are charts in the LMH5401 datasheet that show performance as the Vocm voltage is shifted from mid supply.  Since you are driving a comparator it is likely that you don't care about distortion, if so the TINA simulations are going to be fine. 

    I'm not expert on importing SPICE models into simulation tools.  There should be online instructions for importing SPICE models into both TINA-TI or LTSPICE.  You should pick the tool that is most suitable for your purposes and follow the instructions for importing the necessary models.  I have included a link that shows how to import a model into TINA-TI

    http://www.digikey.com/videos/en/v/Importing-SPICE-models-into-TINA-TI/1680490923001

    http://www.ti.com/lit/an/slva527/slva527.pdf

    Regards,

    Loren