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LMH6554: Design query on LMH6554

Poh Kok Keng
Genius 5512 points
Part Number: LMH6554
Other Parts Discussed in Thread: LMH5401, THS4541

hi,

Query for LMH6554.

  1. In the datasheet all examples are using feedback resistor of 200 Ohms. Can I use other resistor values e.g. 50 or 100 to adjust/reduce gain?

I would like to get flat Freq Response from 1MHz to 2.8GHz as my design bandwidth is 1MHz to 2.8GHz.  I want as much flat response till 2.8GHz. Gain is not important, I can accept 0dB Gain.

Application usage case:

Case 1 : I would like to set gain of 0dB and also maintain input differential impedance as 100 Ohms

Case 2 : I would like to set gain of -6dB and also maintain input differential impedance as 100 Ohms

Are those possible?

Hope to hear from you soon.

Best Regards,

kpk

  • 0
    TI__Genius 15975 points

    Hi kpk,

    The LMH6554 is a current feedback amplifier which has been optimized for feedback resistor of 200ohms. If you look at "Figure 1. Frequency Response vs Rf" in the datasheet, a feedback resistor of 200ohms gives around 1dB of peaking and this peaking reduces with increase in value of the feedback resistor. As a result, reducing the feedback resistor from an Rf = 200ohms will only increase the peaking and cause the LMH6554 to be more unstable. I don't think it is recommended to use 50 or 100ohm feedback resistor to adjust/reduce LMH6554 gain, as the amplifier will be unstable.

    Have you looked at the LMH5401 for your application? Although it is stable for Noise Gain of 2V/V, I think you should be able to achieve a flat frequency response of 1MHz to 2.8GHz for gain of 0dB with this part. You can refer this TI Design for further details:

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Genius 5512 points
    hi Rohit,

    I am currently using LMH6554LEX/NOPM with single Supply +5v.
    The VCM Pin 2 is driven by ADC with voltage of 0.9v. The Amplifier does not work correctly as the Common Mode voltage is out of range.

    Is there a drop in replacement part that will work with 0.9v Common Mode voltage. Pls advise if LMH5401 will work?

    best regards,
    Kpk
  • 0 in reply to Poh Kok Keng
    TI__Genius 15975 points

    Hi Kpk,

    I don't think we have a drop in replacement part to the LMH6554 that will work with 0.9V Common Mode voltage. Sorry, but the LMH5401 wont work. For what you are trying to do, it looks like you would need a rail-to-rail output FDA. The fastest we have is the THS4541 (BW ~620MHz) but is not a drop-in replacement to the LMH6554.

    I think the only solution to this is to operate the LMH6554/LMH5401 on dual supplies +3.5/-1.5V, if you want to stay with 2.8GHz of -3dB BW. 

    Best Regards,

    Rohit