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LMH6624 and low gain apps (<20dB)

Grzegorz Grzeda
Prodigy 10 points
Other Parts Discussed in Thread: LMH6624, LMH6629

Hello,

currently, I'm working on a project with low noise preAmp (need to sense signals about 200uVpp). I've been gave from my supervisor (I'm a 2nd year student of electronics) the LMH6624. The project requires an input stage (G = + 23 V/V non-inv), Sallen-Key LowPass on f0=4 MHz (G= 1 V/V) and output buffer (gain settable from a voltage follower to desired value). I send my schematic project in PDF (from Altium Designer 09).

 7853.PSCtusPreamp4.pdf

My question is:

should I give RC between inputs of the filter and out buffer OpAmps (I assume, that I don't need to do that to the first stage casue of G=27dB)? And aroud what values should those RC be?

I know, that there are unity gain stable OpAmps but for now I need to use these.

Please help

Grzegorz Grzeda (PL)

  • 0
    TI__Mastermind 22825 points

    Hi,

    As you already know, the LMH6624 is not stable with gains less than 20dB. So, the stages which have "noise gain" less than 20dB at "high frequency" would need some sort of compensation (or you need to switch to unity gain stable devices). And, as you noted, your 1st stage won't need any compensation since you're running it at a gain of 23 V/V.

    For some discussion on the range of values that might work for you to increase your high frequency noise gain, and general compensation information, here are some reference materials:

    1. http://www.ti.com/lit/an/snoa486a/snoa486a.pdf

    2. http://www.ti.com/lit/an/sloa020a/sloa020a.pdf (Page 18)

    3. LMH6629 Figure 4 along with the equations provided

    Hope this helps.

    Regards,

    Hooman