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Original question:

make stand alone buffer

make stand alone buffer using LMH6321

Ke Huang
Prodigy 40 points
Other Parts Discussed in Thread: LMH6321

Hi Sir/Madam,

I have a problem when using LMH 6321 as a standalone voltage buffer, as in the below figure shows, what I want to do is extracting a voltage signal from a electrical oscillator (which oscillates at 5 MHz) and then filter the DC component and finally drive another load oscillator. Since I do not want the load and the DC block to affect my electrical oscillator (which is the source), so I hope to use LMH6321 to realize unidirectional signal transfer.

But now I have problem: every time when I apply power to the LMH6321 (V+ and V-) before i turn on the electrical oscillator, I can see parasitic oscillation (~14 MHz) for both the output of electrical oscillator and the output of the DC block. When I turn on the electrical oscillator, the intrinsic output of the electrical oscillator and the output of the DC block all deformed and become noisy.

I do not know the output impedance of the electrical oscillator and meanwhile it has to be changed for our using. But I know the DC block and the load almost have 50 ohm input impedance.

since i do not have much eletrical background, I appreciate if anybody can help me to solve this problem.

 

best

 

Ke Huang

  • 0
    TI__Mastermind 24610 points
    Hi Ke,

    Does the electrical oscillator and the LMH6321 have the same DC bias? If they don't then that could be part of the reason your output is deformed. I would recommend adding a DC blocking capacitor between the oscillator and the LMH6321.

    Also can you upload a picture of the output signal when the electrical oscillator is on and off?

    Let me know if that improves the output signal.

    Regards,
    Robert Clifton
  • 0 in reply to Robert Clifton56
    Prodigy 40 points

    Hi Robert,

    Thanks a lot i have uploaded a test file (pdf) with this email, I am very appreciated if you can give me anymore further suggestion.

    Best

    Ke

    test of LMH6321.pdf

  • 0 in reply to Ke Huang
    TI__Mastermind 24610 points
    Hi Ke,

    It's not everyday I get such a nice presentation report! I'm impressed!

    Now based on your report, by adding the DC blocking 100nF capacitor between the EO and the LMH6621 the parasitic oscillations are gone. You also no longer see any clipping in the signal anymore.

    Correct me if I'm wrong but the only issue you now have is that the EO frequency changes with different loads? Do you have a specific range that the load will change by? You could set up the output to where the load will appear the same to the amplifier by putting a set load in parallel with the load that changes.

    Regards,
    Robert
  • 0 in reply to Robert Clifton56
    Prodigy 40 points

    Hi Robert,

    Thank you for your quick reply, you are correct, my current issue is the 50 ohm load changed the intrinsic frequency of the EO, as you can see in the test report. the intrinsic frequency of the EO is 5.74 MHz (frequency of the separated EO), if i connected it with 50 ohm DC block through op-amp, the frequency of the EO becomes to be 5.215 MHz, if i connected it to a 50 ohm DC block in series with a 50 ohm load through op-amp, the frequency of the EO becomes to be 5.875 MHz. 

    Actually, i think the 50 ohm DC block can be removed, however we need to keep the 50 ohm load. i do not know if removing the 50 ohm DC block can help to maintain the 5.74 MHz frequency of the EO. I will test it later. 

    the load does not change, it is the same load with 50 ohm impedance in my using.

    by the way what is the output impedance of the op-amp?

    Thanks

    Best

    Ke

  • 0 in reply to Ke Huang
    TI__Mastermind 24610 points
    Hi Ke,

    If you look at page 3 of the LMH6321 datasheet you will notice that it says the output resistance of the amplifier is 5ohms.

    Best Regards,

    Robert