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LMH6654: Does this amplifier work with AC coupled loads?

Gabriel Meier
Prodigy 10 points
Part Number: LMH6654

I'm working on a wide bandwidth signal amplifier. I want to use the LMH6654MF/NOPB as the final output stage. The last amplifier I used had an issue where it worked normally with a 50 ohm DC load, but when the output was AC coupled, it had severe zero-crossing issues as shown below. Will this chip behave normally when AC coupled, or does it still need the 50 ohm DC load?

TLDR: This should work fine. v

But will adding this capacitor break things? v

  • 0
    TI__Expert 6050 points

    Hi Gabriel,

    Can you please provide some additional information on how you are measuring the output. What point are you probing from and what is the capacitance of the probe?

    Another thing to consider, for a unity gain configuration, the recommended feedback resistor value is 25 ohms and you are using 402 ohms. Are you able to try swapping out that resistor? This may have an effect on the output, though it is not likely the entire problem.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Sorry for not adding more detail. The current design uses a MAX4201 buffer as the output amplifier. The amplifier goes directly and solely into a 50 ohm cable and then into a 50 ohm terminated oscilloscope. That oscilloscope produced the waveform shown above. The chip works fine when terminated with 50 ohms DC. It has issues when the terminator is AC coupled, and produces the waveform in yellow shown in the post. I'm currently redesigning it and hope to use the LMH6654MF/NOPB instead, but I'd like to make sure that the chip won't have the same issue if C1 is added as shown in the second schematic.

  • +1 in reply to Gabriel Meier
    TI__Expert 6050 points

    Hi Gabriel,

    Thank you for providing more details on your measurements. I am not familiar with the MAX4201 buffer design so it is hard to pinpoint a reason for why the ac coupling capacitor is causing output distortion. 

    I have created a model using the LMH6654 and it shows no output distortion when an AC-coupling capacitor is place on the output.

    Please let me know if you have any other questions or reach out if you face any issues with your redesign and I will be glad to assist further.

    Thanks,

    Nick