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LMH6626: Confusion about LMH6626 amplifier circuit interference problem

bi zelin
Prodigy 60 points
Part Number: LMH6626

Tool/software:

Hello TI engineers:

I built a 128 channel amplifier circuit with 64 pieces of LMH6626 with 20 layers of PCB. I want to use this circuit to amplify uV signals to mV level.During the test, I found that even if the output signal is not given, the output of the amplifier circuit will have an output signal, as shown in the figure

I tried connecting the GND of the board to a large iron plate to minimize the interference, but it didn't have much effect, just a slightly thinner waveform curve.

The signal is very stable, the frequency is around 3.1MHz and the amplitude doesn't change much!

I would like to know where this signal is coming from and how I can eliminate it.

Below is the circuit design of my single LMH6626, I am looking for guidance from engineers!

Thanks

Bi Zelin

  • 0
    TI__Genius 16715 points

    Hi Bi Zelin,

    Thank you for sharing what you are seeing as well as a schematic. Everything is pointing to a stability problem in the circuit which is resulting in an oscillation. I was able to simulate the circuit and there is nothing that stands out however simulation does not capture everything in the real-world circuit which can be affecting results. Just to isolate this stage could you debug the circuit without the next stage, meaning removing the two 10uF capacitors going out the ADC stage. When probing if you could probe on the right side of your 50ohm output resistors. This will isolate any capacitance from the probe and ensure we aren't causing an oscillation by probing.

    There could be a few things at play with this circuit as it is a decompensated device which requires a minimum gain, which you have so I don't think that's a concern. However, one thing that can be a problem with larger feedback resistors is their interaction with the input capacitance of the device as well as parasitic capacitance from the layout. One debug step would be to drop the feedback resistor value to say 1000ohms and see if this changes the behavior of the device. Although it is not the gain you are targeting it would be good to see if the stability problem is due to the very large feedback resistance. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 60 points

    Thank you for your help! Indeed the large feedback resistance is a problem. But I want to ask you a question that if I minimize the feedback resistor, how can I get the 60dB gain?

    Thanks

    Bi Zelin

  • 0 in reply to bi zelin
    TI__Genius 16715 points

    Hi Bi Zelin,

    For such a large gain we would typically recommend splitting the gain into more stages. However, it seems like that option is not available to us. Would you be able to maybe adjust the feedback network values? It is hard to say how low we would need Rf to be as the device was designed for 500ohms but starting by adjudging the Rf lower and therefore Rg as well to maintain the required gain is worth trying. One other debug step can be to add some feedback capacitance to the device. This can be done by stacking a capacitor on top of the feedback resistor already in place. Would you be able to also try adding say 1pF Cf to the circuit and see if this changes the response?

    Best Regards,

    Ignacio