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THS3095: Noise Optimization

Alexander Slepoy1
Prodigy 105 points
Part Number: THS3095
Other Parts Discussed in Thread: LMH6702, LMH6609

Tool/software:

The THS3095 is advertised as a low noise amplifier with 1.1nV/sqHz voltage and 14pA/sqHz current noise. The problem is that the datasheet recommends using a feedback resistor on the scale of 1Kohm. With these values the total noise of the circuit is much higher. For example, a G=-1 amplifier will have output noise of 15nV/sqHz due to the effects of the current noise. If I understand correctly, in order to achieve significant improvement in the output noise, a much smaller feedback resistor has to be used, something in the order of 100ohm. The datasheet does mention that smaller than 1Kohm resistors are possible, but warns from possible stability issues. Yet, there is no diagram that shows how exactly the stability is affected by a smaller feedback resistor.

Is there something I'm missing in my analysis?

Can you provide the missing information? 

If 15nV/sqHz is unavoidable, is there a different amplifier that you would recommend to achieve better overall noise characteristics?

Thank you,

  • 0
    TI__Genius 16245 points

    Hi Alexander,

    That is correct, accounting for total noise due to current noise you must consider the feedback resistor network. It is the parallel of the feedback resistor and the gain resistor that is used to convert the current to voltage spectral density. This means that in cases with a low noise gain, current noise will likely dominate in a current feedback device. We would not recommend adjusting the feedback resistor much more than the datasheet highlights as this will ultimately lead to stability issues if reduced. With a value of 100ohms, I am almost certain the device would be unstable. For your circuit, if you are using a low gain configuration, a voltage feedback device might result in better noise performance compared to a current feedback device. Are there performance requirements you can share to zero in on a different amplifier?

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 105 points

    Hi Ignacio,

    Thank you for the quick response. My requirements are:

    1. Gain configuration: G=-1
    2. Signal: Sine, 6VPP, 30MHz
    3. Supply: +-5V
    4. Noise: minimal
    5. I need the gain at 30MHz to be relatively consistent, so BW>>30MHz is desired.

     Thanks

  • 0 in reply to Alexander Slepoy1
    Prodigy 105 points

    So far, the best option I could find is LMH6702, because it can work with 237ohm resistors. Total output noise: 6.4nV/sqHz

    I didn't find voltage feedback amplifier that can do better, but I'm open for suggestions.

  • 0 in reply to Alexander Slepoy1
    TI__Genius 16245 points

    Hi Alexander,

    The LMH6702 should have better noise performance than the THS3095 as your Rf and Rg values are smaller like you mentioned. I would also consider the LMH6609 as a potential solution.

    Best Regards,

    Ignacio