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OPA847: Negative feedback amplifier stability

Nuno Martins
Prodigy 60 points

Part Number: OPA847

I am using the OPA847 as front-end amplifier for an EMAT acquisition module, as in the schematic below:

InputStage.png

It works as expected without C4, but how we need to limit the bandwidth due to external noise, by adding it the amplifier becomes unstable. In that case, just by connecting a 50R termination resistor in CN1 it oscillates.

I tried to use C8 (220pF) as described in the datasheet to compensate C4, but the result is even worse.

According with the Spice simulation it should work in that conditions. What may be causing that unstability?

  • 0
    TI__Mastermind 22860 points

    Hi Nuno,

    This device is decompensated which means it requires a minimum gain to remain stable. It does involve some tuning to get the right noise gain shape. I was able to adjust the values you would need for a Cf of 10pF and I calculated around 1.6nF that is needed for C8 to shape the noise gain correctly. Is this something you could try adjusting to see if this helps the response? As far as the model, it is likely not modeling the higher frequency poles which is why it is showing it is stable.

    Best Regards,

    Ignacio

  • 0
    Prodigy 60 points

    Hi Ignacio,

    I tried your suggestion of adding 1.6nF in C8 and it looks quite stable with gain ~20V/V at 10MHz and 12.5V/V at 20MHz.

    How can it be calculated?

    About the gain, from CN1 is 20V/V, but with the 50R source impedance is 10V/V and below the specified minimum 12V/V. It should also be considered?

    Best regards,

    Nuno

  • 0 in reply to Nuno Martins
    TI__Mastermind 22860 points

    Hi Nuno,

    I used the equations in the datasheet on page 12. I set the formulas in excel and kept adjusting NG2 for a feedback capacitor of 10pF which is the value you chose. In the datasheet it has you calculate Cf for a given noise gain at higher frequencies but with your scenario you are fixing Cf so I just adjusted the other inputs. The idea with the Cf and Cdiff is to shape the noise gain at high frequencies. At low frequencies, the noise gain is the 20V/V or 10V/V when using 50R source impedance. The formulas that were used depend on NG1 (noise gain at low frequencies). I went ahead and adjusted the values for a NG1 of 10V/V and the resulting Cdiff was around 1.3nF. I would check to see if this works for you in this scenario.

    Best Regards,

    Ignacio