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THS3491: about the output capacitance and stability

Joyce Li
Intellectual 2170 points
Part Number: THS3491

Hi Dear expert,

My customer used THS3491 and they need to add a capacitor at the output which causes the loop stability issue. Below is the schematic, V+ connects to 10V reference voltage, V- is the input signal. Vs- connects to GND, Vs+ connects to 30V.

They add a 10pf cap at the ouput side and found amplifier is self oscillated. I suggest them to add resistor before cap to improve the stability issue, while it will impact the output gain and efficiency.

IS there any other way to solve the self oscillation?

Below is the EVM design. What is the function of C4? C6 is not NC. Will the layout also impact the stability? 

  • 0
    Guru 140200 points
    Hi Joyce,

    unfortunately, your schematic is incomplete.

    Yes, inserting Riso as discussed in section 9.1.1 of datasheet will help.

    C4 is a decoupling cap providing three terminals for lowest parasitic inductance. This results in enhanced decoupling performance.

    Yes, using a wrong layout can considerably impact stability. It's recommended to exactly adopt the example layout of the EVM.

    Kai
  • 0 in reply to kai klaas69
    Guru 48395 points

    Hi Joyce, I worked some on this - that 3 terminal cap is what is called an X2Y cap and has a very high self resonant frequency - wonder why it is not on the plus supply also.

    Cload is always an issue, there is some guidance in the datasheet, I also just published a CFA stability article giving you a clear path to assess phase margin as you make adjustments, I worked on the THS3491 TINA model, it is pretty good small signal wise - there were large signal things that were too tough to model, but basic slew rate is there.

    www.planetanalog.com/author.asp

  • 0 in reply to Michael Steffes
    Guru 140200 points
    Hi Michael,

    I also wondered why the 3-terminal decoupling cap is only used in the -supply. Using it this way makes one believe that the THS3491 has a stability issue...

    Kai
  • 0 in reply to kai klaas69
    Guru 48395 points
    No, probably just an oversight - yes, 1GHz amps are touchy but this one is fine with the right Rf value.