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OPA2322: to increase capacitive load

Other Parts Discussed in Thread: TINA-TI

Dear all,

I would like to drive 2nF or more capacitive load with this device.

I read data sheet how to increase capacitive load.(inserting resistors)

However I could not understand how value should I insert.

Is there the way to calculate the calculate resistances?

Or..How value should I insert to increase capacitive value to 2nF at unity gain, for example?

Best regards,

  • Dear all,

    Could you answer this ASAP?
    I am looking forward hearing from you.

    Best regards,
  • Hi Takao San

    I am really sorry for the delayed response

    I recommend to go though the stability series videos in the link below 

    You will get a deep and better understanding of designing the value of resistor for different value of capacitance 

  •  Please make sure  click on the stability series .

  • Takao,

    The sum of the open-loop output impedanece (R=90ohm) and Riso resistor toghether with Cload creates a pole in the close-loop vs frequency transfer function while Riso resistor alone and Cload form a cancelling zero - since in order to prevent exessive dipping in phase the pole and cancelling zero MUST occur within a decade in frequency domain, the minimum required value of Riso resistor must be 10ohm (90ohm+10ohm vs 10ohm) - see below.

    Simulating the circuit with and without Riso resistor shows a great improvement in the capacitve load driving 2nF load (small-signal overshoot should be kept below 25%) - see below simulation results as well as the attached schematic you may use for your own simulations.

    You may dowload a free version of Tina-TI using link following: 

    OPA322 cap drive.TSC

  • Dear Marek,

    Thank you for your support.

    I would like to check one thing.

    Your recommendation is keeping overshoot within 25%, but Vout_rsio have 6.5V overshoot that is over 30%.
    Is this OK?
    Should we use 20Ω instead of 10Ω?

    Best regards,
  • Takao,

    Only small signal overshoot (10mV signal)  indicates whether the system is stable or not .  A large signal overshoot tells you nothing about stability. Having said that, a higher value of Riso resistor increases isolation level and thus improves the stability but at the price of higher gain error in case you have also  resistive load.

  • Dear Marek,

    Thank you for your advice, and I am sorry, I had mistake at previous post.

    mistake = 6.5V
    correct = 6.5mV

    So, I thought that Vout_rsio have overshoot over 30%.
    Is this OK?

    Best regards,