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OPA1611: Isolation Resistor

Part Number: OPA1611
Other Parts Discussed in Thread: BUF634A, OPA1656, , TINA-TI, BUF634
Hi,
Should the isolation resistor be inserted between OPA1611 and BUF634A when replacing OPA1656 with OPA1611 and using with the combination OPA1611 and BUF634A?
I am concerned about the issue that the high capacitive load decreases the phase margin of OPA6111 and can lead to gain peaking or oscillations.
Best regards,
Kato
  • Kato-san,

    I'm sorry for delayed response.

    As you pointed out, I agree the isolation resistor between OPA1611 input and BUF634A output helps to improve the phase margin on OPA1611, since BUF634A input should behave as cap load to OPA1611; i.e. it create a pole and the isolation resistor can create a zero.

    If you would like to eliminate the gain peaking, you can also consider an additional the phase compensation circuit in as below.

    If you can use TINA-TI, you can find its simulation file.
    OPA1611+BUF634A01.TSC

    Pleas let me know if you have any further questions.

    Best regards,
    Iwata Etsuji

  • Hi Iwata-san,

    Thank you for the detailed information.

    According to the following application note, I found that the phase shift resulting from this low-pass could cause the entire circuit to oscillate when inserting the series resistor between OPA1611 and BUF634A.
    Should the Isolation Resistor Riso be Inserted into the Output of BUF634A?
    Could you please give me your advice again?

    COMBINING AN AMPLIFIER WITH THE BUF634
    https://www.ti.com/lit/an/sboa065/sboa065.pdf

    Best regards,
    Kato

  • Hello Kato-san,

    I'm sorry for delayed response.

    As you pointed out, any additional series resistors between amplifiers in cascaded configuration should cause the phase shift. However, if you would like to eliminate any ringing, the double feedback configuration could help to optimize the phase margin and the settling behavior.

    Please note that BUF634A is not exactly same as BUF634 and OPA1611 has not same output topology as opamps in that application note; OPA603/04/71/27: emitter follower but OPA1611: collector follower.
    Therefore, that application note is not always effective. 
    If you are afraid of the stability problem, the phase margin analysis should be important.

    Best regards,
    Iwata Etsuji

  • Hi Iwata-san,

    Thank you for the explanation.

    As you mentioned, the topology of each output stage seems to be different as follows.

    I understand.

    Best regards,
    Kato