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OPA548: Is there any limitations on the value of CF with integrator?

Shinichi Inoue
Guru 20536 points
Part Number: OPA548
Other Parts Discussed in Thread: TINA-TI

Dear Specialists,

My customer is evaluating OPA548 as a integrator and has a question.

I would be grateful if you could advise.

---

Operation has been confirmed using OPA548 as an integrator.(Schematic is listed below)
The oscillation occurred when 10nF was used for CF. (R=1kohm)
Then I changed to 150pF, the oscillation stopped.

Ri=1kohm, Co=10nF/150pF, Rload=160ohm(Eo to GND)

Is there any limitations on the value of CF?

I could not find any description.

Could you please let me know? 

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Guru 56491 points

    Hi Shinichi-san,

    Shinichi Inoue said:
    Is there any limitations on the value of CF?

    CF in integrator should not result oscillation issues, unless the circuit is driving capacitive load (including leads and parasitic capacitance). 

    Could you provide us the input  frequency, input amplitude and output integrated range? I can take a closer look a the actual circuit.  

    I provide an example per an integrator circuit, though I do not have all your operating parameters. I believe that the circuit is operating in a single supply rail. In the example, I limit the DC gains at approx. 16V/V. If I have your design parameters, I can optimize the integrator. 

    OPA548 Integrator 02152025.TSC

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Guru 20536 points

    Hi Raymond,

    Thank you for your reply.

    I understand there is no limitation regarding CF and it needs to consider load capacitance.

    I shared your suggestion with the customer and requested detailed circuit information.

    When I obtain, I'll share with you.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 20536 points

    Hi Raymond,

    The customer confirmed their original circuit in the TINA-TI and no oscillation.

    But in the actual circuit oscillation was observed. 

    I could obtain the customer's schematic and waveform.

    I'd like to share with you.

    Could you please send a friendship request?

    I appreciate your great help and cooperation.

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Guru 56491 points

    Hi Hi Shinichi-san,

    The friendship request is sent. Please send me the schematic and waveforms. 

    When RF gain resistor is inserted, it does two things. One is that it limits the gains of the integrator. Without it, the DC gain is extremely high. The Vout will rail to the positive supply rail. Two is that is limited the BW of the OPA548, which it makes the circuit more stable. You do not have to use 16kΩ. You could use 20kΩ or similar. It depends on the input frequency signal, but the pole of the integrator may be optimized.  

    Once we are establishing contact via the private messaging or internal email, I will close this inquiry. 

    Best,

    Raymond

  • +1 in reply to Raymond Zhang1
    TI__Guru 56491 points

    Hi Shinichi-san,

    I am going to close this inquiry, since we have established communication via E2E Private messaging. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Guru 20536 points

    Hi Raymond,

    I agree.

    Let's talk about Private message.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi