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PSPICE-FOR-TI: Apparent unexpected instability of integrator with low-value bias resistor​

Marvn Arcangel
Genius 13970 points
Part Number: PSPICE-FOR-TI
Other Parts Discussed in Thread: OPA891

Hi Experts,

Can you help with the inquiry below?

I'm looking for some clarity if what I'm seeing is real, or a simulation artifact due to an imperfect model. I'm simulating a PLL response, and was hoping to use OPA891 as my main op amp in my active loop filter. As such, I have built the followinng loop filter, but my simulation seems to enter a radically different behavior as soon as I push R10 <1 kohm. The suddenness of the transition makes me think this is a simulation error (R10 being 919 ohm is normal, 918 ohms is not), but I thought it best to check.

Please let me know if additional information would be helpful. This simulation is in ADS, after having imported the SPICE model in. The general amplifier stages using this op amp seem to be behaving normally. Any input would be appreciated.

image (9).png

Regards,

Marvin

  • 0
    Prodigy 10 points

    Hi Marvin - I asked the original question, and at least at the surface level, I'm able to get rid of the problem by increasing the sweep density of my AC simulation (from 10 points/decade to 100 or 1000). It's not clear to me why this would fix an issue like this, but the urgency of finding a solution has decreased for me.

  • 0 in reply to Michael Toennies
    TI__Genius 17905 points

    Hi Michael,

    I was able to put together your circuit in TINA and tried different simulations and could not cause the simulation to fail so I would agree with Marvin that it is ADS. The circuit is in an open loop which might be why the simulator is struggling as it might not be finding a good bias point and therefore not converging. There is not much support we can offer with ADS as our models are designed and simulated in TINA and PSpice for TI but I am glad you were able to find a solution.

    Best Regards,

    Ignacio