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OPA604: OUTPUT IS OSCILLATING WITH THIS IC

Part Number: OPA604

hi,

   i am working the below attached circuit for my input voltage range of 9mV to 14mV so that i can get the output from 0 to 2.7V . i am feeding 10mV at the input,the output is oscillating from .5 to 1V which i have observed in scope.i dont know why.please confirm if my New Doc 2020-05-27 14.32.03_1.pdfcircuit and thNew Doc 2020-05-27 14.32.03_1 (1).pdf

  • Hello Uma,

    Can you provide information about the oscillation such as the frequency, amplitude and where you are observing it in the circuit? An image of the oscillation from a DSO would be helpful. Be sure to include the DSO scale information.

    I evaluated the OPA604 circuit from both dc and ac standpoints. I am not finding anything about the circuit that would explain an oscillation. There is the large 100 nF C10 capacitive load at the U8 output, but it isolated by R26 the 1 k resistor which should assure that there is sufficient phase margin and remain stable. I did a stability analysis and the circuit and C10 and R26 do create a second pole in the gain vs frequency response, but it occurs well beyond where the loop gain drops below zero dB. Although we don't have a modern OPA604 simulation model (the OPA604 is a very mature product) the simple model that it has indicates a 47 degree phase margin. Even if the model is off by quite a bit in term so its ac characteristics the actual phase margin shouldn't be off too much and the circuit should be stable even if the actual phase margin is somewhat less.

    If the application circuit has worked properly in the past with OPA604 op amps, all indicators are it should work correctly with valid OPA604 devices. Did you obtain the OPA604 op amps you are evaluating directly from TI, or from an authorized TI distributor?

    Any other information you can provide about the circuit, devices, construction may provide us a clue why there is oscillation.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • hi,

      i am checking the output at 6th pin of U8 at which the output is oscillating.by the way i am using this circuit on bread board.please look at the output that i captured in  oscilloscope.

  • hi,

      i have not  worked previously with this ic.we have purchased from digikey. also confirm is the gain that i have kept will provide the stable output? 

  • Hi Uma,

    Thank you for the OPA604 circuit DSO image. It is helpful to see the oscillation you are observing.

    The ~170 kHz of the oscillation doesn't readily relate to anything about the circuit such as the closed loop bandwidth, or gain bandwidth of the OPA604. My simulations indicated that ideally the system should have sufficient phase margin and remain stable; however, there is another possibility that could affect stability.

    You mention that the circuit is built up on a breadboard. I assume it is one of those common white boards that ICs, passive components and connecting wires can be plugged into. This could be a problem. Another way in which an op amp can become unstable is when the feedback resistor and capacitance at the op amp introduce enough phase shift around the feedback loop that the phase margin is degraded and the feedback becomes positive. The white board capacitance, along with the very high 100 k feedback resistor may be creating an unstable situation.

    First off, try reducing the U8 input (R31) and feedback (R30) resistors by a factor of 5x to 10x. Doing so will move the pole in the feedback loop upward in frequency by the same factor. That will reduce the amount of phase shift contributed by the feedback resistance and total capacitance at the OPA604 inverting input. Check to see if the oscillation is still present, or if it has ceased. If it is gone then you have found the source of the problem.

    Ultimately, you really want to test the circuit on a well planned and laid out PC board where the capacitance at the OPA604 inverting input is minimized. When such a board is used you may find that the U8 resistances can be increased by an higher factor and stability is still maintained.

    Regards, Thomas

    Precision Amplifiers Applications Engineering