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Frequency response of LPF in TINA

Sira Rao1
Intellectual 260 points
Other Parts Discussed in Thread: OPA2314

1263.2ndStageLPF-LTS -stabilityanalysis.TSC

The attached TINA schematic is a multiple feedback low pass filter (2nd order), and it should have a passband gain of 0dB. However, frequency response plotted with TINA yields a different results (approx -72dB).

What is the reason for this behavior? Strangely, the behavior also appears to be dependent on the DC voltage supplied to the op-amp.

Please let me know how to fix this issue.

Thanks,

Sira

  • 0
    TI__Mastermind 22825 points

    Hello,

    If you apply a negative voltage to the VEE pin of at least -0.2V or greater, then the AC analysis runs fine. This may have something to do with the fact that you are running the part single supply and you expect 0V input to result in 0V output which is not possible even with a rail-to-rail output device like the OPA2314!

    In the real application also, you do not want to operate the device the way you have shown it as the output will be close the VEE rail. You need some means of centering the output voltage with biasing.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    TI__Expert 3555 points

    Sira,

    I was looking at your circuit in TINA and realized a few things that you might want to change.

    • This is a single supply circuit. The correct configuration for this circuit is to supply an input signal that has a DC bias between the negative and possitive supply. this voltage value turns out to be 1.65 V in your circuit.
    • The non-inverting input of the amplifier must also be between the power supply voltages. Again, the DC voltage that you should apply to the non-inverting input should be 1.65 V.
    • I have gone through your LPF circult and found a few problems with the resistors and capacitors. I think tha your resistor values tend to be about 10x too high and your C1 value is too low.

    Since I don't know what your filter approximation value is (Butterworth, Bessel, Chebyshev, or etc) I can not give specific values. Can you please supply that information?

    I am curious, how did you generate the componenet values? TI does have a tool that will generate your values easily, WEBENCH Filter Designer. As I said, this program will give you the component values, allow you to select your amplfier for your circuit, and give you simulation results using TI-Spice.