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-3dB phase and gain margin

Abhijit Pethkar
Prodigy 190 points

I was trying to design the 4th order low pass filter of Sallen Key architecture in FilterPro. I have ported the values from FilterPro to TINA software and tried to measure the -3dB gain margin and phase margin but found that both the margins are zero. which means that the closed loop system is just on the mark of getting unstable.

When I had simulated the Low pass filter in the FilterPro the report says that the design is done for -3dB.

I am enclosing the TINA bode plot for your reference.

Kindly revert

  • 0
    TI__Expert 5515 points

    Abhijit,

    Not quite sure I understand the concern here. What you've plotted in TINA is the transfer function of the filter. You are not looking at gain and phase margin, really. 

    You may be concerned about using this filter in a closed loop system, but without the details of that system it's hard to say what kind of stability issues you may face. If you're concerned about the filter itself being stable, you'd have to look at how each stage's transfer function interacts with the open-loop gain of the op amp you use to realize that stage. As Neil pointed out in another post, a Sallen-Key filter should be quite stable as long as the Q is in a reasonable range; from the looks of your transfer function, that seems to be the case, here.