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    Need for safe phase margin when dealing with sin input signals

    Hello

    If I am using the instrumentation or operational amplifier to read sinusoidal input sensor signal, why do I need a good phase margin,
    in my opionion, low phase margin below 45 make ring oscillation when we apply pulse signal, but if the input is sin we dont have ringing problem, os even 45 degree should be enough

    Thank you

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    Re: Need for safe phase margin when dealing with sin input signals

    Phase margin also protects from component drift causing control loops
    to break out into oscillation. Power up transients can take longer times
    to settle to final value.

    Phase margin is good :)


    Regards, Dana.


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    Re: Need for safe phase margin when dealing with sin input signals

    For sine signal you are ok to use about 60deg of PM. 45deg is probably acceptable too but I won't recommend. 60deg is about max flat response of your amplifier.


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    Re: Need for safe phase margin when dealing with sin input signals

    Quote Originally Posted by sutapanaki View Post
    For sine signal you are ok to use about 60deg of PM. 45deg is probably acceptable too but I won't recommend. 60deg is about max flat response of your amplifier.
    Indeed 60 phase margin is also optimum value for pulse signal, this because it offer safe phase margin plus it is the fastest output settle response,

    I was expecting you to say that value of 45 is more than enough for sin signals



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    Re: Need for safe phase margin when dealing with sin input signals

    As already stated 45 degree phase margin will be accompanied by gain peaking in closed loop response. If it's no problem in your application, you can implement your amplifier with even lower phase margin. But why would you?


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    Re: Need for safe phase margin when dealing with sin input signals

    Quote Originally Posted by Junus2012 View Post
    Indeed 60 phase margin is also optimum value for pulse signal, this because it offer safe phase margin plus it is the fastest output settle response,

    I was expecting you to say that value of 45 is more than enough for sin signals
    Actually, for pulsed signals optimum PM is about 75deg. Then you will have fastest settling.


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    Re: Need for safe phase margin when dealing with sin input signals

    Quote Originally Posted by FvM View Post
    As already stated 45 degree phase margin will be accompanied by gain peaking in closed loop response. If it's no problem in your application, you can implement your amplifier with even lower phase margin. But why would you?
    Ok right, gain peaking I should consider
    Thank you FvM

    - - - Updated - - -

    Quote Originally Posted by sutapanaki View Post
    Actually, for pulsed signals optimum PM is about 75deg. Then you will have fastest settling.
    Thank you Suta

    In behzad book 60 is complained to be the optimum fastest



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    Re: Need for safe phase margin when dealing with sin input signals

    What page of Razavi's book? However, 60 is not the fastest for switched caps amplifiers which are working with pulses and steps. 75deg is. Here is a plot of the settling time vs. PM

    Click image for larger version. 

Name:	Capture.PNG 
Views:	3 
Size:	76.7 KB 
ID:	159526



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    Re: Need for safe phase margin when dealing with sin input signals

    Click image for larger version. 

Name:	Phase Margin Trans Response.JPG 
Views:	5 
Size:	38.3 KB 
ID:	159527


    Regards, Dana.



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