Need for safe phase margin when dealing with sin input signals

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Junus2012

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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
 

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.
 
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.
 
sutapanaki said:
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.
Click to expand...

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
 

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?
 
Junus2012 said:
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
Click to expand...

Actually, for pulsed signals optimum PM is about 75deg. Then you will have fastest settling.
 
FvM said:
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?
Click to expand...

Ok right, gain peaking I should consider
Thank you FvM

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sutapanaki said:
Actually, for pulsed signals optimum PM is about 75deg. Then you will have fastest settling.
Click to expand...

Thank you Suta

In behzad book 60 is complained to be the optimum fastest
 

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

 
Besides providing a cushion against component drift, there is a more fundamental reason.
What you call sin(2pi_f_t) is actually sin(2pi_f_t)u(t). In other words, you can get persistent ringing on startup.
 

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