Hi Expert,
I have a question that when we measure bode plot and see gain across zero not only one time, shall we measure 1st or 2nd point for phase margin?
For example we did test on TPS54917 with Cff=680pF and see 1st Fc=28K and 2nd Fc=45K(File: Vim-4YE-220P). Which one is what we need?
Sometimeswe see phase across zero not only one time for example file: Type 2. Which one is what we need to measure for gain margin?
Could you please have review if the result is good? Thank you.
Hi Yann,
For example we did test on TPS54917 with Cff=220pF and see magnitude 1st Fc=~28K and 2nd Fc=~45K(File: Vim-4YE-220P). Which cross point for phase margin is what we need?
Sometimes we see phase across zero not only one time for example file: Type 2. Which one is what we need to measure for gain margin?
Thanks,
Allan
Hi Allan,
In this particular case these measurement seems to have effect of random noise on the measurement.
The accuracy of the bode plot results depends on the measurement setup, and the frequency analyzer setup. These two blog post show how it could affect the bode plot measurement
I would recommend to retake the measurements with verifying the setup, and check the results. If you still get the same curve, please share the schematic, layout of the application along with the measurement setup.
On a separate note to check, if the stability in your system is good enough,I would recommend to also look at the worst case load transient response in your system. If this is acceptable and still is within the application requirements, it should be a stronger decision point than phase margin.
Regards,
Sabrina
