sheldon
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Navin,
Phase Margin is a figure of merit for the frequency response of the op-amp and it can not be calculated from time domain characteristics of the circuit, for example, settling time. Phase Margin is the measure of how close the phase of the loop gain is to inverting at unity gain. Or the phase margin is the difference between the phase of the loop gain and inversion, phase=180 degrees for unity gain of the loop gain. It is described in detail by the books in the Reference books section, for example, Gray and Meyer. If you have access to Spectre, stability analysis will calculate it for you directly.
Best Regards,
Sheldon
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