# settling time in opamp

1. ## settling time in opamp

hi
in Saxena thesis(INDIRECT FEEDBACK COMPENSATION TECHNIQUES FOR MULTI-STAGE
OPERATIONAL AMPLIFIERS) is written that t_s =.35/f_un for a 90 degree phase margin,where t-s is 99.9% settling time and
f_un is unitary frequency.
how can this relation be obtained ?
thanks  Reply With Quote

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2. ## Re: settling time in opamp

I think it is wrong. 90° phase margin means simple RC behaviour. For an RC filter the 99.9% settling means the exponential part is
exp(-t_s/T) = 0.001, thus
t_s = -ln(0.001)*T, where T is 1/(2*pi*f_un).

From above relations the t_s = -ln(0.001)/(2*pi*f_un) ~ 1.1/f_un, where [f_un] is Hz.  Reply With Quote

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3. ## Re: settling time in opamp

Surely not 99.9 % settling. The expression is usually applied as an estimation of 10% to 90% rise time.  Reply With Quote

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4. ## Re: settling time in opamp

I see you have quoted the thesis correctly, but unfortunately the author is mistaken in this point.

Modifying the calculation by frankrose, you get

t_s = (ln(0.9)-ln(0.1))/(2*pi*f_un) ~ 0.35/f_un

The incorrect value of 99.9% is claimed without a calculation or literature reference, no good example of scientific writing.  Reply With Quote

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