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Getting the Settling time in Cadence
This page describes how you can setup an expression for the settling time without using the Cadence settling time function in the calculator. I have found that the settlingtime
function in the calculator is not that reliable and its hard to debug it because I don't know how it operates internally. Here I document a technique I use to calculate the settling time of a waveform using simple functions like value
To find the settling time of WAVE1 to P% of final value:
- Clip the WAVE1 at the extremes of where the settling event occurs. Make sure only 1 settling event occurs in the clipped waveform and the end point is what the final value you are looking to settle to.
- Now the initial value is INI and final value (also the final value of the clip) is FIN.
- Let WAVE2 = abs(FIN-WAVE1)
- Now setup a cross function for abs(FIN-WAVE1) to cross the threshold of abs(FIN-INI)*P/100 and measure the last edge (-1 edge number) and the edge can be "either" (instead of rising or falling). This cross function now gives the time point when the signal enters the P% range and settles. Another threshold could be P% of the absolute final value and not the difference i.e. abs(FIN)*P/100. Depending on what kind of settling you are measuring pick the appropriate threshold.
- Now subtract the initial stimulus time from this cross function to get the settling time.