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Getting the Settling time in Cadence


Overview

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, clip and cross.

Technique

To find the settling time of WAVE1 to P% of final value:
  1. 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.
  2. Now the initial value is INI and final value (also the final value of the clip) is FIN.
  3. Let WAVE2 = abs(FIN-WAVE1)
  4. 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.
  5. Now subtract the initial stimulus time from this cross function to get the settling time.


Copyright 2018 Milind Gupta