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Design >> Analog Design >> A question for Slew Rate Function in Cadence Calcu
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Message started by ethan on Jul 27^th, 2005, 7:50am

Title: A question for Slew Rate Function in Cadence Calcu
Post by ethan on Jul 27^th, 2005, 7:50am

Hello everybody,

When I tried to use "slewRate" function in waveform calculator of Cadence Analog Design Environment , I found I could get two different SR values for the same ramp of the same curve by using two options seperately, which are either

"initial value Y at X" and " Final value Y at X"
or
"Initial value Y (value)" and "Final value Y (value)"

All others are under same configurations (opamp structure, curves, and the same ramp). For example, below show what I got for two cases:

slewRate(VT("/Vod") 1.96e-5 t 2.5e-5 t 10 90) = 6.018M
slewRate(VT("/Vod") -2.52 nil 2.04e-7 nil 10 90)=-2.02M

In above two equations, the Y values are the exact values for those at the time being.

If we look at the abosulte value of the above two results, they are 3 times different.

Can someone point out what 's reason for this?

In addition, I have looked up the "Overshoot Function", "Settling Time Function", and "Slew Rate Function" in Cadence "Waveform Calculator User Guide", I found that the definitions for "Initial Value" and "Final Value" are all same in the above three cases. Am I right? Because, in user guide, it didn't show the figure for Slew Rate function, and it only shows other two figures. I would like to be confirmed that I have done correctly.

Thanks a lot.

ethan

Title: Re: A question for Slew Rate Function in Cadence C
Post by Andrew Beckett on Jul 31^st, 2005, 9:05pm

Well, without knowing what the waveform looks like, it's hard to tell exactly what you've done wrong here - but perhaps if I outline what the meaning of the values are in each case, that might help you spot the problem.

In the first example, you are saying that you are measuring the slew rate between the Y values corresponding to an X axis value of 1.96e-5 and 2.5e-5. These are the 0 and 100% values, and so from the Y values here, it will determine the 10 and 90% values (as you've specified 10 and 90) and figure out the slew rate between those two Y values.

In the second example, you specified that the 0% Y value is -2.52, and the 100% Y value is 2.04e-7 (i.e. 0 to all intents and purposes). So this means that the 10% value would be -2.268 and the 90% value would be -0.252 - and that's where it would measure the slew rate.

So my guess is that the 10% and 90% Y values don't match up in the two cases.

Regards,

Andrew.