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Debdut

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question on Elmore delay
May 20^th, 2016, 4:18am

I understand that Elmore delay is a model for interconnect networks, the networks being represented by cascaded RC network.

I am trying to create a delay line with RC networks and calculating the R-C values using Elmore delay model. But after simulation I am not getting the correct delay at the nodes. For smaller networks I have seen that Elmore delay is different than delay calculated using Laplace transform. But for large network the Laplace transform is becoming very complicated.

Can there be a simpler way predict R-C values that produces delay which matches with simulation?

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: question on Elmore delay Reply #1 - May 20^th, 2016, 1:45pm How you are finding delay with Laplace transforms? By using time constants?
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Debdut Junior Member Offline Posts: 10	Re: question on Elmore delay Reply #2 - May 20^th, 2016, 10:37pm I have calculated the transfer function and did inverse laplace for step input. Then finding the time taken for the output to reach 50 percent of input.
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raja.cedt

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Re: question on Elmore delay
Reply #3 - May 21^st, 2016, 1:24am

HI-
Yes that is called time constant based calculation. Elmore delay is based on dominate time constant, means in the transfer function ignore all s power terms except constant and s-coefficient. Hence this pretty accurate if systems is dominted by single time constant. For a given network if one R and one C domintes this works generally. Please post which circuit is creating problem.

Best Regards,
Raj.

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Debdut Junior Member Offline Posts: 10	Re: question on Elmore delay Reply #4 - May 21^st, 2016, 11:10pm "Elmore delay is based on dominate time constant" - thanks for the info. The circuit has three low pass RC networks just connected in cascade. It does not have a dominant pole. All the RC values are comparable. So what can I do to generate correct delays ... I mean how do I calculate?
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Debdut Junior Member Offline Posts: 10	Re: question on Elmore delay Reply #5 - May 22^nd, 2016, 1:55pm Thank you again, raja.cedt. Is there any way to rate your comments. It saved me. I added a large time constant circuit and I am getting the precise delays that I want. Thank you.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: question on Elmore delay Reply #6 - May 23^rd, 2016, 2:37am Which book you are using for Elmore delay. If you are interested better look for 8th chapter in the following book or just read open circuit time constants method. The Design of CMOS Radio-Frequency Integrated Circuits Good Luck, Raj.
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