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Simulators >> AMS Simulators >> have ac analysis pick up node/net ic conditions from a transient run in AMS
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Message started by trond on Nov 6^th, 2008, 6:55am

Title: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by trond on Nov 6^th, 2008, 6:55am

I have an analog circuit connected to a digital control block. I would like to check the interaction between digital and analog.

Now when I run an AC analysis the simulator picks up the wrong net conditions as the digital verilog block needs a little while to output the correct states to the analog block. For example, after power on the digital block provide filter and gain settings to the analog block.

I figured I would run a short transient first, save the ic conditions to a file and then have the AC analysis read in this file and use it. However, the AMS simulator options are different from the regular Spectre ones. I cannot force the AC to read in ic nodes as I could in a conventional ac analysis.

Does anyone know whether there is a way to do this?
Thanks

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by Peruzzi on Nov 6^th, 2008, 11:08am

Trond,

Try running AMS through the GUI and if you mouse around in the options, transient analysis menus and you'll find your answer. (At the moment I don't have access to it or I'd give you a specific answer, but I know the answer is in the menus.)

Good luck,

Bob

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by trond on Nov 7^th, 2008, 1:58am

Hello Bob,

I am using the AMS simulator via the ADE gui. In the AC analysis option I can specify the file to read in. When running the sim it does read the file (as it complains about some nodes) but it seems to performs a dc analysis anyways.

In Spectre I would just tell it to FORCE the ic conditions but in the AMS simulator under AC options there is no such option. Even when I manually edit the netlist to "force=node" . Weird.

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by trond on Nov 7^th, 2008, 6:53am

SOLVED!

I found the explanation in the sourcelink cadence page. I copied it below for easy reference.

Solution Number: 11089964
Difference between prevoppoint and readns options

Problem statement:
What is the difference between prevoppoint and readns options?

Solution:
prevoppoint - causes the dc analysis to be skipped completely by ac and use the previous solution (last point of transient) as a starting point.
readns - is just a starting point for the dc analysis to bias it towards a certain solution. With this option, nodes are initialized to the given values and then dc computed. This is entirely different from the above. Since the transient operating point was computed with some parameters changing in the circuit, it maybe entirely different from the actual dc stable solution. As such, dc could converge to a different stable solution independent of the nodesets.

This explains the difference in behavior between when readns is used and when prevoppoint is used.

Since ac does not have readic or skipdc option, If you want to use transient simulation information for your ac analysis then you have to run transient analysis before every ac analysis run and use prevoppoint option for ac analysis.

See also Solution 11218088.

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by Geoffrey_Coram on Nov 10^th, 2008, 9:49am

trond wrote on Nov 7^th, 2008, 6:53am:

Since ac does not have readic or skipdc option, If you want to use transient simulation information for your ac analysis then you have to run transient analysis before every ac analysis run and use prevoppoint option for ac analysis.

That seems wasteful. Have you looked at "readforce" ? Also, are you sure ac doesn't have a skipdc option (in the current version of the simulator) ?

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by trond on Nov 11^th, 2008, 3:46am

Geoffrey,

I agree, it is somewhat time consuming. But the ac analysis does not provide a skip dc option or a force node/readforce option with the AMS simulator set.

Cadence IC5141ISR20080523,
Spectre version 7.0.1

Regards,

Title: Re: have ac analysis pick up node/net ic conditions from a transient run in AMS
Post by Andrew Beckett on Jan 5^th, 2009, 8:00am

Given that usually with an AMS simulation you're going to want to do a time domain simulation anyway (otherwise why use AMS?), having to run the transient first is not really a problem, I'd have thought - since that's why you'd be running AMS...

You'd need to be a bit careful about using readforce (in a DC sim) followed by an AC analysis anyway, because I think you'd see the effect of the rforce resistors in the linearisation of the circuit (assuming you were able to do this with AMS).

Regards,

Andrew.