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MC in cadence

RAO VINAY

Hi all,

I am new to monte carlo analysis features and I am unable to interpret results from that simulation in cadence. I have designed band-gap current reference and I want to know how the resultant current varies with process and mismatch. I have a set-up and had few runs. For some runs, it generates Gaussian distribution but for some other runs it generates only a single bin in histogram where all the samples are in one particular range. I am having few following doubts about MC analysis.

1> How do we go about MC analysis? Whether we have to start from some definite seed, where we have to do some trial and error and find out which seed gives the best? I think in my case, it may be generating all those samples which are very close, so all the samples' response is concentrated in one point.

2> What is child analysis? Whether every general analysis such as DC, AC, Tran, SP etc. are called child analysis?

3> How samples are taken when I have a selected child analysis. Ex: When I am varying temperature from -10 to 100 Deg C, each other samples are considered at which temperature? It is important because response is a scalar quantity and don't know those responses are w.r.t which temperature value.

4>How to plot or record responses for AC, Tran, SP etc for different samples ? Ex: I want my resulted current to be plotted w.r.t temperature along the x-axis but for different MC samples.

Thanks and regards,

Vinay Rao.

Parents

  • I'm assuming you're running Monte Carlo via ADE XL (you didn't give any information such as the version you're using, the tool you're using - you might not even be running spectre for all I know).

    So first of all, I'd recommend you start with the Rapid Adoption Kit for ADE XL. Go to and then at the top of the page, Resources->Rapid Adoption Kits. Then Custom IC and Signoff, and finally you'll find a workshop for ADE XL.

    Then let me answer your questions:

    1. In general you shouldn't need to worry about the seed. Random is random and one seed is no better than any other. The only reason seed is exposd is to allow you to repeat an experiment - and even that is a bit pointless because the seed from ADE XL is by default set to 12345 and so will give you repeatable results anyway. Still, some people seem to want to be able to set the seed, despite the fact that it is pretty unimportant.
    2. In the simulator, monte carlo is a type of analysis which effectively performs a sweep, where a set of statistical variables are varied in each point in the sweep. For each point in the sweep you want to have a set of analyses run - these are the "child" analyses. If you look at a monte carlo netlist statement (as seen from a netlist produced by ADE XL), or from "spectre -h montecarlo" in the examples, you'll see analyses specified within curly brackets - these are the child analyses. With ADE XL, all the analyses set up in the test are put within the {} for the monte carlo analysis - so all analyses are child analyses.
    3. Temperature is not generally a statistical parameter. So if you are varying temperature via a corner analysis in ADE XL (or a surrounding sweep if running spectre standalone) - it will run the simulations at whatever temperature that corner is at - not sure where you are coming from with this question.
    4. For this, you might want to see my answer earlier today: (which seems to have vanished - so I will need to post it again).

    Kind Regards,

    Andrew.

Reply

  • I'm assuming you're running Monte Carlo via ADE XL (you didn't give any information such as the version you're using, the tool you're using - you might not even be running spectre for all I know).

    So first of all, I'd recommend you start with the Rapid Adoption Kit for ADE XL. Go to and then at the top of the page, Resources->Rapid Adoption Kits. Then Custom IC and Signoff, and finally you'll find a workshop for ADE XL.

    Then let me answer your questions:

    1. In general you shouldn't need to worry about the seed. Random is random and one seed is no better than any other. The only reason seed is exposd is to allow you to repeat an experiment - and even that is a bit pointless because the seed from ADE XL is by default set to 12345 and so will give you repeatable results anyway. Still, some people seem to want to be able to set the seed, despite the fact that it is pretty unimportant.
    2. In the simulator, monte carlo is a type of analysis which effectively performs a sweep, where a set of statistical variables are varied in each point in the sweep. For each point in the sweep you want to have a set of analyses run - these are the "child" analyses. If you look at a monte carlo netlist statement (as seen from a netlist produced by ADE XL), or from "spectre -h montecarlo" in the examples, you'll see analyses specified within curly brackets - these are the child analyses. With ADE XL, all the analyses set up in the test are put within the {} for the monte carlo analysis - so all analyses are child analyses.
    3. Temperature is not generally a statistical parameter. So if you are varying temperature via a corner analysis in ADE XL (or a surrounding sweep if running spectre standalone) - it will run the simulations at whatever temperature that corner is at - not sure where you are coming from with this question.
    4. For this, you might want to see my answer earlier today: (which seems to have vanished - so I will need to post it again).

    Kind Regards,

    Andrew.

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