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So You Want to Be a Supermodel-ler

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Having a circuit simulator with tons of models is a great thing. But, what do you do when you need to quickly create a new model for a component that is not in your library? Well, you could break out your Spice or VHDL-AMS modeling manual and start hand-coding your model. But wait. You are a circuit designer, not a “Supermodel-ler.” You would rather focus on getting your circuit working than spending time writing models.

What you need is an easy way to generate a simulation model that represents key component characteristics found in the component's datasheet. One way to generate such a model is to create a look table or piece-wise linear (PWL) model that computes an output (y) based on an input (x). Creating a model by hand this way can be tedious and error-prone. First, you need to look at the curve on paper, move to several points on the x-axis, draw a line up from the x-axis to where it intersects the curve, and then determine the y-value of the curve for that value of x. You copy this information to paper or a spreadsheet and then hand-enter these values into an interpolating look-up table or PWL model.

An easier, faster, and less error-prone approach would be an automated tool that would overlay the datasheet curve with an automatic curve tracing program. In this program, you would then use the mouse to select points along the curve. The program would capture the points and automatically generate the appropriate model. After capturing the curve data you should be able to define what physical relationship the curve is modeling. For example, your curve might represent such relationships as:

  • Current versus voltage
  • Voltage versus current
  • Voltage versus time
  • Current versus time
  • V/I resistance curves
  • I/V conductance curves
  • Gain blocks and voltage limiters where current is not flowing between ports
  • Voltage versus Voltage-Voltage curves where a separate port selects the curve such as a multi trace where the different curves represent different operating temperatures

Once the model has been created you can bind the model to a schematic symbol and use this custom model for simulations. The particular approach being discussed here generates custom VHDL-AMS models that can be combined with SPICE models, schematics and VHDL digital models in a unified simulation environment.

Watch a video showing you how to create your own custom VHDL-AMS models here. Then let us know if this proves useful to you.

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12 comments on “So You Want to Be a Supermodel-ler

  1. bjcoppa
    June 19, 2013

    SPICE models noted in this blog are critical and a normal function for design and process integration engineers in the microchip industry. It is a handy software skill to own as it is predominant across the board. There is less and less hand-waving these days in circuit design but making the designs a reality is a struggle amid the lack of extreme UV lithography for next-generation devices.

  2. RedDerek
    June 19, 2013

    I believe PSpice had this type of model input, but not with as nice interface the video shows. A few additional comments as to what I see could improve things.

    1. Use a cross-hair type cursor so that one know where the marker will b placed.

    2. Is one able to enter data at different temperatures to get a model that reflects temperature data as well?

    3. Will this work for BJT, MOSFETs, and other active devices? Granted that one will have to specify the conditions of each of the curves on the transistor.

    Would one be able to export out the model to be ported into other simulators?

    Be even better if this could be something interactive on line where by one can upload the curve, click on the points and have a model to use spit out. With everyone entering in data based on part numbers and allowing people to pull up the model, it would be a killer on-line crowd-sourcing hit for those into simulations. I would jump in and add in data. Heck, even differences between manufacturers on a MMBT2222A could be shown on one curve. The possibilities become endless.

    Oh, if you do get this started on line, can I get some credit. I would love to start something like this for people to use. Parts can be broken down by manufacturer, process, etc. Then, once enough special data is developed, a full model set could be sold designers so they could then simulate with accurate models.

  3. eafpres
    June 19, 2013

    It is interesting how rare a good UI capability is to simply draw the behavior and capture it as a model (table).  Recently I was doing some work on dynamic simulation models, mainly for business analysis, although in principal the tools could be used for anything.  I used a very simple web-based tool that had this feature.  You could just draw a dynamic behavior and it would store it in a table.  You could edit and fine tune it.  You did not need a closed for equation or set of equations.

    How many times have you been talking over your favorite adult beverages when someone draws a response curve on a napkin and says something like “if I could get that I would buy a milliion of them”?  Now at least you can get that into a simulation post-haste to see if it would be well behaved with the rest of the circuit.

    Now, if you could just give us a tool that takes that parametric sketch on a napkin and designs the corresponding analog part…

  4. Via Monger
    June 19, 2013

    @RedDerek – 

    1) Great idea on the cross-hair type cursor. Might be nice to have a cursor that spanned all the way the the x & y axes & showed the x&y value at the cursor…

    2) Yes, you can enter different data at different temperatures. This is the multi-port model with one of the inputs being temperature that selects the appropriate temperature curve. This other input can also be some curve selecting paramater other than temperature.

    3) Yes, you can us this kind of curve tracing for BJT, MOFSETs and other active devices. If it gets too complex a model should probably be hand coded but this is a great way to create a simple table or curve-trace model for parts “not in the library.”

    The generated models will work in any VHDL-AMS compliant simulator.

    Tool isn't online at the present time. At present, ViaDesigner is a PC desktop tool.

    Design and model sharing is supported at community.viadesigner.com.

     

  5. Davidled
    June 19, 2013

     Long time ago, I was told that owner's major was mechanic engineering.  He invented model in the Napkin and calculated all dynamic behavior based on thermal dynamic theory. Now, he is CEO in the company that a few thousand employees are working.

  6. Vishal Prajapati
    June 20, 2013

    This can be a ice breaking tool for the simulation softwares. We never had this kind of freedom and ease to work with simulation models. Manual work can take up long hours to make a similar model.

     

    The same functionality should be available with SPICE also. And again scope of improvement will always be there in all the products, but having a breakthrough like this is a definite welcom.

  7. Brad Albing
    June 20, 2013

    @eafpres – Now we need that desktop 3D printer we discussed in another blog comment thread a few weeks ago so we could go from napkin sketch to device.

  8. Via Monger
    June 24, 2013

    @Brad – “Desktop-like 3D printing” of analog chips seems a bit far off (for now, I know you were joking). We do want to provide software tools and configurable analog technology that makes analog integration as easy, affordable and timely as printed circuit board design.

    • If you could design a mixed-signal circuit on your desktop
    • Simulate the design
    • Auto place and route the analog and digital
    • And, have prototypes fabricated in 4 weeks
    • For a few hundred dollars per prototype

    That would be pretty cool.

    We just want to be the “3D Printing of Analog Chip Design Company”

  9. Brad Albing
    June 24, 2013

    @Via Man – you're right, at that price and that time frame for delivery of parts, that would be pretty cool.

  10. WKetel
    June 26, 2013

    I see a problem with generating a table or equation from a printed curve, or perhaps any curve that has existed as a PDF file. That problem is distortion and inaccuracy. In addition those curves may not accurately match real operation at extreme conditions. And once again, the results of a simulation are usually no more accurate than the input data and the simulation model.

  11. Via Monger
    June 27, 2013

    @WKetel – You make good points. A model created with curve tracing is certainly no more accurate than the curved used for tracing. And, such a model may or may not comprehend extreme corner cases. The ViaDesigner model generator is a fast, easy and reasonably accurate way to generate an initial high-level model. You are right, this type of model should not be considered as a golden, sign-off ready model but such models are very handy when you don't have a model that even approximates the behavior you are looking to simulate.

  12. WKetel
    June 27, 2013

    Via Man, you are right. It is important to keep those limitations in mind.

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