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Are Analog Design Tools Turning Engineers Into Simulation Jocks?

Over the years, the introduction of more complex tools has enabled much more capability in the circuit simulation arena. These new tools enable exploring circuit interaction to arrive at solutions to complex issues. The combination of the complexities of the models for these sub-micron technologies and the need to come closer to the idea of first-pass success is driving the need for simulation tools to do more and to explore various simulations not possible in the past.

In addition, the cost of producing products in these nanometer technologies is limiting the number of spins of silicon to obtain the correct yielding design at adequate margins for the cost of the product. The cost of production in these nanometer technologies is becoming prohibitive for the luxury of several spins of silicon to fix circuit issues. Getting it right the first time is even more important.

Issues that come from interactions on the IC and the complexity of the analog functions integrated along with large areas of digital circuits cannot be underestimated. These interactions create an environment where the probability of success on the first pass of production diminishes as we move down the technology path.

As a result of the integration of these complex analog and digital circuits into a single IC (in an effort to create embedded solutions on a single substrate), the need to simulate more of the embedded system and individual block interfaces more thoroughly is critical to first-pass success. These complexities require the need to simulate the statistical variations, as well as other complex circuit parameters, before the first silicon is released.

The desire to simulate these various interactions for higher first silicon success places more demand on using accurate simulation test benches as well as accurate transistor models. In my blog, Where Are Your Analog Design Models Accurate? I speak about how models do not always fit the size of transistors used in analog design. This means that the large number of simulations the designer chooses to run is only as accurate as the models allow — and trust me, even accurate models do not catch all of the interactions. Because of the inaccuracies in the transistor models and insufficient simulation test benches with proper interfaces and/or operating conditions, all of this tends to give weight to the old saying regarding simulations: “garbage in equals garbage out.”

Therefore, we as designers cannot be simulation jocks, but must understand the trends and have a clear understanding of what the circuit sensitivities are for the blocks being developed. In one of my previous blogs, What Happened to Napkin Sessions? I pointed out the need to complete back-of-the-envelope-style calculations to gain this understanding.

The use of these first-order calculations may not be accurate, but it can provide the engineer with critical insight into the trends expected in the circuit. Moreover, these first-order insights may provide some guidance to question the results of the simulations and to question whether the results make sense, or whether there are potential modeling issues.

What do I mean by simulation jocks? I, like many of you, have been in design reviews that show massive simulation results generated by complex tools. The engineer may present massive quantities of data without understanding the trends the data are suggesting about the sensitivities simulated in the circuit blocks. This is who I would call a simulation jock. Having said this, I do not mean to insult, but to call to our attention the potential for all of us at various times in our design cycles to become simulation jocks.

The tools and horsepower of the computer farms used to simulate circuits have enabled massive simulations once thought impossible to complete. These trends are great, but I fear that this capability can turn engineers into simulation jocks. How do we avoid becoming simulation jocks? I would like to suggest several ideas:

  1. We can learn a lot from looking at operating points in an analog circuit. Explore the transistor operating points and try to understand critical parameters where you know your circuits are sensitive to variation.
  2. Think temperature. How does you circuit vary with temperature? Can problems be avoided by using the correct temperature-compensated bias current or voltage?
  3. How far are the transistor operating points from unwanted regions once the circuit is operating in normal mode? Do you have adequate margins for node variations in normal operation?
  4. When you run your simulations, ask yourself if the results make sense under normal conditions. Do a mental “what if” exercise. For instance, what if this transistor's operating point changed by 10%-20%? Would the circuit performance be susceptible?
  5. Do not launch extensive corner simulations until you understand the sensitivities of the circuit under nominal operating conditions.
  6. Based on the understood sensitivities explored under nominal conditions, try to focus your simulations to explore those corners where you expect are most sensitive to variation.

Furthermore, we must always avoid the tendency to rely too much on the simulations and not spend the time analyzing what the data is suggesting when looking at the results. As engineers, we should accept that a little pessimism goes a long way in making great designs that are robust and that work in silicon the first time.

Have you seen similar issues at your company? How do you think we can avoid becoming simulation jocks?

17 comments on “Are Analog Design Tools Turning Engineers Into Simulation Jocks?

  1. TM123
    November 19, 2014

    Yes, well, if you have no idea of how analogue circuitry works, no number of simulation tools will assist you with a high success rate.

    Design simulation tools are just that, simulation tools. Nothing beat the real thing. However, being able to cobble together some virtual components to start the process without having to do heaps of calculations first, enables a quick proof of concept to be validated without having to spend too much time on it. After all time to market is the key thing these days.

    If you have no idea of what you want to achieve from an analogue circuit, or indeed, how analogue circuits work, you would find it very difficult to quicky design a new concept in any simulation tool.

    I find these tools invaluable. Firstly I am lazy, let the tools do the number crunching. Secondly, getting back to the design specs is something I do later. I must explain that when I design something it is usually a “one off” and will probably never be touched again – well by my hands only. I can write all the specs I like but few around here know what I am talking about anyway.

    Love these tools, use them often. I generally use LTSpice IV…

  2. Davidled
    November 19, 2014

    Most analog designer will work the circuit of small voltage range from 0 V to around 12 V. Any simulate tool supports any model with the injection of real environment. If designer might design high power electrical system, engineer should deal not only the system component but also cable's electric characteristics such as tension force, crush, bending and etc, when designer plans cabling system in real environment. It would be expected that tool provides the worst-case scenario worst –case level parameters of cable.  

  3. analogspiceman
    November 21, 2014

    Hi TM123, I like your post and its sentiment, especially how you note that efficiently crafted simulation allows the lowest possible entropy design process with the least possible expenditure of personal effort.  You seem to “get it,” but the author of this article, although not a nitwit, has apparently not yet developed a full apreciation of the power of analog simulation properly done and does not yet seem to fully appreciate the potential power of a well and realistically crafted simulation (which can replace one or two itterations of physical breadboarding).  He does, however, seem to have a well honed appreciation for the shortfalls of a half-a$$ed effort.

     

  4. Netcrawl
    November 21, 2014

    @Brandt great post! thanks for that, while tools for analog design are anything but simple, and while progress is constantly being made to improve such tools, the fact remains that analog has eluded the levels of automation achieved by their digital cousins. The master of level of description remains the transistor, and for this reason most functional verification efforts are performed at this level of abstraction, contrast this with standard digital methodologies where spinoff functional verification is performed at RTL level, relying on correct-by-construction methodlogies.

  5. Netcrawl
    November 21, 2014

    @Daej I agree with you, traditionally analog and digital design were separate and highly specialized disciplines with success determined by a clean handoff between an analog and digital teams.

    And there's something new and its coming, a new discipline called “mixed-signal verification” is emerging that requires skills in mixed-mode simulation, behavioral model generation and tes bench development.

  6. chirshadblog
    November 23, 2014

    @netcrawl: Well it's a good sign indeed. I guess it has the potential for reducing the unwanted noise behind the signals and make sure the signals do get transmitted are in high quality

  7. Myled
    November 24, 2014

    “Design simulation tools are just that, simulation tools. Nothing beat the real thing. However, being able to cobble together some virtual components to start the process without having to do heaps of calculations first, enables a quick proof of concept to be validated without having to spend too much time on it. After all time to market is the key thing these days.”

    TM123, you are right. It will display the system performance and its variants in output based on various inputs. In test mode its very interesting to carry out simulations at various levels, so without much efforts we can collect the outputs for different set of inputs.

  8. TM123
    November 24, 2014

    Myanalog, so very true – Often when designing for the environment I work in, it is impossible to breadboard to try things out. Simulation is the only way forward for me.

    Inside our vessels (where much of our electronics sit) is hostile (up to 10MV – yes 10 Million Volts), is full of gas (SF6), and takes a day or so to get in there after the nasties have been taken away. As I said, simulation is the only thing that works for me – bearing in mind I always need a fallback option should things turn to toast…

  9. Myled
    November 25, 2014

    “Inside our vessels (where much of our electronics sit) is hostile (up to 10MV – yes 10 Million Volts), is full of gas (SF6), and takes a day or so to get in there after the nasties have been taken away. As I said, simulation is the only thing that works for me – bearing in mind I always need a fallback option should things turn to toast…”

    TM123, this is of what type testing. Is it EMI or radiation testing? Normally for space devices we used do similar testing's and simulations in such environments. 

  10. geek
    November 25, 2014

    @Netcrawl: When you mention Mixed-Signals, are you referring to the combination of Digital and analog signals that will be processed at the same time? Or does the mix here refer to some other categorization of signals?

  11. TM123
    November 25, 2014

    Myanalog, we look after ion accelerators. We have 4: 10MV, 6MV, 2MV, 1MV. All of these are used for either IBA (Ion Beam Analysis) or AMS (Accelerator Mass Spectography) in some way or form for scientific research (60%) or commercial work (40%).

    SF6 is an insulating gase used widely in the electrical transmission industry, but we use it as teh insulating gas inside our tanks. While not poisonous, it will asphyxiate you if you breathe it in, and you need to hang upside down to remove it from your lungs. We take no chances, and don't go into a tank until air is certified breathing quality.

    Hope this clarifies for you.

  12. Netcrawl
    November 25, 2014

    @tzubair thanks for that, its all about connectivity testing between digital and analog design using very low level Analog behavioral model, mixed-signal design and verification are not entirely new ballgame. Engineers have been doing mixed-signal for years, however, it seems today that neither analog nor digital engineers are completely prepared to enter each other's area of expertise. 

  13. TM123
    November 26, 2014

    Netcrawl, digital is a simplified form of analogue, so why not go there?

  14. nasimson
    November 29, 2014

    @ Brandt: 

    Your six ideas are very simple yet overlooked. Will definitely apply these to my area of work in addition to sharing these with some engineer friends.

    Sometimes simple checks escapte the attention.

  15. geek
    November 29, 2014

    “. Engineers have been doing mixed-signal for years, however, it seems today that neither analog nor digital engineers are completely prepared to enter each other's area of expertise. “

    @Netcrawl: Is it because of the fact that each type of engineers have been too specialized in their own fields and haven't had a chance to study the alternate areas? Is it a gap in the way cirriculum has been designed, or is it because of some other reasons?

  16. Myled
    December 1, 2014

    ” we look after ion accelerators. We have 4: 10MV, 6MV, 2MV, 1MV. All of these are used for either IBA (Ion Beam Analysis) or AMS (Accelerator Mass Spectography) in some way or form for scientific research (60%) or commercial work (40%).”

    TM123, thanks for this info and so far am not familiarized with IBA and AMS. Thanks for educating us.

  17. Myled
    December 1, 2014

    “SF6 is an insulating gase used widely in the electrical transmission industry, but we use it as teh insulating gas inside our tanks. While not poisonous, it will asphyxiate you if you breathe it in, and you need to hang upside down to remove it from your lungs. We take no chances, and don't go into a tank until air is certified breathing quality.”

    TM123, hanging upside down…. horrible. These are certain situations where test engineers are struggling for perfection. As a space engineer we are also facing  similar issues during the vibration ad EMI testing.

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