Category Archives: Electronics Part Models
Is Your Circuit Simulator Just A Pretty Face? Five Reasons Why Simulations Are Not Sufficient For Design Validation
Jerry Twomey recently pointed out some pitfalls with math-based circuit analysis (“Academic Simplifications Produce Meaningless Equations,” 13 June 2012, Electronic Design.com.)
I agree with the general sentiments of Mr. Twomey, but would like to point out that there is a simple solution to avoiding the pitfalls he mentions: develop equations from component data sheets, not from academic simplifications. This is straightforward and will be discussed further in a future post.
Also, it should be noted that simulations are not some miracle cure-all elixir. Indeed, simulators are also math-based creatures: SPICE and its cousins simply grind out numerical solutions to the multitude of hidden equations that are buried beneath their pretty graphical interfaces.
So what’s the problem with simulators? A lot. For example,
1. Because simulator math is hidden behind the user interface, simulators don’t promote engineering analysis (thinking). To the contrary, they promote lazy tweak-and-tune tinkering.
2. Because simulator component models are typically very complex, the interactions between important variables are usually obscure, if not downright unfathomable. Obscurity does not promote engineering understanding.
3. Simulator results typically do not provide insight into important sensitivities. For example, can your simulator tell you how sensitive your power supply’s thermal stability is to the Rds(on) of the switching Mosfet, including the effects of thermal feedback?
4. A simulation “run” is not an analysis, but is instead a virtual prototype test. Yes, it’s better to check out crappy designs with a simulator rather than wasting time and money on building and testing crappy hardware. So simulators have their place, particularly when checking out initial design concepts. Eventually, however, hardware testing is required to verify that the simulator models were correct. And you will still need to do a worst case math analysis to determine performance limits, and to confirm that desired performance will be maintained in the presence of tolerances and aging.
- Proper Design Validation = Testing/Simulations + Analysis
5. Simulators don’t really do worst case analysis. Yes, you can use a simulator to implement a bunch of Monte Carlo runs, but valid results requires (a) identification of all of the important parameters (such as Rds(on)), (b) assignment of the appropriate distributions to those parameters (such distributions are typically not available), and (c) the generation of enough runs to catch errors out in the tails of the overall resultant distribution (and how many runs should you do? Hmmm…).
- Monte Carlo is not a crystal ball. It only shows you the production performance you will get if all of your assumptions were correct, and if you did enough runs.
- The knowledge required to determine the number of runs requires an exhaustive study of the circuit’s parameters, distributions, and interrelationships (not practical), or a knowledge of the limits of performance.
- But if you know the limits of performance, then why do you need a Monte Carlo analysis? You don’t. You can skip it altogether and go directly to a math-based Worst Case Analysis.
For further insights into math-based Worst Case Analysis versus simulations, please see “Design MasterTM: A Straightforward No-Nonsense Approach to Design Validation.”
(c) 2011 Design/Analysis Consultants, Inc.
Newsletter content may be copied in whole or part if attribution
to DACI and any referenced source is prominently displayed with the copied material
This Issue: NEWS BITE: Astronomer Discovers Giant Eye Staring At Earth! / DESIGN MASTER: New Pricing Structure / COMPONENTS: The Most Popular Op Amps? It’s A Secret! / CONCEPTUAL DESIGN: Look At The Big Picture / COOKING & MEASUREMENTS: Why Engineers Get It Right
NEWS BITE: Astronomer Discovers Giant Eye Staring At Earth!
The Helix Nebula, “Hubble’s Best Photos” by Lauren Effron, Discovery News
DESIGN MASTER™: New Introductory Pricing Structure
Effective for 2011, we’re changing our introductory price structure for DM to a flat fee of $85/copy, single user ($25 for upgrades), and providing modeling packages in DMX template format at an added charge. DMX is a DM feature that allows expertly-designed “fill in the blanks” templates to be used for thorough and efficient worst case analysis. Click here for details.
With the introductory pricing structure, DM will only be supported via email, and only with regard to DM operation, not circuit or part modeling. For those who may need help with modeling, please see DM V8’s expanded help content, and refer to the available DMX templates. If more extensive assistance is required, please contact us with regard to a DM training seminar, or for the provision of custom circuit or part models.
Offer expires 30 April 2011.
We were looking for some guidance in building up op amp models for our DMX templates, so we asked several major semiconductor vendors this question: “What are your most popular op amps?” Surprisingly, although the technical contacts said they would like to provide the data, they couldn’t; their sales departments considered the data proprietary, due to competition concerns.
Therefore it seems our alternative is to create our own “favorite” lists. If you have some preferred op amp (or other analog IC) part numbers that you would like to share with the DM design community, please send them in. We’ll post them in future issues of this newsletter. The data will also provide guidance for our development of part models.
In the 12 December 2010 issue of Electronic Design, Louis E. Frenzel discusses some modern versions of the ubiquitous 555 timer IC (“And You Thought The 555 Timer Was Dead?“).
In that article, Mr. Frenzel also refers back to an earlier article of his , in which he asks, in essence: why in the world does the 555 timer still exist? (“The 555: Best IC Ever Or Obsolete Anachronism?“, 12 Dec 2007, Electronic Design).
Might we suggest an answer? The 555 provides a simple common function in an elegant low-cost manner. This does not mean it is always the best solution (Mr. Frenzel mentions some alternatives), but it certainly is an attractive option, particularly for circuits that don’t contain a uP.
I suspect that Mr. Frenzel was deliberately being provocative to stimulate conversation, particularly with his implication that all modern products contain a uP. If that were true, then yes, why add a part when the uP can provide the timing capability for free? But a huge number of products do not contain uPs, because they simply don’t need uPs. In fact, avoiding a uP has additional advantages in addition to cost avoidance, including better reliability (by keeping things simple), and by eliminating radiated emissions noise. We have saved some of our customers a lot of money by showing how a dirt-cheap non-uP circuit can achieve the functions they need, while also avoiding the noise issues that would require EMC compliance certification.
Mr. Frenzel quotes from a 1997 article, “…the 555 is dated mainly because it is no longer compatible with the mostly low-voltage (less than 5 V) circuits in use today. Furthermore, it consumes excessive power compared to today’s circuits.” But this ignores the fact that the 555 has been continuously reincarnated over the years with better performance, operating at lower voltages and current, and available in various modern packages.
From a broader perspective, we think it’s important to stand back and look at the big picture during the concept design phase. We shouldn’t just indulge our prejudices, and grab the parts and designs we’re familiar with. Rather, we should ask: what’s the optimum design? Among other considerations, this means we need to challenge our assumption that newer means better. Maybe, maybe not. There are lots of situations where older-technology designs are much better for certain applications — all things considered — than their modern counterparts. A few examples:
- 4000 series CMOS: these senior ICs are great solutions for low cost, low speed, noisy industrial applications.
- Mechanical relays: ancient, but like the 555, continuously modernized, and still the best solutions for numerous higher power industrial designs.
- Older IC technologies in general: lower cost, less EMC issues, and less susceptibility to electromigration, which is becoming a significant concern with modern minimal-geometry ICs.
COOKING & MEASUREMENTS: Why Engineers Get It Right
“Cooking is awesome because it’s applied science. Biology, chemistry, physics, they all come into play when you’re cooking. I don’t know that I necessarily do anything specialized. I know that some of my more particular habits come out when I’m cooking. I very rarely cook meat, for instance, without a thermometer. I know there are these old-school cooks who would turn their noses up at me because they can tell by giving their roast a touch or by poking it in the right spot and seeing what color the juices are, they can tell when it’s ready to come out of the oven. Well, I can tell when it’s ready to come out of the oven when it’s the right temperature and that is okay with me. You know, my bread—before I invested in a thermometer, sometimes it was too dry and sometimes it was gummy in the middle, but now it’s right every time. I use a kitchen scale for almost everything. If you think about a cup of flour: If I measure the cup of flour it may weigh 4 ounces; if you measure your cup of flour, it may weigh 5 ounces, because the flour is compressible. So you achieve a degree of precision when you measure things that way. Nothing drives me crazier than when you look at a recipe and they call for 1 large onion. I’m guessing that one large onion in Texas is massively different that one large onion in southeast Missouri. You know I can get an onion from the store here that weighs almost a pound. I have no idea if that’s what they meant. So my life got substantially more precise when I was able to start making notes, you know—this much onion worked, this much onion didn’t.”
–April Woods, as quoted in “Geeks Cooking: April Woods, the Hungry Engineer,” an interview by Susan Hassler in IEEE Spectrum
The DMeXpert™ (DMX) Edition guides the user with pop-up instructions, component selection lists, standard part values, important formulas, and a variety of other tips that are activated when entering a Formula cell. It’s like having a design/analysis expert at your side.
DMX is ideal for the design engineer or project manager who needs quick and efficient “fill in the blanks” design validation in minutes rather than weeks. Based on DACI’s thorough and practical worst case analysis plus (WCA+) methodology, DMX uses expertly designed templates to facilitate analysis of a variety of standard circuits.
In addition to its standalone function, DMX is an integral part of the just-released Version 8 of Design Master’s Professional Edition. DMX facilitates efficient organization and selection of circuit and part files, and also provides easy access to an organized array of tutorials and DMeXpert tips.
Circuit templates, part files, and help files can be accessed and edited by using the Professional Edition. This allows an organization’s design experts to use the Professional Edition to create DMX templates tailored to the needs of the company.
For more information, please click here.