With Today’s Design Tools, You Don’t Have To Copy Your Father’s Schematics

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It had been a long day and my colleague and I were unwinding at the hotel bar. We had visited several customers and were reflecting on how the young engineers we had met that day knew very little about analog.

I pointed out that graduating engineers I had met everywhere didn’t have the same analog knowledge that us old guys had when we graduated. My colleague said, “If they don’t understand these principles, if universities aren’t teaching this stuff, then what will happen? Will these new engineers just be doomed to copy their fathers’ schematics?”

The principles my colleague was referring to weren’t just the basics of designing, for example, an op-amp circuit, but the things that aren’t covered well in academic classes: non-ideal components, accounting for circuit noise, and doing error budgets and error analysis.

With shrinking design cycles, reduced resources, and increased global competition, engineers need to take advantage of all the tools at their disposal to help them realize working designs faster. They need to do this with confidence that the tools they use will steer them right, because they may not have the experience or education to guide them.

The Role Of Design Tools

This is where semiconductor manufacturers have often stepped in with design tools.¹ For example, Texas Instruments has SwitcherPro,² a program that helps people design switching power supplies, and FilterPro,³ a program for active filter design. They aren’t simulators. They’re design generation tools. These are the tools that less experienced engineers need to lean on to help get them past their inexperience, while the more experienced engineers can avail themselves of the time-savings these tools provide.

These design generation tools have only started to become available in the past 20 years or so. Back when I was a freshly minted engineer, design generation for me came from textbooks, or the Burr-Brown series of analog handbooks (now many years out of print), or my highly prized Modern Electronic Circuits Reference Manual,⁴ a 3-in. thick compendium of circuits from electronics publications from all over the world—sometimes called a circuit “cookbook.”

Yes, I was copying someone’s father’s schematics. (My own father was a mechanical engineer, so I couldn’t copy his!) Analog is such a broad area that very few people have expertise in all of its subtopics, so learning from the circuits of those who came before is necessary, as long as you’re equipped with the basic understanding of circuit analysis.

Semiconductor manufacturers also provide reference designs. They go beyond the basic circuit to include a complete bill of materials and printed-circuit board (PCB) layout and may also contain firmware or software needed to make the entire design operate. Drawing upon these reference designs or cookbook circuits can certainly get you a much faster start toward a working design.

Spice Steps In

Since the early 1970s, Spice⁵ has been a powerful tool in the circuit designer’s toolbox. Especially for IC development, where the cost to build prototypes would be prohibitive, the ability to simulate a circuit’s behavior before committing it to production has enabled significant progress.

Its ubiquity is such that most manufacturers of electronic components provide Spice macromodels of their devices to aid board-level designers to simulate their circuits before committing to a PCB fabrication. Evolving from Spice models, Input/Output Buffer Specification (IBIS) models and software that can use them allow PCB designers to validate signal integrity in their PCB design.⁶

In both cases, you’ll note that I said, “simulate” or “validate” a circuit. These tools can help you predict what will happen with a circuit you already have, but they really don’t help you design the circuit in the first place. Engineers still have to design the circuits, yet many expect the simulators to tell them how to do that.

My employer, Texas Instruments, offers a free Spice simulator called TINA-TI.⁷ We get questions about how to best use the program from customers who vary in their level of experience with simulators from experts to novices.

I’ve had some engineers be very upset when TINA-TI won’t tell them that their power supplies are connected backward to an op amp—shouldn’t the simulator know that? The answer is no, the simulator doesn’t know that would be wrong and could result in a smoking part on your PCB.

Spice is nothing more than a computer program that executes the mathematics required to analyze a circuit, usually faster than you could do it with a pencil and paper. I say “usually” because Spice is very good at some things, like linear circuits, and not so good at other things, like switching circuits that you might find in modern power supplies. Simulating switching power supplies in a reasonable amount of time has been such a problem that many simulation programs have been created that are optimized to handle these power circuits.

Other domain-specific simulators have been created to address the unique challenges of RF circuitry. “Analog” covers an extremely wide range of applications, so one simulation solution likely won’t be optimal for a complete system simulation.

Putting It All Together

Even with the benefits that simulation brings, no circuit or system design is complete until a thorough error analysis is done. This involves accounting for all the non-ideal characteristics of the components and environment and determining how they affect the operation of the circuit.

This typically has been done using spreadsheets or hand calculations. Comparing the results of this analysis to the requirements will allow you to be confident that the circuit designed will meet those requirements under all operating and manufacturing conditions, or it will tell you that you need to try a different approach.

Bringing all of these various tools together—design generation, simulation (perhaps using several different simulation engines), and error analysis—into one tool leads to the creation of an expert system.

Using just the basic requirements of the circuit, such a system could create or recommend a circuit or system design, then allow simulation of the circuit and the ability to optimize it by changing circuit element values. Finally, an error analysis of the circuit would be performed, allowing a complete evaluation of the suitability of the circuit to meet the requirements.

Such systems are just now beginning to become possible. In the mixed-signal IC design space, the big CAD vendors have been working on something like this, with an eye toward reducing simulation time.⁸

Some vendors have combined existing tools or mathematical modeling systems to attempt this at a higher level. But it will likely be the semiconductor vendors, and their applications engineers, who will bring these expert systems to board- and system-level designers.

One of the most exciting aspects, for me, of the acquisition of National Semiconductor by Texas Instruments was the chance to get a peek “under the hood” of the WEBENCH Designer Toolset,⁹ which in many areas has become this expert system I described.

WEBENCH dynamically generates a circuit from given requirements, calculates component values, allows simulation (based on mathematical models as well as Spice) and optimization, and performs error analyses. As noted earlier, there are so many areas to cover in the analog world that it may never be “finished.”

Tools like this are likely to become essential components in the board- and system-level designer’s toolbox. There’s nothing wrong with copying your father’s schematics if you have the tools that let you apply those schematics or the underlying design principles in them correctly.

The tools should be able to guide you on how to optimize those circuits to solve your own problem at hand and let you know what to expect when you put those circuits into production—and do that quickly enough so you can explore many different possibilities in the time constraints allowed. We’re not there yet, but it is closer than ever before.

References

“Mixed-Signal Tools Evolve To Accelerate Complex Designs,” by Don Tuite, Electronic Design, June 23, 2011: http://electronicdesign.com/article/analog-and-mixed-signal/Mixed-Signal-Tools-Evolve-To-Accelerate-Complex-Designs.aspx
Download a free copy of SwitcherPro: www.ti.com/switcherpro-ca
Download a free copy of FilterPro: www.ti.com/filterpro-ca
J. Markus, Modern Electronic Circuits Reference Manual, McGraw Hill, 1980
http://en.wikipedia.org/wiki/SPICE
“IBIS and Spice timing mismatches,” by Bonnie Baker, EDN, March 15, 2007: http://www.edn.com/article/460123-IBIS_and_Spice_timing_mismatches.php
Download a free copy of TINA-TI: http://www.ti.com/tinati-ca
“Contrasts Mark Analog Design Tool Use,” by Don Tuite, Electronic Design, October 24, 2011: http://electronicdesign.com/article/analog-and-mixed-signal/Contrasts-Mark-Analog-Design-Tool-Use.aspx
Download a free copy of WEBENCH Designer Tools: http://www.national.com/en/webench/

Rick Downs is the signal chain applications manager for Texas Instruments’ Analog eLab, which provides analog design tools online. He received his BSEE from the University of Arizona and holds four patents. He also has authored more than 45 articles and application notes and has prepared and delivered many seminars on data acquisition.