December 15, 2019

Copy a senior's design schematic to optimize circuit design with simulation tools

My colleague and I chatted for a day at the hotel bar. We have met several customers before. We are all thinking about a question: how can the engineers we met know almost nothing about analog technology? I think that the knowledge of the simulation technology that these recent graduate design engineers have is different from the knowledge that our predecessors learned when they graduated. That's all. My colleague said: "If they don't know these principles, and the university doesn't teach them, what will happen? These newly graduated engineers are destined to only find their predecessors to copy a simulation design schematic? ”

The principles that my colleague refers to are not just the basis for op amp circuit design, but some things that are not explained in depth in the university classroom: non-ideal components, circuit noise causes, error budgets, and error analysis.

With shorter design cycles, fewer resources, and more intense global competition, engineers need to make effective use of the tools at hand to help achieve faster product designs. In the design process, they need to have confidence in the tools they use. They believe that these tools can guide them to complete the design work correctly, because they may not have relevant design experience or may not have received relevant training.

This is why semiconductor manufacturers often get involved in design tools [1]. For example, TI's SwitcherPro [2] is a program that helps people design switching power supplies, while FilterPro [3] is an active filter design program. Neither is an emulator, but a design generation tool. Engineers who lack design experience can use these tools to help them overcome the inexperienced problems, and some experienced engineers can save a lot of time using these tools.

More than 20 years ago, these design generation tools began to be used by people. Looking back, when I was a "novice" engineer, my knowledge of design generation was from textbooks, the Burr-Brown series of simulation manuals (now out of print for many years) or the "modern electronic circuit reference manual" I admire [4] ( A 3 inch thick world electronic publication circuit summary, sometimes referred to as the circuit "design manual") was learned there.

The field of simulation is so vast that few people are familiar with all its branches, so learning the circuits of predecessors is necessary, provided that you have mastered the basics of circuit analysis.

Semiconductor manufacturers will also provide some reference designs. These designs are far from the basic circuits, they include a complete bill of materials and printed circuit board (PCB) layout, and may include the firmware or software needed to run the entire design. There is no doubt that using these reference designs or designing guidelines will allow you to get started quickly.

Since the early 1970s, SPICE [5] has been a powerful tool in the circuit designer's toolbox. This is especially true for the development of integrated circuits, because the cost of manufacturing prototypes is extremely high, and simulating the behavior of circuits before moving to production is a huge advance in circuit design.

It is so common that most electronic component manufacturers offer SPICE macro models of their devices to help board-level designers simulate their circuits before they are put into production. The IBIS model and software developed from the SPICE model allows PCB designers to verify the signal integrity of their PCB designs [6].

In both cases, it is important to note whether the circuit is "simulated" or "verified." These tools can help you predict how well your circuit has been designed, but they won't help you design your circuit. Although engineers still have to design their own circuits, the simulator can tell them how to design.

TI offers a free SPICE emulator called TINA-TI [7]. How to use this program most effectively, because the experience of using the simulator is not consistent, from experts to novices, at all levels. When TINA-TI didn't tell them that their power supply was connected back to the op amp, some engineers would be at a loss—the emulator wouldn't know? The answer is no, the emulator doesn't know if it's wrong, and it doesn't know that doing so will make your PCB components smoke.

SPICE is nothing more than a computer program that performs the mathematical operations required for circuit analysis. It is usually faster than pencil and paper. "Usually" is because SPICE is very good at certain things, such as: linear circuits, while some things are not good at, such as: some of the switching circuits that will be available in modern power supplies. There is a problem with using a fairly long time to simulate a switched-mode power supply: the analog programs that have been programmed need to be optimized to simulate these power circuits.

Other dedicated emulators are used to solve RF circuit problems. The application of "simulation" is extremely broad, so an analog solution may not be the best choice for a complete system simulation.

Although simulation has many benefits, the circuit or system design is incomplete until a thorough error analysis is completed. This requires considering all non-ideal characteristics of the component and the environment and determining their impact on the operation of the circuit. We generally use spreadsheets or manual calculations to do the job. Compare the results of the analysis to the requirements, and let you believe that the circuit being designed will meet these requirements under all operating conditions and manufacturing conditions; or, it will tell you that you need to try other methods.

Combining all of these different tools—design generation, simulation (possibly using several different simulation engines), and error analysis—as a tool creates an “expert system”. Using only the basic requirements of the circuit, the system can construct or suggest a circuit or system design, then allow the circuit to be simulated and optimized by changing the value of the circuit components. Finally, after the circuit error analysis is completed, a complete applicability assessment of the circuit to meet various requirements is completed.

Now, this method of system development is becoming a reality. In the field of mixed-signal IC design, large CAD suppliers are already working on this in order to shorten the simulation time [8]. Some vendors combine existing tools or mathematical modeling systems to try this design approach at a higher level. However, bringing these expert systems to board-level and system-level designers may be semiconductor suppliers and their application engineers.

The most exciting thing about Texas Instruments' acquisition of National Semiconductor is that we are finally able to see the true content of the WEBENCH® Designer tool suite [9], which in many respects is the expert system I have described. It can dynamically generate specific required circuits, calculate component values, perform simulations (based on mathematical models and SPICE) and optimize, and perform error analysis. As mentioned earlier, there is so much to learn in the analog world that it is “endless”.

Such tools may become the primary tool in the board-level and system-level designer toolbox. There is nothing wrong with "copying your father's design schematics", provided you have the tools to apply these schematics or to properly understand the underlying design principles. These tools should guide you through how to optimize your circuits to solve the problems you are experiencing and give you an idea of ​​what happens when you put these circuits into production—and be able to do so quickly enough so that you can allow them. Try a number of different methods within the time limit. Although there is still a way to go, it is not far from our goal.

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