Triple play
Production test, bench work, and Web-based simulation deliver quality op amps and applications support.
Rick Nelson, Chief Editor -- Test & Measurement World, 8/1/2004
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Read the August 2004 features: Find reflections in glass Analog performance guaranteed by test For more information on ATE, visit www.tmworld.com/ate. |
It's the applications support portion of the picture that's readily visible to outsiders such as customers and editors. Everyone's familiar with the detailed data sheets that provide a complete picture of device operation, along with circuit examples that guide a customer step by step toward a successful design: You just choose an appropriate circuit type and plug your specifications—such as gain and frequency response—into some simple formulas to choose the resistors and capacitors you need. Then, you breadboard your design and use your bench instruments to verify that it works.
This paper data-sheet part of the process is now giving way to Web-based applications tools that threaten the honored place that bench instruments have earned in the analog designer's lab. In a visit to Test & Measurement World late last year to introduce the Amplifiers Made Simple tool for the company's Webench design environment, Phil Gibson, National's VP for Web business and sales automation, and Erroll Dietz, the company's VP for amplifiers, intimated that a lab bench cluttered with scopes, meters, soldering irons, and hard-copy data-sheet volumes would be history, with jumbled bench tops giving way to tidy, disembodied simulations accessible via desktop computers (Ref. 1).
Concerned for the future of the equipment we at Test & Measurement World make our living writing about, I visited NSC's headquarters here to find out what role test equipment plays. Gibson provided reassurance: "None of us would advocate that our Web tool is a replacement for bench work. The idea is to make the engineer as productive as possible." Web tools can handle 90 to 95% of design decisions, he said, freeing the engineer to focus on the 5 to 10% of decisions that demand more skill and creativity.
Instrumentation's roleNSC's customers aren't the only ones who use a range of instrumentation to ensure quality designs: NSC itself deploys a range of test tools, beginning with initial op-amp design. To learn what roles instrumentation plays, I asked a team of NSC engineers to describe the evolution of an op amp, from product conception through high-volume manufacturing and applications support.
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Wanda Garrett, technical communications manager; David G. Wheelwright, product and test engineering manager; and Benjamin Lee, applications and systems development engineer, of National Semiconductor. |
The design process takes place under the management of design director Arie Van Rhijn. Van Rhijn, who has a Master of Science degree from Delft University of Technology in the Netherlands, joined NSC in 1992 as a circuit design engineer. In 1996 he created an R&D center in the Netherlands, which currently consists of more than 20 engineers. In 2002, he assumed his present position, in which he is responsible for the company's worldwide amplifier R&D activities.
The process begins, he said, with specification of an amplifier product that's realizable from a technology standpoint and feasible from a marketing standpoint. The engineers then deploy an automated design flow—Cadence Design Systems is the company's primary EDA vendor—to perform transistor-level simulations and physical layout.
Verification is critical at this stage, Van Rhijn said, because of mask-set costs extending beyond $200,000. Complicating the process is the analog world's lack of the digital world's formal design tools that can provide a nearly foolproof conversion of a high-level description into a physical transistor layout. For op amps, performance and package constraints often dictate custom layouts that defy automation.
The receipt of prototypes is when test and measurement begins in earnest—at the bench and on production test equipment. Lab tests verify that early prototypes behave as expected. Tests on production equipment verify that the company can test the devices at high volume. Often, said Van Rhijn, initial low-volume prototypes are produced using multi-die reticles, in which one wafer contains different prototype dice from various NSC groups.
Production testWhile awaiting silicon prototypes, said David G. Wheelwright, product and test engineering manager, engineers begin developing a prototype production test system. Based on a device description and preliminary data sheets, engineers develop test programs and circuits that will enable high-volume test of the op amps on the company's high-end Teradyne and newer, lower-cost Eagle Test Systems test platforms.
Wheelwright, who joined NSC in February 1980 and has 20 years of experience developing test solutions for amplifiers as well as other analog devices, said he often relies on ATE systems' high-quality analog instrumentation. At times, though, he finds it more effective to add what he called "analog accelerators" to a DUT board. Such functions might include precision amplifiers that allow the transfer of relatively high-voltage signals to and from an ATE system over rugged, low-cost cables. In addition, he might employ a tester's digital outputs with DUT-board filtering to generate a sine-wave test input signal.
Complicating the production-test process is the fact that a general-purpose op amp may end up being used in any number of configurations—and one test circuit is seldom ideal for all of them. Consequently, NSC engineers often employ relays on DUT boards to switch in and out optimized test circuitry for each potential end use. Ultimately, the engineers strive for a simple ATE interface. Said Wheelwright, "As far as manufacturing personnel are concerned, they shouldn't have to know whether they're testing a performance part or an LM324"—a venerable multisourced, low-voltage, low-cost device that I recall designing into control systems in the 1970s.
While the development engineers are designing an op amp, applications engineering gears up to support the new part. According to Albert Kelsch, customer applications manager, "An applications engineer is involved from the time a business case is approved. He or she is learning about the part, in great detail, on the bench but from the viewpoint of the user, asking, 'What would I do with this part?' By the time the product is released, the applications engineer is an expert on using that part."
As the applications engineers do their jobs, test engineers develop sets of curves to characterize the new part over, for example, power-supply voltage ranges and temperature ranges. Unlike absolute maximum and minimum guaranteed specs, which must map to specific production-test measurements, the characteristic curves don't necessarily reflect guaranteed performance, but they do provide insights valuable to customers and applications engineers.
NSC engineers automate the generation of characteristic curves to the extent possible, providing data extracted from hundreds or thousands of devices, enabling data that's more representative than what could be obtained from just a few samples on a bench. Said Wheelwright, "In 60 seconds we can generate 30 or 40 curves on maybe 60 different parameters simultaneously. Then, we use data-analysis tools to generate graphs, which help us evaluate the part itself, looking at linearity, for example. The curves generated can also help us identify problems with the test system," because, as Wheelwright emphasized, it's not always the part that causes abnormal test results.
With characterization complete, the test developers map their test hardware and software to the multisite configurations that will be employed at the company's production facilities. At that point, said Kelsch, "The designers are happy, the test developers are happy, and the marketers are happy. We know we have a manufacturable part that's not going to change," and the applications engineering team begins developing the Spice models that will enable customers to simulate their op-amp based designs. "The applications engineer who has already become the expert on the new part will evaluate the Spice model to ensure that it truly represents real-world performance," he added.
Of course, Spice models from the original design flow are available, but these aren't ideal for applications support, said Van Rhijn. "From a user perspective, the transistor-level models from the design stage are not particularly useful. They require too much compute power and don't provide the type of information a user is ultimately looking for." Customers, he said, benefit from a model that offers a higher level of abstraction. Added Kelsch, "We take a user viewpoint. We don't try to reflect what's inside, but rather what the user will see."
