Designing “Off Data Sheet” - Risks
Murphy’s Law and the Risks of Designing “Off Data Sheet”
Abstract: This application note considers off-data-sheet operation of integrated circuits (ICs). It discusses the pitfalls awaiting engineers who have not experienced Murphy’s Law, which is anything that can go wrong, will go wrong at the worst possible time. The article reviews the thought process of an experienced and inexperienced engineer, and how they can make a circuit immune to leakage from environmental contamination, radio frequency interference, and electro static discharge.
I believe in Murphy’s Law as a general rule in life. Murphy’s Law says that anything that can go wrong, will go wrong at the worst possible time. Wikipedia illustrates the history of Murphy’s Law with the image shown in Figure 1. Murphy sometimes produces train wrecks.
Wikipedia explains that the 1895 derailment occurred when the Granville-Paris Express overran the buffer stop. The accident was caused by a faulty brake and the engine drivers who were trying to make up for lost time. The engine careened across almost 30m (98ft) off the station concourse, crashed through a 60cm (24in) thick wall, shot across a terrace, sailed out of the station, and plummeted onto the Place de Rennes 10m (33ft) below, where it stood on its nose. For much more on Murphy’s Law see: Murphy’s laws site.
Standard Device Testing and Data Sheet Specifications
Consider now how Murphy’s Law can affect ICs that have been tested and meet published specifications. It is important to understand that there is a difference between the way an IC is tested and the way the IC is used in a customer’s application. Test time on the automatic test equipment (ATE) is very costly; it is paid for in portions of a second. Since there are usually many possible applications for an IC and just one test, the manufacturer sets specifications to meet the majority of the applications. Then a test procedure is defined to test those specifications with minimum test time. A well-known adage of the IC industry says that: the data sheet describes the test conditions and specifications. All manufacturers guarantee that the part meets the minimum and maximum values by a combination of tests and/or simulation (this is what is meant by “guaranteed by design.”).
An example of a parameter that is commonly “guaranteed by design” is the operating temperature range. The part is tested at one temperature, “room,” meaning +25°C; it is not tested for every temperature in the device’s published operating range. Rather, design simulation allows us to predict the device’s operation over the whole range of temperature and process variations. Most manufacturers, moreover, set guardbands around the parameters to allow for these variations. Statistically many manufacturers set a safety wall at “six sigma,” a commonly accepted way to state that the standard deviation has a 99.9997% probability of meeting the specification for that parameter. Guaranteed by design is used for the operating temperature range instead of increasing test time by three times or more.
How Can a Part Be Used “Off Data Sheet?”
All parts have limitations. We just cannot design a single part for all possible uses. Even if someone tried, no one could afford to pay for such a part. That is why there are literally thousands of logic parts and device families as well as operational amplifiers (op amps) and analog-to-digital (ADC) and digital-to-analog (DAC) data converters.
To illustrate, some years ago CMOS (complementary metal oxide semiconductors) logic was invented. It was designed to be inexpensive digital logic. Engineers soon started using the digital logic in a linear analog way for the endless applications that would benefit from less expensive IC devices. In Figure 2a a logic inverter with two resistors is used as a crude amplifier. The specifications of the resulting device are barely good enough to work. The data sheet says nothing about this application, and the IC manufacturer does not guarantee any parameters for this use. Thus this crude amplifier is, in fact, the definition of “off-data-sheet” use. Clever engineers could use all six inverters in the IC package. Figure 2b adds larger current-drive capabilities to an op amp. While Figure 2c is an oscillator, Figure 2d is the same as Figure 2a with more gain. Figure 2e is a 4-bit DAC. If we drive Figure 2e with a counter, we get a staircase generator. Add a windowed comparator and we can make a successive-approximation ADC by stopping the counter when the DAC output matches the incoming voltage.
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