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Guide to reading data sheets

27 Aug 2012  | Doug Grant

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You have just finished the block-diagram version and perhaps a rough schematic of your new design. There are some op amps, maybe some analog switches, a voltage reference or two, some other analog interface ICs, a couple of A/D or D/A converters, and some power management. Now you need to pick the components that together will meet your performance and cost objectives.

Maybe you're constrained to use only the chips that are available in your CAD library, on a qualified-products list, or maybe you are free to use a search engine or other online tool to start finding chips that you can use. In any case, you'll have to go through the data sheet for each component to figure out whether it will work in your application.

This article will examine some of the issues and how to read a data sheet and get everything out of the exercise.

Let's say you find a chip that looks interesting, but there's a note on the vendor's Web page that says, "Contact factory," and there's no data sheet openly available. That could mean several things.

Maybe the chip is too new, preannounced by an overly zealous marketing department, and the apps engineers aren't done with the final version of the data sheet. It happens. Talk to the vendor, get the inside scoop on the timing, and decide whether you can still use the new chip.

Another possibility is that the part is intended for a very specific application or set of customers. If you're in the target market, great; if you're not, don't try to talk the vendor into letting you use a chip that really doesn't fit your needs (or the vendor's). For example, a power-management chip designed for the latest server reference design from a processor vendor may indeed have the voltage and current outputs you want, but it may be very hard to use without the intended processor. You may not get delivery if supply falls short and the customers in the intended application need inventory. The chip may become obsolete when the intended application moves to the next generation. Maybe the vendor can steer you to something better suited to your needs.

But let's assume that the chip you want to use has a data sheet available on the Web. Look for a revision number/letter and date. If the data sheet is Revision G, dated six years ago, maybe this chip is past its prime and there's something newer that is faster/better/cheaper/smaller/lower power. You don't want to be caught gearing up for production only to find out that the key chip is being discontinued. On the flip side, if the data sheet is "Revision Preliminary A", dated last month, there may be some risk associated with using the component. The device might be not quite ready for prime time; the specifications might change. Most analog ICs hit their peak sales after about two years on the market, so that's kind of a nice vintage.

Data sheets for multipurpose ICs (this article focuses mostly on analog and mixed-signal IC data sheets) have not evolved very much from the early days of the industry (1970s). Compare any modern data sheet with that for the venerable NE555 timer chip. I chose to use the 555 as an illustrative example to pay homage to the designer, Hans Camenzind, who died recently.

The front page
The first page of the data sheet should give you an overview of the chip's key features and a few critical specs. Look there for any obvious red flags, such as power consumption that is too high, a key spec that falls short of your needs, or stated compliance with an old revision of an industry standard. A good data sheet also has the block diagram of the chip right there up front. Does it have the inputs and outputs you need? Any features missing? Anything cool you didn't expect that you could use to reduce the circuit's cost?

On the 555 data sheet, note the specification for temperature stability: 0.05%/°C. We'll come back to that in a moment.

Look also at the suggested applications. If one of them sounds like what you are doing, great. If not, don't worry too much; many of the listed applications are a marketer's wish list of where he hopes to sell the device. The longer the list, the broader the intended market, and you probably will not find anything that limits your ability to use the chip. On the other hand, if there is only one application listed, and you're not doing it, move on.

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