[des ignnews.com] 10.24.05 DESIGN NEWS 25

Rarely Asked QuestionsStrange but true stories from the call logs of Analog Devices

S P O N S O R E D B Y

R A Q ’ sS P E C I A L A D V E R T I S I N G S E C T I O N

Have a question

involving a

perplexing or

unusual analog

problem? Submit

your question to:

A. With some dif-ficulty. The ostensi-ble purpose of adata sheet is toprovide the userand potential userwith the fullest pos-sible informationabout the functionsand technical char-acteristics of thedevice concerned.But the realitiespractically ensurethat there are toomany cooks con-cocting these doc-uments, producing spoiled broth indeed.

The IC designer who writes the first draftwants to emphasize the genius of his or hercreation. The marketing manager wants tostress competitive advantages while soft-peddling any drawbacks. The test engineerwants to minimize the time and cost of pro-duction testing, and tries to remove all maxi-ma and minima from the table of characteris-tics, instead replacing them with “typical”values. Corporation lawyers want to makecertain that potential (mis)users of the devicehave no grounds for suing the Corporation.Corporate communications wants the docu-ment shrunk from 60 pages to four. Andapplications engineers (ahem!) want the datasheet so clear and simple that even a soft-ware engineer can understand it and theycan sleep away their afternoons without theapplications enquiry phone ringing. The finalproduct is a “compromise”, and not alwaysas helpful as it could be. And because datasheets are always produced in a hurry whenthe product is ready for release they alwayshave some mistakes.

What’s an engineer to do? First, know whichspecifications are most important to yourapplication. If you don’t know, consult designguides or screw up your courage and asksomeone. Second, conduct parametric search-

es among manufac-turers to find candi-date devices. Third,despite your misgiv-ings, read the d***eddata sheets. (I reallydid once have some-one call to ask howmany pins there areon an 8 lead mini-DIP.)

When reading datasheets, at the veryleast watch out for:

• “Vdd � Vss” or,better “|Vss| � Vdd”These are subtleways of saying that if

you supply the negative supply before thepositive, the device will destroy itself.

• Specifications which appear similar ontwo data sheets but are not—such as small-signal bandwidth versus full-power band-width, or settling time to 1 lsb (12-bits) ver-sus settling time to 1 percent.

• “Typical” versus maximum and minimumspecifications. The meaning of maximumand minimum is clear and well-understood.The meaning of typical is open to manyinterpretations.

The “compromises” involved in meeting adata sheet’s conflicting requirements meanthat much more can be learned from a datasheet than simply the overt facts and specifi-cations. The website below analyzes thesethings in considerable detail. It would be awonderful thing if the industry could agree ona standardized format for data sheets andthey all told the truth, the whole truth, andnothing but the truth. But I’m not holding mybreath until it happens. Caveat emptor!

To learn more about howto read data sheets,

Go to:http://rbi.ims.ca/5696-103

Q. How do I make sense out of a data sheet?

raq@reedbusiness.com

Contributing Writer

James Bryant has

been a European

Applications Manager

with Analog Devices

since 1982. He holds a

degree in Physics and

Philosophy from the

University of Leeds. He

is also C.Eng.,

Eur.Eng., MIEE, and an

FBIS. In addition to

his passion for engi-

neering, James is a

radio ham and holds

the call sign G4CLF.

Caveat Emptor!

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