Page 214 - Troubleshooting Analog Circuits

F. How to Get the Right Information From a Data Sheet 20

A Point-By-Point Look Another example is the application hint for the LF156 family:
Let’s look a little more closely at the data sheet of the Na- “Exceeding the negative common-mode limit on either input
will cause a reversal of the phase to output and force the
tional Semiconductor LM34. whiih happens to be a temper- amplifier output to the corresponding high or low state. Ex-
ature sensor. ceeding the negative common-mode limit on both inputs will
Note 1 lists the nominal test conditions and test circuits in force the ampliier output to a high state. In neither case
which all the characteristics are defined. Some additional does a latch occur. since raising the input back within the
test conditions are listed in the column “Conditions”, but common-mode range again puts the input stage and, thus
Note 1 helps minimize the clutter. the amplifier, in a normal operating mode.”
Note 2 gives the thermal impedance, (which may also be That‘s the kind of information a manufacturer should really
shown in a chart or table). give to a data-sheet reader because no one could ever
Note 3 warns that an output impedance test if done with a guess it.
long pulse, could cause significant self-heating and thus, Sometimes, a writer slips a quirk into a characteristic curve,
error. but it’s wiser to draw attention to it with a line of text. This is
Note 6 is intended to show which specs apply at all rated because it’s better to make the user sad before one gets
temperatures. started, rather than when one goes into production. Con-
Note 7 is the definition of the “Accuracy” spec, and Note 8 versely, if a user is going to spend more than 10 minutes
the definition for non-linearity. Note 9 states in what test using a new product, one ought to spend a full five minutes
circuit the quiescent current is defined. Note 10 indicates reading the entire data sheet.
that one model of the family may not be available at the time FINE PRINT
of printing (but happens to be available now). and Note 11 is What other fine print can be found on a data sheet? Some-
the definition of Absolute Max Ratings. times the front page may be marked “advance” or “prelimi-
Note-the “4 seconds” soldering time is a new standard nary.” Then on the back page, the tine print may say some-
for plastic packages. thing such as:
’* Note-the wording of Note 11 has been revisd-this is “This data sheet contains preliminary limits and design
the best wording we can devise, and we will use it on all specifications. Supplemental information will be published
future datasheets. at a later date. The manufacturer reserves the right to make
APPLICATIONS changes in the products contained in this document in order
Another important part of the data sheet is the applications to improve design or performance and to supply the best
possible products. We also assume no responsibility for the
section. It indicates the navel and conventional ways to use
a device. Sometimes these applications are just little .&as use of any circuits described herein, convey no license un-
der any patent or other right and make no representation
to tweak a reader’s mind. After Wing at a couple of appli- that the circuits are free from patent infringement.”
cations, one can invent other ideas that are useful. Some
applications may be of no real interest or use. In fact, after a device is released to the marketplace in a
In other cases, an application circuit may be the complete preliminary status, the engineers love to make small im-
definition of the system’s performance; it can be the test provements and upgrades in specifications and characteris-
tics, and hate to degrade a specification from its first pub-
circuit in which the specification limits are defined, tested
and guaranteed. But. in all other instances. the perfmance lished value-but occasionally that is necessary.
of a typil application circuit is not guaranteed, it is only Another item in the fine print is the manufacturer’s tele-
typical. In many circumstances, the performance may de phone number. Usually it is best to refer questions to the
pend on external components and their precision and local sales representative or field-applications engineer, be-
matching. Some manufacturers have added a phrase to cause they may know the answer or they may be best able
their data sheets: to put a questioner in touch with the right person at the
“Applications for any circuits contained in this document are factory.
for illustration purposes only and the manufacturer makes Occasionally, the factory‘s appliicationa engineers have all
no representation or warranty that such applications will be the information. Other times, they have to bring in product
suitable for the use indicated without further testing or modi- engineers, test engineers or marketing people. And some-
fication.” times the answer can’t be generated quicklydata have to
In the future, manufacturers may find it necessary to add be gathered, opinions Wii or policies formulated be-
fore the manufacturer can answer the question. Still, the
disclaimers of this kind to avoid disappointing users with
circuits that work well, much of the time, but cannot be easi- telephone number is the key to getting the factory to help.
ly guaranteed. ORIGINS OF DATA SHEETS
The appliitions section is also a good place to look for Of course, historically. most data sheets for a class of prod-
advice on quirks-potential drawbacks or little details that ucts have been closely modeled on the data sheet of the
may not be so little when a user wants to know if a device forerunner of that class. The first data sheet was copied to
will actually deliver the expected performance. make new versions.
For example, if a buffer can drive heavy loads and can han- That’s the way it happened with the UA709 (the first mom
dle fast signals cleanly (at no load), the maker isn’t doing lic op amp) and all its copies. as well as many other simi-
anybody any favors if there is no mention that the distortion lar families of circuits.
goes sky-high if the rated load is applied.