How to Read Datasheets

How to Read Datasheets
For every electronic component or series of components, th
manufacturer or designer produces a data sheet. In its early stages
a data sheet might be the specifications the designer works from
but, by the time the device is released, the data sheet is th
essential piece of information that describes exactly what th
component does. Everything from the smallest resistor to the mos
elaborate processor needs a datasheet. Datasheets focus on electrica
properties and the pin functions of the device; usually the inne
workings of the device are not discussed. This is partly to mak
industrial espionage more difficult, and also because the user shoul
not need to know the internal workings of the device. In practice, i
you find that you need to know how a particular product work
internally, you can often call the manufacturer and find out what yo
need to know.
In addition to datasheets, devices with complex configurations o
applications may have related documents to help the designer wor
with their products. These are called ?°application notes,?
?°user?És guides,?± ?°designer?És guides,?± ?°packag
drawings,?± etc. These documents are usually just as necessary as th
datasheet. In general, it is best to get the datasheet directly fro
the company website relatively often because occasionally there i
errata or new information to be found in the datasheets. Datasheet
are invariably covered with legal disclaimers as to the accuracy
permanency, and utility of the document. Below are some of the kind
of things you might find in a datasheet and explanations of thei
usual meaning.
Official name of the part or series, part numbers and part numbe
variations and manufacturer release date of the datasheet. Par
number variations usually indicate alternative packaging o
temperature tolerance. For component families, a chart might b
provided to helpfully graph all of the family members. Price i
usually not indicated on a datasheet.
An overview of the parts purpose and features is usually included nea
the beginning. This is what you scan to see if the part is what yo
think it is.
The electrical operating characteristics section of a datashee
indicates the minimum and maximum voltage and current parameters fo
the chip as a whole and for individual pins. Power requirements fo
the chip as a whole are essential. The power supply circuitry of th
device must be able to support all of the components. Operatin
frequencies of clocks or information are often indicated here. Pi
electrical characteristics are important when using the pin to driv
larger loads. Lower power integrated circuits are not always capabl
of driving an LED, for example. Noise tolerance of the power supply
or noise created by the component might also be found here
Capacitance, inductance, and resistance caused by the component migh
also be found here. These are especially important for high-spee
circuit analysis.
The datasheet should also note the tolerances of the device. While th
above operating characteristics indicated what conditions are neede
for the device to operate as promised, the tolerances indicate th
maximum and minimum conditions the component can handle withou
permanent damage. Both the operating conditions and the tolerance
should indicate the nature of the testing experiments. Voltage
temperature, moisture, air pressure, ultraviolet radiation, an
physical stress are possible tolerance conditions.
The arrangement and name of each pin on the chip is a necessity on an
integrated circuit (IC) datasheet. The diagram should specify whethe
the diagram is from a point of view above or below the chip and lis
the pins by name or number. Pin functional descriptions shoul
accompany the pin map diagram to explain the basic purpose of eac
pin. If this short description is insufficient, a more elaborat
explanation is usually included later in the datasheet. Often th
short descriptions may not be 100% clear to a novice data shee
reader because of abbreviations or conventions. If the data shee
doesn?Ét fully explain what the short description means, th
term is probably common enough to be found elsewhere on the Internet.
A block diagram of architecture might be included for more complex
devices. Other internal descriptions might be provided, but usually
the description is limited to the parts of the system that the user
can access.
Waveforms of input or outputs are common. This is especially true for
explaining bus operation and data formats. Timing diagrams and
information are also essential for finding interoperable devices.
Just because two components use the same bus protocol does not always
mean they can talk to each other. Checking this information is always
a good idea.
Graphs of I/V curves, noise profiles, input response, performance
descriptions are very common. For system/control behavior this can be
useful, but the testing conditions are not always entirely clear. This
kind of information is the basis of the analysis many engineers do,
but the graphs are rarely a good substitute for prototyping.
Many kinds of components only work if accompanied by necessary passive
components. Usually these systems provide an ?°example?± configuration
that will produce a known behavior. Examples are very useful if you
have the same needs the example configuration claims to meet, but the
datasheet should also include the formulas and explanation necessary
to pick your own accompanying components.
Distribution information and manufacturing or assembly advice might
also be found in a datasheet. For example, a crystal clock might
specify that it should be soldered to the circuit board for no more
than 10 seconds at 400 degrees. For commercial design this kind of
information is useful, as adhering to such recommendations improves
yield.
Mechanical drawing and footprints are the last piece of essential
information a datasheet includes. This will be a drawing of the
physical form of the device, with measurements specified in metric
and American units. For designing a board, it is very important to
understand the drawings because incorrectly interpreting the
relationship among the pins will waste an entire revision of the
board. The usual way of describing the dimensions is to specify a pin
width and spacing precisely, but to give broader tolerances on the
exact length and width of the overall device. What this means is that
the process of manufacturing the ICs tries to control the width and
spacing of the pins, but everything else is somewhat flexible.
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