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LM34 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM34
Description  Precision Fahrenheit Temperature Sensors
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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 6 page
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Typical Performance Characteristics (Continued)
Typical Applications
The LM34 can be applied easily in the same way as other
integrated-circuit temperature sensors. It can be glued or ce-
mented to a surface and its temperature will be within about
0.02˚F of the surface temperature. This presumes that the
ambient air temperature is almost the same as the surface
temperature; if the air temperature were much higher or
lower than the surface temperature, the actual temperature
of the LM34 die would be at an intermediate temperature be-
tween the surface temperature and the air temperature. This
is expecially true for the TO-92 plastic package, where the
copper leads are the principal thermal path to carry heat into
the device, so its temperature might be closer to the air tem-
perature than to the surface temperature.
To minimize this problem, be sure that the wiring to the
LM34, as it leaves the device, is held at the same tempera-
ture as the surface of interest. The easiest way to do this is
to cover up these wires with a bead of epoxy which will in-
sure that the leads and wires are all at the same temperature
as the surface, and that the LM34 die’s temperature will not
be affected by the air temperature.
The TO-46 metal package can also be soldered to a metal
surface or pipe without damage. Of course in that case, the
V
terminal of the circuit will be grounded to that metal. Alter-
natively, the LM34 can be mounted inside a sealed-end
metal tube, and can then be dipped into a bath or screwed
into a threaded hole in a tank. As with any IC, the LM34 and
accompanying wiring and circuits must be kept insulated and
dry, to avoid leakage and corrosion. This is especially true if
the circuit may operate at cold temperatures where conden-
sation can occur. Printed-circuit coatings and varnishes such
as Humiseal and epoxy paints or dips are often used to in-
sure that moisture cannot corrode the LM34 or its connec-
tions.
These devices are sometimes soldered to a small,
light-weight heat fin to decrease the thermal time constant
and speed up the response in slowly-moving air. On the
other hand, a small thermal mass may be added to the sen-
sor to give the steadiest reading despite small deviations in
the air temperature.
Capacitive Loads
Like most micropower circuits, the LM34 has a limited ability
to drive heavy capacitive loads. The LM34 by itself is able to
drive 50 pF without special precautions. If heavier loads are
anticipated, it is easy to isolate or decouple the load with a
resistor; see
Figure 3. Or you can improve the tolerance of
capacitance with a series R-C damper from output to
ground; see
Figure 4. When the LM34 is applied with a 499
load resistor (as shown), it is relatively immune to wiring ca-
pacitance because the capacitance forms a bypass from
ground to input, not on the output. However, as with any lin-
ear circuit connected to wires in a hostile environment, its
performance can be affected adversely by intense electro-
magnetic sources such as relays, radio transmitters, motors
with arcing brushes, SCR’s transients, etc., as its wiring can
act as a receiving antenna and its internal junctions can act
as rectifiers. For best results in such cases, a bypass capaci-
tor from V
IN to ground and a series R-C damper such as 75Ω
in series with 0.2 or 1 µF from output to ground are often
useful. These are shown in the following circuits.
Noise Voltage
DS006685-31
Start-Up Response
DS006685-32
DS006685-6
www.national.com
6




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