LM135/LM235/LM335, LM135A/LM235A/LM335A
Typical Performance Characteristics
Thermal Response in Stirred Oil Bath
(Continued)
Forward Characteristics
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Application Hints
CALIBRATING THE LM135
Included on the LM135 chip is an easy method of calibrating
the device for higher accuracies. A pot connected across the
LM135 with the arm tied to the adjustment terminal allows a
1-point calibration of the sensor that corrects for inaccuracy
over the full temperature range.
This single point calibration works because the output of the
LM135 is proportional to absolute temperature with the ex-
trapolated output of sensor going to 0V output at 0˚K
(−273.15˚C). Errors in output voltage versus temperature are
only slope (or scale factor) errors so a slope calibration at
one temperature corrects at all temperatures.
The output of the device (calibrated or uncalibrated) can be
expressed as:
To insure good sensing accuracy several precautions must
be taken. Like any temperature sensing device, self heating
can reduce accuracy. The LM135 should be operated at the
lowest current suitable for the application. Sufficient current,
of course, must be available to drive both the sensor and the
calibration pot at the maximum operating temperature as
well as any external loads.
If the sensor is used in an ambient where the thermal resis-
tance is constant, self heating errors can be calibrated out.
This is possible if the device is run with a temperature stable
current. Heating will then be proportional to zener voltage
and therefore temperature. This makes the self heating error
proportional to absolute temperature the same as scale
factor errors.
WATERPROOFING SENSORS
Meltable inner core heat shrinkable tubing such as manufac-
tured by Raychem can be used to make low-cost waterproof
sensors. The LM335 is inserted into the tubing about
1
⁄
2
"
from the end and the tubing heated above the melting point
of the core. The unfilled
1
⁄
2
" end melts and provides a seal
over the device.
where T is the unknown temperature and T
o
is a reference
temperature, both expressed in degrees Kelvin. By calibrat-
ing the output to read correctly at one temperature the output
at all temperatures is correct. Nominally the output is cali-
brated at 10 mV/˚K.
Typical Applications
Basic Temperature Sensor
Calibrated Sensor
Wide Operating Supply
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*Calibrate for 2.982V at 25˚C
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5
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