LM135/LM235/LM335, LM135A/LM235A/LM335A
Application Information
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:
Calibrated Sensor
569809
*Calibrate for 2.982V at 25°C
Wide Operating Supply
where T is the unknown temperature and T
o
is a reference
temperature, both expressed in degrees Kelvin. By calibrating
the output to read correctly at one temperature the output at
all temperatures is correct. Nominally the output is calibrated
at 10 mV/°K.
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 pro-
portional 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 �½″ from
the end and the tubing heated above the melting point of the
core. The unfilled �½″ end melts and provides a seal over the
device.
569810
Minimum Temperature Sensing
569804
Average Temperature Sensing
Typical Applications
Basic Temperature Sensor
569818
569802
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