Datasheet

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 al-
lows 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
(
b
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:
V
OUT
T
e
V
OUT
T
o
c
T
T
o
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 out-
put 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 resist-
ance 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 er-
ror proportional to absolute temperature the same as scale
factor errors.
WATERPROOFING SENSORS
Meltable inner core heat shrinkable tubing such as manu-
factured by Raychem can be used to make low-cost water-
proof sensors. The LM335 is inserted into the tubing about
(/2
×
from the end and the tubing heated above the melting
point of the core. The unfilled (/2
×
end melts and provides a
seal over the device.
Typical Applications
Basic Temperature Sensor
TL/H/5698–2
Calibrated Sensor
TL/H/5698–9
*Calibrate for 2.982V at 25
§
C
Wide Operating Supply
TL/H/5698–10
Minimum Temperature Sensing
TL/H/5698–4
Average Temperature Sensing
TL/H/5698–18
Remote Temperature Sensing
TL/H/5698–19
Wire length for 1
§
C error due to wire drop
I
R
e
1mA I
R
e
0.5 mA*
AWG FEET FEET
14 4000 8000
16 2500 5000
18 1600 3200
20 1000 2000
22 625 1250
24 400 800
*For I
R
e
0.5 mA, the trim pot must be deleted.
4