When the
LM335
measurement is calibrated, C1 and C2 uncalibrated,
and
the temperature accuracy is typically
+O.5
OC
over the rated operating range. With
C1
and
C2
calibrated also, the temperature accuracy is typically
-
+0.2
OC
over the rated operating range. The temperature accuracy may
be
improved over a narrow temperature
-
range by a two-point calibration of
C
1 and
C2.
However, the LM335 is not perfectly linear, and even
with
C1
accurately known (and
C2
uncalibrated) there is a
non-linear absolute temperature error associated with the device. This non-linearity caused error is typically
+0.3
OC,
with the error associated with C1 assumed to
be
zero.
-
If
a maximum absolute error of tO.5
"C
is tolerable, no calibration of
C1
or
C2
is required, just set
C1
=
0,
C2
=
1.
If
a maximum absolute error of
s.3
OC is tolerable, the one point calibration of C1 should
be
used.
If
a greater
accuracy is desired, the two point method of determining C1 and C2 should used. Note however, the absolute error
associated with the constant C2 may vary over different temperature ranges.
One
Point Calibration Method
This procedure will work for any linear temperature sensor. The accuracy of this procedure depends on the
accuracy of the known temperature, externally measured. It is
used
to determine the zero offset of the device, and it
assumes that the gain offset (slope) is known and is correct.
1.
Allow the LDC-3700 Series Laser Diode Controller to
warm
up for at least one hour. Set
the
SENSOR SELECT switch for the desired sensor
type,
and RECALL the constants for the particular
device to
be
calibrated.
2.
Select the C1 parameter. Read and record the value of
C1.
3.
Place the sensor at an accurately known and stable temperature,
T,.
Connect the sensor to the
TEC
module of the LDC-3700 Series Laser Diode Controlier for normal Constant temperature operation.
Allow the LDC-3700 Series Laser Diode Controller to stabilize at the known temperature,
T,
and read
the displayed temperature,
Td.
4.
Determine the new value of C1, Cl,, from the formula:
and replace C1 with C1, by selecting the C1 parameter and entering
the
new
C1,
value.
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