Appendix
C
AD590 and LM335 Sensor Calibration
Introduction
The LDC-3700 Series Laser Diode Controller Modular Laser Diode Controller uses two constants (C1 and C2) for
calibrating linear thermal sensing devices, such
as
the AD590, and the LM335. C1 is used as the linear or zero
offset value, and C2 is used as the slope or gain adjustment. Therefore, C1 should
be
set to
a
nominal value of
0,
and
C2
should be set to a nominal value of 1, when the
SENSOR
SELECT switch is in the AD590, or LM335
positions.
In order to calibrate a linear sensor device, the sensor must
be
operated at an accurately known, stable temperature.
For example, the sensor may
be
calibrated at 0°C
if
the sensor is placed in ice water until its temperature is stable.
A
highly accurate temperature probe, thermometer, environmental chamber, etc., may also
be
used to determine
the known temperature for calibration. This appendix contains one and two point calibration methods for linear
sensor devices. These methods will work for either
type
of
device.
AD590
Sensor
The AD590 is a linear thermal sensor whch acts
as
a constant current source. It produces
a
current,
i,
which
is
directly proportional to absolute temperature, over its
useful
range (-50°C to +150°C). This nominal value
can
be
expressed as:
-where i is the nominal current produced by the AD590, and
K
is in Kelvin.
The LDC-3700 Series Laser Diode Controller uses i to determine the nominal temperature, T,, by the formula:
The
temperature, T,, which is displayed by the LDC-3700 Series Laser Diode Controller is first calibrated as
follows:
-where C1 and C2 are the constants stored by the user in the LDC-3700 Series Laser Diode Controller for
the AD590.
The AD590 measurement is calibrated, at the factory, with C2
=
1 and C1
=
0 (nominal values). The AD590
grades of tolerance vary, but typically tlus means that without adjusting C1 or C2, the temperature accuracy is
+1
"C
over its rated operating range.
If
C1 and C2 are also calibrated, the temperature accuracy
of
9.2
OC
over
its
-
rated operating range.
However. the AD590 is not perfectly linear, and even with C1 accurately known there is a non-linear absolute
temperature error associated with the device. This non-linearity is shown in Figure C.l, reprinted from Analog
De\.ices specifications, where the error associated with Cl is assumed to
be
zero.
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