User's Manual

38 LDT-5525
CHAPTER C

T
d
= C1 + ( C2 * T
n
)
- where C1 and C2 are the constants stored by the user in the LDT-5525
Temperature 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 this means
that without adjusting C1 or C2, the temperature accuracy is
+1
o
C over its rated
operating range. If C1 and C2 are also calibrated, the temperature accuracy is
+0.2
o
C 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.1,
reprinted from Analog Devices specifications, where the error associated with C1
is assumed to be zero.
Figure C.1 AD590 Nonlinearity
If a maximum absolute error of 0.8
o
C is tolerable (over the entire temperature
range), the one point calibration of C1 should be used (see page C-5). If C1 is
calibrated at 25
o
C, and the intended operating range is 0 to 50
o
C, a maximum
error of about
+0.2
o
C may be expected over that operating range. If a greater
accuracy is desired, the two point method of determining C1 and C2 should used
(see page C-6). Note however, the absolute error curve is non-linear, therefore the
constant C2 will vary over different temperature ranges.
LM335 Sensor
The LM335 is a linear thermal sensor which acts as a constant voltage source. It
produces a voltage, v, which is directly proportional to absolute temperature, over
its useful range (-40
o
C to +100
o
C). This nominal value can be expressed as:
v = 10mV / K
- where v is the nominal voltage produced by the LM335 and K is in Kelvin.
-55 150
DEGREES C
0
-0.8
0.8
ABSOLUTE
ERROR
1.6
-1.6
(DEGREES C)