60 Chapter 4 Temperature Controller Module Operation
Figure 28 - AD590 Nonlinearity
If a maximum absolute error of 0.8°C 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 be used. Note however, the absolute error curve is non-linear,
therefore the constant C2 will vary for different measurement points.
4.5.2.3 LM335 Sensor
The LM335 is a linear thermal sensor which acts as a voltage source. It produces a
voltage, v, which is directly proportional to absolute temperature, over its useful
range (-40°C to + 100°C). This nominal value can be expressed as:
v = 10mV / K
where v is the voltage produced by the LM335 and K is Kelvin.
The 8000 uses v to determine the nominal temperature, T
n
, by the formula:
T
n
= ( v / ( 10mV / K) ) - 273.15
where T
n
is in °C.
The temperature, T
d
, which is displayed by the 8000 is calculated as follows:
T
d
= C1 + (C2 * T
n
)
where C1 and C2 are the constants stored in the 8000 for the LM335.
When the LM335 is calibrated to 25°C, C1 = 0 and C2 = 1, and the temperature
accuracy is typically ±0.5°C over the rated operating range. However, the LM335 is
not perfectly linear, and even with C1 accurately known there is a non-linear absolute
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