Lightwave Communications LDT-5525 manual LM335 Sensor, Figure C.1 AD590 Nonlinearity

C H A P T E R C

Td = C1 + ( C2 * Tn )

-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 oC over its rated operating range. If C1 and C2 are also calibrated, the temperature accuracy is +0.2 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.1, reprinted from Analog Devices specifications, where the error associated with C1 is assumed to be zero.

1.6

0.8

ABSOLUTE

ERROR 0 (DEGREES C)

-0.8

-1.6

-55	150
DEGREES	C

Figure C.1 AD590 Nonlinearity

If a maximum absolute error of 0.8 oC 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 oC, and the intended operating range is 0 to 50 oC, a maximum error of about +0.2 oC 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 (-40oC to +100oC). 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.

38 LDT-5525