Page 176 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

Stability and Tempco Issues 169

TABLE 8.1 Temperature Sensors
2
LM19 Analog, (-3.88 ¥ 10 ¥ T ) +
-6
-2
(-1.15 ¥ 10 ¥ T) + 1.8639V
LM34 Analog, 10mV/°F
LM35 Analog, 10mV/°C
LM135 Analog, 10mV/K
AD590 Analog, 1mA/K
DS1620 Digital, 9-bit coded serial
LJK SMT160-30 Digital, ª250Hz, duty-cycle coded
Silicon transistor V BE , Analog ª-2.2mV/°C or
collector connected to base -0.04 percent/°C

Precision current
source (e.g., LM334) or
12V DC
voltage ref. (e.g., REF195)
plus resistor
10k
+ V(T)
Voltage Ref.
(e.g., Zetex -
ZRA124Y 1.24V) LED Instrumentation amp.
(e.g., INA114)
Figure 8.5 The terminal voltage of the source LED itself
provides an effective local temperature measure. The
LED current should be stabilized to high precision with
a current source or voltage reference and low tempco
resistor.

Instead of trying to place a discrete temperature sensor as close as possible
to the LED chip, an obvious alternative approach is to use the LED junction
voltage itself as the temperature sensor (Fig. 8.5). Operated at a constant
current, the LED terminal voltage makes a superb internal temperature sensor.
Where the LED is current-modulated for use with synchronous detection, as
it will almost always be, the terminal voltage can be simply low-pass filtered
to obtain a high-resolution temperature-dependent average voltage (Fig. 8.6).
Alternatively the AC signal could itself be synchronously detected, reducing
low-frequency interference and perhaps improving the signal to noise as with
the optical receivers.
Once we have a high-precision, reliable measurement of temperature near or
even in the optoelectronic device, there are many techniques to compensate for
temperature variations. Figure 8.2 showed a simple open-loop analog technique.
Where microprocessor control is available, compensation is more flexible and
convenient in digital circuitry. If the long-term drift of LED characteristics
means that the power/temperature relationship is not constant, the micro-
processor can be reprogrammed with an updated look-up table.

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