Page 177 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

170 Chapter Eight

Precision
modulation source
5V pk
Low-pass
180R filter
+ V(T)
~1.5V pk 100k
-
1μF
LED
Precision opamp
(e.g., OP177)
Figure 8.6 Temperature estimation of a modulated
LED is also possible by smoothing (or synchro-
nously detecting) the terminal voltage variations.

Precision modulation
voltage source 5V pk
180R
OP177 10k
+ V(T)
Set - 1μF Peltier element
point LED on heat sink

PI CONTROLLER +ve
-
+
Instr. 10k cools
Amp. - 10k
Error + "Proportional" + ± 3A bipolar
-
power opamp

10k
-
+
"Integral"
Figure 8.7 Precision temperature control of an LED source can be effected
using a PI-controller and Peltier element heat pump. Temperature fluctua-
tions of 1mK are relatively easy, 10mK possible with care.

8.4 Temperature Regulation
8.4.1 Simple circuitry
Instead of providing temperature compensation, we can use the brute-force
approach of temperature regulation. All of the above temperature sensors are
suitable, but for stabilization of the LED temperature the terminal voltage
technique appears to be superior. Conventional analog or digital feedback
controllers can be used, with heating-only to a regulated temperature above
the maximum operating temperature, or heating and cooling using a Peltier
element heat pump (Fig. 8.7). Peltier elements allow stabilization near to room

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