Page 181 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

174 Chapter Eight

R L1
Current
control Beam-
splitter - Signal
+A
LED
PD1
Cuvette
Monitor -
+A
PD2
Figure 8.11 One channel of ADC can be saved if the reference is
not measured, but used to regulated the source intensity via
analog feedback control.

R1
Set-point
10k
100nF
PD2 100k IC2 1N4148 330
S1337
2 7 AD711
3 7
3 6 + 6
-
4 1.5k ZTX551
+
IC1 2 - 4
LMC7101
22k LED
1k
-12V
LED
DC
PD2 Optical feedback

Figure 8.12 LED output intensities can be stabilized by closely coupling
source and reference photodiode, e.g., by gluing, and using a simple
analog feedback controller.

as the Hamamatsu S1337 series, with <100ppm/°C. It is sometimes adequate to
glue the photodiode directly to the side of the LED where it collects a little of
the light, as shown in the insert of Fig. 8.12. Most of the light passes by to the
experiment.
In these two-channel measurements it is not strictly necessary to have pho-
todetectors with very low temperature coefficients of output current. It is the-
oretically only necessary to have two detectors which have similar temperature
coefficients, and a similar temperature. It is nevertheless highly advisable to
use photodetectors with a low tempco. Thermally coupling the two photodiodes
will aid temperature tracking and improve precision. It still won’t be perfect,
but an obsessive approach to matching helps. The use of similar diodes has
already been mentioned, meaning the same manufacturer, type, model and

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