Page 143 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

136 Chapter Six

give the best dynamic range. In a conventional opamp design the output voltage
is limited to not closer than about 1.2V (two diode-drops) from the positive rail,
and this can be a big loss with a 3.3 or 5V supply. Luckily, you aren’t on your
own in needing these characteristics; almost the whole electronics industry,
driven by applications in mobile phones, digital cameras, personal digital assis-
tants, MP3 music players, and notebook personal computers is pressing for new
opamp designs that stress low-voltage, low-power, rail-to-rail inputs and outputs
and operation from the same 3.3 or 5V supply used for the digital circuitry.
Hence there is a reasonable and rapidly expanding choice. The downside is that
these requirements make design for high speed, low offset voltage, good output
drive capability, etc. much more difficult. In transimpedance configuration some
rail-to-rail opamps show severe distortion and instability with low output volt-
ages. As your laboratory design is unlikely to be built or abandoned depending
on the cost of a couple of chips, it is often worth just using split supplies, gen-
erating split supplies using a specially designed voltage regulator, or generat-
ing a low-current negative supply from the +5V rail using an ICL7660 inverter.
Figure 6.11 shows a few possibilities. These can be used to supply either the
whole opamp or just the photodiode’s reverse bias. Note that the oscillation fre-
quency of the standard devices is fixed in the audio band and can give a lot of
in-band interference which is difficult to screen perfectly. Alternative devices
offer either much higher operating frequencies or adjustable frequencies, which
can help to avoid the biggest problems.

6.8 Use of Audio Outputs
In many of the simple systems discussed we have modulated the source in the
audio range, above the region of worst industrial noise, say 500Hz to 20kHz.
Because of this, a receiver with an audible output is a very useful piece of equip-
ment for optical system testing. This can be used to find out whether the trans-
mitter LED is being modulated, or even if the invisible infrared LED is actually
still alive. In debugging field installations, it may save lots of time and face. In
multifrequency systems the human spectrum analyzer can even differentiate
between sources and allows extremely effective detection in the presence of
noise. You can of course make up a complete battery-powered transimpedance
receiver, small audio amplifier chip such as the LM386, and a speaker or ear-
phone. This is the proper approach for testing weak signals. However, even a
trivial receiver can be useful.

6.8.1 TRY IT! Minimalist audio photo-receiver
A large area photodiode or small solar cell connected to a pair of headphones, per-
sonal audio player earphone, or even a piezobuzzer does a great job. Ideally you would
like the most sensitive device you can find. Try to find an old high-impedance ear-
phone; 600W or 2kW are best, but even a modern 16W personal stereo earpiece works,
albeit with lower sensitivity. Just connect it to a silicon photodiode. An eco-
nomical BPW34 is fine, but so is almost anything else. I am lucky to have a twenty-

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