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Subject: Re: Photodiodes in photovoltaic mode, speed considerations.
Date: Wed, 16 Oct 2002 08:17:37 +0200
Organization: Universite des Sciences et Technologies de LILLE, France
X-Mailer: Mozilla 4.78 [fr] (Windows NT 5.0; U)
My two cents...
If you want speed you may buffer your photodiode with a transimpedance
That's the usual way. But still you'll need negative supply.
At least you can try to put a low impedance load to your diode at the
expense of lower gain.
Mind that in voltage mode the diode response is logarithmic with light
Chuck Simmons a =E9crit :
> I get a lot of 780 nanometer and 650 nanometer lasers with built in
> power monitor diodes for laser power control. There are two types I see=
> One type has the cathode of the laser and the cathode of the photodiode=
> connected together. The second type has the photodiode completely
> independent. Looking inside with a microscope, I can see that because
> the monitor diode is a separate die, the two types are a bonding and
> package difference. The diode specifications I always get are reverse
> bias specifications for both types. A systm limitation is that there is=
> only +5v and +12v available. I cannot generate a negative voltage and
> remain inside system design parameters.
> The independent photodiode devices are simple to work with. The reverse=
> biased photodiode is very fast so that laser power can be controlled
> with a DAC, counter and comparator scheme (which is part of an IC in th=
> system) with any counter clock we might use (likely not above 13 MHz).
> The common cathode devices, of course, must be used in photovoltaic mod=
> and the photodiode is as slow as molasses in January. The problem is
> that I don't know how slow. That is, I can find a clock frequency for
> power control for a particular unit but I can't predict from reverse
> characteristics how a photodiode will behave in photovoltaic mode. The
> problem exists because I use lasers from more than one manufacturer and=
> I don't have any control over that.
> The problem is predicting the clock to use given a laser from any maker=
> who delivers the common cathode arrangement. Does anyone know how to
> predict the speed of a photodiode from reverse characteristics or
> possibly from dimensions which I can measure under a microscope?
> ... The times have been,
> That, when the brains were out,
> the man would die. ... Macbeth
> Chuck Simmons firstname.lastname@example.org
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