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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Photodiodes in photovoltaic mode, speed considerations.
References: <3DACB831.F814C3D6@webaccess.net> <3DAD0481.A4004E95@NOSPAM.univ-lille1.fr>
Date: Wed, 16 Oct 2002 06:41:38 GMT
NNTP-Posting-Date: Tue, 15 Oct 2002 23:41:38 PDT
> 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
A negative supply is outside of allowed parameters. If it costs $0.10 it
is outside of allowed parameters. A load on the diode is not allowed
because because I cannot specify a buffer that operates that close to
the negative rail. I figure I can just live with 100mV off the rail and
since I see, in devices so far, 150 to 200 mV at desired power, I'm OK.
The problem is speed for clock selection for the digital loop which
depends on the diode speed in photovoltaic mode. I know some clock
limits for some devices but I can't predict because I only get reverse
specs and I don't know how these or area relate to speed.
BTW, we did use loading for a while but it did not work with a
population of parts we had because the effect of the load on voltage was
less predictable and caused laser damage (i.e., the things failed).
Unloaded photovoltaic is safe it would seem from our experience. Some of
the population of lasers tend to require close to burnout current
levels. This is within spec for the lasers but a real pain for me.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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