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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> <3DAD0A0A.1F7BF2DD@webaccess.net> <firstname.lastname@example.org>
Date: Wed, 16 Oct 2002 13:14:35 GMT
NNTP-Posting-Date: Wed, 16 Oct 2002 06:14:35 PDT
fred bartoli wrote:
> Chuck Simmons a =E9crit dans le message :
> > "s.d." wrote:
> > >
> > > My two cents...
> > >
> > > If you want speed you may buffer your photodiode with a transimpeda=
> > > amplifier.
> > > 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 t=
> > > expense of lower gain.
> > > Mind that in voltage mode the diode response is logarithmic with li=
> > > power.
> > A negative supply is outside of allowed parameters. If it costs $0.10=
> > 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 a=
> > since I see, in devices so far, 150 to 200 mV at desired power, I'm O=
> > 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 revers=
> > 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 =
> > less predictable and caused laser damage (i.e., the things failed).
> > Unloaded photovoltaic is safe it would seem from our experience. Some=
> > 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.
> > Chuck
> > --
> What is your laser power (current) ?
> If it's low enough, you could simply sightly offset the laser & photodi=
> pair form the ground by inserting one diode (or two) in the lower branc=
> Then you'll have 0.7V (1.4V) room for a transimpedance amplifier with s=
> about 0.5V (1.2V) full scale output.
I thought about it but the manufacturers of the optical units connect
the laser cathode to power ground. I don't control the circuits on the
optical units. I live with them. This means that I definitely have a
photovoltaic diode referenced to system ground. The question is
predicting the speed of the diode given inadequate data and not one of
redesigning some other company's circuits. I want to select a clock
frequency for the power controller for each type of laser rather than
design additional circuits.
... The times have been, =
That, when the brains were out, =
the man would die. ... Macbeth =
Chuck Simmons email@example.com
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