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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Photodiodes in photovoltaic mode, speed considerations.
References: <3DACB831.F814C3D6@webaccess.net> <3DAD0481.A4004E95@NOSPAM.univ-lille1.fr> <3DAD0A0A.1F7BF2DD@webaccess.net> <3DAD2632.9E6DDA83@earthlink.net> <3DAD5AC6.642D726C@webaccess.net>
Date: Thu, 17 Oct 2002 05:38:01 GMT
NNTP-Posting-Date: Wed, 16 Oct 2002 22:38:01 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Chuck Simmons wrote:
> Robert Baer wrote:
> > Chuck Simmons wrote:
> > >
> > > "s.d." wrote:
> > > >
> > > > My two cents...
> > > >
> > > > If you want speed you may buffer your photodiode with a transimpedance
> > > > 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 the
> > > > expense of lower gain.
> > > > Mind that in voltage mode the diode response is logarithmic with light
> > > > power.
> > >
> > > 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.
> > >
> > > Chuck
> > One can bias the diode, either forward or reverse; millivolts or volts
> > (at least reverse) and have zero need for a negative supply.
> > At least with a reverse bias, the varactor-like capacitance is a lot
> > lower than forward (photo voltaic generator) mode.
> > Another advantage is that this gives a capacitance less dependent on
> > doping and other process parameters, and closer to packaging parameters.
> > The diode resulting voltage shift can drive a comparitor, or opamp
> > acting as a comparitor; or in a bridge-like circuit driving the opamp
> > (with or withot feedback).
> > And there are a lot of opamps that go rail to rail with common mode
> > input specs wider tan the supplies (many are SOIC).
> The issue is not redesigning what I have which works but rather
> predicting performance of the monitor diode in photovoltaic mode given
> only reverse specifications for the diode and possibly dimensions that I
> can measure under a microscope. In fact, the question is still one of
> predicting the speed of the diode so that I can select a clock for the
> power control device. The possible clock range is large so I have
> considerable freedom to select it for the common cathode devices.
> BTW, the reason I want to select the clock before powering a type of
> laser is because the lasers are easy to burn out. I have piles of dead
> ones some of which are victims of the clock being set too fast for them.
> Most of the other dead ones seem to be ESD failures but that's another
> ... The times have been,
> That, when the brains were out,
> the man would die. ... Macbeth
> Chuck Simmons email@example.com
Well, if you insist on that mode and the variations, be my guest.
There is absolutely *NO* way to determine the speed from datasheet
specs, unless that is given as a measured (ie: specified) parameter
under identical conditions that you use (an i do not think *that* will
Therefore, use the absolutely *slowest* clock speed that is useful for
the slowest device you can find, and then slow down the clock by a
factor of 10 if practical; but no less than a factor of 2 slower for
safety (since you indicate a fast clock rate is nasty).
If that does not suit the application, too bad; it is a problem you
created by refusing to use circuitry that is faster and less dependent
on diode doping and size.
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