Phil,   
      
   Any chance you can share a reference circuit with us?   
      
      
      
   >   
   >   
   > >>> Phil,   
   >   
   > >>   
   >   
   > >>>   
   >   
   > >>   
   >   
   > >>> I own a copy of your book. It is fantastic great job.   
   >   
   > >>   
   >   
   > >>>   
   >   
   > >>   
   >   
   > >>> My current system, I sense an object that is far away using a   
   >   
   > >>> very weak laser. (For various reasons, it needs to be weak and it   
   >   
   > >>> is pulsed at a low frequency of 1MHz) My distance of sensing is   
   >   
   > >>> 400mm.   
   >   
   > >>   
   >   
   > >>> Now for a next version, I need to extend my distance of sensing   
   >   
   > >>> to 4Meter. I currently use a si-pd and I have done a lot to make   
   >   
   > >>>  this current system work at 400mm, now I need 10x the   
   >   
   > >>> improvement, based on my experience I don't think I can get this   
   >   
   > >>> done, hence the Avalanche PD idea.   
   >   
   > >>   
   >   
   > >> The thing about APDs is that they multiply the shot noise of the   
   >   
   > >> ambient light as well as the signal, so they're really only useful   
   >   
   > >>  when you'd otherwise be in the Johnson noise limit.  A gain of M   
   >   
   > >> lets you drop the feedback resistance by a factor of M**2, so in   
   >   
   > >> some situations that's a win.   
   >   
   > >>   
   >   
   > >> Advanced front ends are much cheaper, though--usually only a few   
   >   
   > >> dollars in onesies.   
   >   
   > >>   
   >   
   > >> How much photocurrent are you expecting to have, and how big is   
   >   
   > >> your detector?   
   >   
   > >>   
   >   
   > >> Any possibility of putting some retroreflecting tape on the target?   
   >   
   > >> That's good for a factor of about 3000 (70 dB electrical) compared   
   >   
   > >> to a Lambertian scatterer.   
   >   
   > >>   
   >   
   >   
   >   
   > > Hi Phil...   
   >   
   > >   
   >   
   > > On my current system I get about 5nA from a 7mm2 PD. My current   
   >   
   > > system gain is about 6M times.   
   >   
   > >   
   >   
   > > No reflector tape is possible, however I will push the laser to a   
   >   
   > > higher power while reducing the duty cycle to that average power is   
   >   
   > > preserved, the issue is even a low frequency of 1-2MHz rise time   
   >   
   > > becomes a problem . I will probably give a higher negative bias to   
   >   
   > > reduce the capacitance so that the rise time could be gained back. I   
   >   
   > > expect to gain 6db by way of pushing laser power and some other   
   >   
   > > optical means, if I can squeeze the duty cycle to half or more   
   >   
   > > another additional 3db lurking somewhere there, so all in all   
   >   
   > > perhaps I can gain another 10db but I think I need more like 25db   
   >   
   > > improvement to achieve a good signal SNR.   
   >   
   > >   
   >   
   > > Anyway, I will spin a board to see if I could solve the problems   
   >   
   > > this way. Last resort is APD.   
   >   
   >   
   >   
   > One method is to use an integrating front end, with a DC restore loop to   
   >   
   > get rid of the background photocurrent, and then differentiate again   
   >   
   > afterwards.  I did that with an IR remote control for Samsung Research   
   >   
   > about 4 years ago.  Using a couple of BPV22NF diodes, which are probably   
   >   
   > similar to the ones you're using, I got about a factor of 4 more light   
   >   
   > than a single diode of the sort you're using.  Due to the moderate   
   >   
   > capacitance of the parallelled PDs, it ran about 10 dB above shot noise   
   >   
   > at 1 MHz and 10 nA.  (The one I did with JL & Co. is a few decibels   
   >   
   > better than that, and has a nice flat response, but the design is   
   >   
   > tweaked to the eyeballs and it needs some production adjustments.)   
   >   
   >   
   >   
   > The Samsung one used a pair of BF862s as a bootstrapped bootstrap, with   
   >   
   > another pair running as a peaked common-source amplifier with a low   
   >   
   > noise BJT cascode stage (300 uH || 2.4k in the collector circuit).   
   >   
   > There was a 1-pF feedback cap across the common-source stage, and   
   >   
   > capacitive coupling into the second stage.  It wound up with a broad   
   >   
   > peak around the 1-MHz subcarrier frequency.   
   >   
   >   
   >   
   > Since you know where your laser is pointing, a lens on the receive side   
   >   
   > would be a big help!   
   >   
   >   
   >   
   > Cheers   
   >   
   >   
   >   
   > Phil Hobbs   
   >   
   >   
   >   
   > --   
   >   
   > Dr Philip C D Hobbs   
   >   
   > Principal Consultant   
   >   
   > ElectroOptical Innovations LLC   
   >   
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   > hobbs at electrooptical dot net   
   >   
   > http://electrooptical.net   
      
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