From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 1/12/2014 12:51 PM, haiticare2011@gmail.com wrote:   
   > Oz: When you control both the emitter (eg led) and sensor (eg PD with   
   > transimpedance amp) with same reference signal.   
   >   
   > I have seen some very simple designs and more complex. The more   
   > complex generally have phase lock and other features.   
   >   
   > Going for the simple, I have seen circuits which just use a square   
   > wave to excite an led and the same reference signal is read by an ADC   
   > on a mcu. In the simplest case, the "off" reading is subtracted from   
   > the "on" reading. Alternatively, this can be done in the analog realm   
   > by switching an op amp with a cmos switch and a low pass filter on   
   > the output.   
   >   
   > Anyone know any drawbacks to the simple? (ISTM you won't get the full   
   > SNR benefits w/o phase lock, and also a square wave has odd   
   > harmonics.)   
   >   
   > An interesting twist on this lock-in is the algorithm described by   
   > Horowitz in his Art of Electronics, ca. p. 1027. The algorithm is so   
   > simple it is mind-bending: Just add many measurements, and the   
   > "grass" on top of the mountain will become visible. No averaging   
   > involved in its execution. I know this has been used in the Hubble   
   > telescope, and wonder if anyone has used this in more mundane apps.   
   >   
   > Horowitz explains this by noting that the noise increases as the sqrt   
   > of number of observations, but the buried signal increases in a   
   > linear fashion. So, if the measurement value is 100 with noise   
   > fluctuation 5, and the signal is say .01, then after 10exp6   
   > measurements, the signal is now 10,000 and the noise fringe 5,000.   
   >   
   > The signal value will be 100 million from the addition, so a   
   > microprocessor seems a good way to do this.   
   >   
      
      
   The only difference between adding and averaging is dividing by the   
   number of measurements.  The SNR improvement is unchanged.  This has   
   been known since the time of Gauss, at least.   
      
   Cheers   
      
   Phil Hobbs   
      
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   Dr Philip C D Hobbs   
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