PSN-L Email List Message

Subject: Re: Seismic sensors
From: Tangazazen@.......
Date: Fri, 29 Jun 2007 13:52:18 EDT


Hi Chris,
 
      Each of the photo detector quadrants  generate about 0.7 microamp which 
are fed into virtual earth charge  sensitive amplifiers.
There are numerous noise sources but as you point out the shot noise  
associated with the photo current is dominant. The quadrant detector is 8mm dia,  in 
fact a larger detector means a larger capacitance which in turn increases the  
noise. It is the  'lock-in amplifier'  approach that controls the  noise 
level. For an integration time of a few milliseconds the effective  bandwidth at 
10KHz is 100 - 200 or so Hz. Lock-in systems can pull out a signal  that is 
substantially less than the noise level.
 
The noise level of the LED has not been noticeable. The  random conversion to 
photons is offset by the use of a diffuse  encapsulation, a bit like an 
integrating sphere. The important requirement of  the LED is uniformity of the 
light spot and reasonable linearity when modulated.  The feedback from the sum of 
the quad elements is dynamic but there is a limit  to how much the amplifier 
loop can correct for nonlinearity.
 
LED temperature dependence under constant current conditions is non  linear 
but -0.7% over 20C to 80C is an approximate figure for a Gallium Arsenide  
Phosphide at 670nm. The feedback loop as mentioned overcomes any  temperature 
dependance. Interestingly LED temperature coefficients  seems to get smaller at 
the shorter wavelength but the quantum yield of the LED  and the response of the 
Silicon diode decrease, it's a question of  optimisation.  
 
If I had to choose between full capacitive bridge and a quad photo detector  
I would choose the latter, It's a much more elegant solution.
 
 
Regards Martin
 



   





Hi Chris,
 
      Each of the photo detector quadrants=20 generate about 0.7 microamp which are fed into virtual earth charge=20 sensitive amplifiers.
There are numerous noise sources but as you point out the shot noise=20 associated with the photo current is dominant. The quadrant detector is 8mm=20= dia,=20 in fact a larger detector means a larger capacitance which in turn increases= the=20 noise. It is the  'lock-in amplifier'  approach that controls the=20 noise level. For an integration time of a few milliseconds the effective=20 bandwidth at 10KHz is 100 - 200 or so Hz. Lock-in systems can pull out a sig= nal=20 that is substantially less than the noise level.
 
The noise level of the LED has not been noticeable. The=20 random conversion to photons is offset by the use of a diffuse=20 encapsulation, a bit like an integrating sphere. The important requirement o= f=20 the LED is uniformity of the light spot and reasonable linearity when modula= ted.=20 The feedback from the sum of the quad elements is dynamic but there is a lim= it=20 to how much the amplifier loop can correct for nonlinearity.
 
LED temperature dependence under constant current conditions is no= n=20 linear but -0.7% over 20C to 80C is an approximate figure for a Gallium Arse= nide=20 Phosphide at 670nm. The feedback loop as mentioned overcomes any=20 temperature dependance. Interestingly LED temperature coefficients= =20 seems to get smaller at the shorter wavelength but the quantum yield of the=20= LED=20 and the response of the Silicon diode decrease, it's a question of=20 optimisation.  
 
If I had to choose between full capacitive bridge and a quad photo dete= ctor=20 I would choose the latter, It's a much more elegant solution.
 
 
Regards Martin
 

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