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