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