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need help

HELLO, we r in great trouble for my senior design project in school,
I will ask for some advice. my project is to make a device to find
polarization axis of a lens. we are passing laser light(using laser
pointer) through the lens of which axis needs to find and a analyzer
lens. if 2 lens's axis are perpendicular thn they block light. as we
know the axis of the analyzer, we will rotate the lens with a stepper
motor unless all the light is blocked. We r trying to detect the
light by using a photodiode. built atransimpedence amplifier without
any Cf using lm741. now we see theoutput is very noisy. gives very
very different datas at differnettime . i took data from the TIA and
tried to find fft of data andfound all signal is in DC and noises on
higher freqs. but spec saysoutput will be 100 level. i dont know y i
am getting output at DC. Ihave built a filter also, cutoff at 50 Hz.
Is filtering with this willhelp?? we need to detect measurable voltage
change in TIA with every 1degree rotation of the lens by the stepper.
we gonna use ADC to changethis voltage to use in microcontroller. my
question is whether i should use filter or need to use lock in amp.Is
modulating the signal or demodulating is easy?? i need to finishthis
part in 1 week. as i am a student dnt have tht much idea and timeat
all. I still have to work on the rest of the project by april. So,
what will be less time consuming ?? money is not a factor. if Ican buy
good less noisy transimpedence amp, filter or lock in amp itwill work
too. All i need less time consuming direction. If anybody give me some
suggestion I will be obliged. will eagerlywait for reply.
 
J

Jamie

Jan 1, 1970
0
HELLO, we r in great trouble for my senior design project in school,
I will ask for some advice. my project is to make a device to find
polarization axis of a lens. we are passing laser light(using laser
pointer) through the lens of which axis needs to find and a analyzer
lens. if 2 lens's axis are perpendicular thn they block light. as we
know the axis of the analyzer, we will rotate the lens with a stepper
motor unless all the light is blocked. We r trying to detect the
light by using a photodiode. built atransimpedence amplifier without
any Cf using lm741. now we see theoutput is very noisy. gives very
very different datas at differnettime . i took data from the TIA and
tried to find fft of data andfound all signal is in DC and noises on
higher freqs. but spec saysoutput will be 100 level. i dont know y i
am getting output at DC. Ihave built a filter also, cutoff at 50 Hz.
Is filtering with this willhelp?? we need to detect measurable voltage
change in TIA with every 1degree rotation of the lens by the stepper.
we gonna use ADC to changethis voltage to use in microcontroller. my
question is whether i should use filter or need to use lock in amp.Is
modulating the signal or demodulating is easy?? i need to finishthis
part in 1 week. as i am a student dnt have tht much idea and timeat
all. I still have to work on the rest of the project by april. So,
what will be less time consuming ?? money is not a factor. if Ican buy
good less noisy transimpedence amp, filter or lock in amp itwill work
too. All i need less time consuming direction. If anybody give me some
suggestion I will be obliged. will eagerlywait for reply.
you need to modulate the laser with a set frequency and use a decoder or
pass filer in the op-amp stage.
the photo detector should be set up to detect sudden changes of light
and not the DC component.
i guess you could do that a couple of ways.
Capacitive decoupling or common mode input to both + & - of the op-amp
having one input using a bypass cap to remove the DC. also you will need
to use the Nulling inputs of the 741 since the Cmr (common mode
rejection) isn't really that good and the offset is a little off.
 
R

Rich Grise

Jan 1, 1970
0
HELLO, we r in great trouble for my senior design project in school, I
will ask for some advice.

Next time get an earlier start.

Good Luck!
Rich
 
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