Time to collect some data and let it guide development! I was using the setup shown in the earlier picture, and wanted to verify that I could still get a valid signal from the Lock-In Amp. Earlier experiments with a smaller LED showed things to be working.
All the necessary functions are under control of the Teensy 3.0. The Lock-In Amp (ADA2200) needs both configuration and a clock. Configuration is done using the SPI port. The ADA2200 data sheet gives the codes for configuration. (The Arduino sketch for all of this will be attached.) The FrequencyTimer2 library is used to create a 10KHz clock.
The ADA2200 will divide the 10KHz clock by 8 to get a Sample Clock and by 64 for the Reference Clock. The Reference Clock modulates the LED driver and the Sample Clock is fed back to the Teensy to trigger A/D sampling of the signal produced by the ADA2200.
With all the signals hooked up, I tried collecting data. Nothing! The signal was at the maximum and nothing I could do would make it change!
Time to get out the scope. Checking the clocks showed them all to be as expected. Probing the signal from the Lock-In Amp showed it to be pegged as expected. But then further probing made the expected signal appear. Seemed that just touching the Lock-In Amp board made the signal appear. WTFO?! At first I suspected that the capacitance of my finger was affecting things, but further tests showed this wasn't the case. To make a long story short, I realized that when I blocked light from the LED that wasn't coming through the slit, I got better results.
And of course that makes perfect sense! Should of realized it much sooner. Any light from the LED that reaches the photodiode is sensed as signal! That's because it's modulated by the Reference Clock and measured by the Lock-In Amp as signal. That is, it's correlated noise - indistinguishable from noise. Experimenting further with a shield over the sensor showed significant improvement in getting a reasonable measurement.
Time to build a new enclosure to properly shield the sensor!