Observations (yay!):

1. 50 Hz noise still can easily couple into the system

2. LEDs with a larger bandgap (for 400nm, off by 0.3eV) can still detect some 450nm light from white LEDs

3. there's other noise present, possibly because of the USB 5.0V power, microcontroller and UART activity - and it's correlated among the channels

200 samples/sec, 200 pC range:

plot for [i=2:9]'20170102-16 capture20.txt' u ($0/200):i smooth kdensity bandwidth 0.01 t sprintf("ch%d",i-2)