I have a python example here[1] of how to setup the Beagle to do continuous analog in and plot it via gnuplot.
The example also shows which device tree to edit to set the sampling rate to 200k samples/second.
I hope someone finds it helpful.
–Mark
[1] https://elinux.org/EBC_Exercise_10a_Analog_In#Analog_in_-_Continuous