Analog sampling on the Beaglebone Black only reads noise

Question

I want to write a script in python that will do data acquisition with the beaglebone black. However, I'm only getting noise when I read in values.

I have a circuit set up to feed in a signal (I was running about a .1-10 Hz 12v square pulse), the voltage regulator limits the voltage to 5v, and the voltage divider drops it down to a safe voltage (~1v) for the analog pin on the BBB.

Circuit:

enter image description here

Then using a python script I wanted to log the values into an array and plot them once the program finished.

In ipython shell:

In [1]: import Adafruit_BBIO.ADC as adc
In [2]: import time
In [3]: AIN0 = "P9_39"
In [4]: T = 0.1                           # or some other value
In [5]: adc.setup()
In [6]: a = []
In [7]: while True:
            a.append(adc.read(AIN0)*1800) # pin value times 1.8 ref voltage
            time.sleep(T)

After running for a while, I crash the script and write a to a file to be scped to my desktop. But when I plot it, it's only noise. I've hooked up on o-scope between AIN0 and ground and the wave form is what I expect. I also set up a potentiometer like so:

enter image description here

And I am able to read in the correct values through python. So, I figure it must be something about doing a continuous sampling that's creating problems.

Unless there are connections you're not showing, the circuit you're using doesn't make sense. The AIN0 and ADC_GND nodes would be at the same potential. — user149341

Claudio Claudio · Accepted Answer · 2013-09-27T12:46:27

If you don't connect the power supply ground to your Beaglebone ground it will not work, and you should indeed see only noise, since AIN0 will be sampling the Beaglebone's ground. You have to connect them in order to AIN0 see the signal of interest.

fixed schematic

See the "missing connection" wire. If you don't have that you have no return path for the current coming from the power supply (blue arrows), so there can't be any voltage across the pontentiometer (remember Ohm's law: voltage = resistance x current. If current is zero, the sampled voltage must also be zero).

As for the script part, you can directly write the sampled data to a file with this:

with open('sampled_data.csv', 'w') as f:
  while True:
    f.write(','.join(str(adc.read(AIN0)*1800)))
    time.sleep(T)

When you interrupt the script you'll get the sample_data.csv file, with all values separated by commas (,), which is easily importable to a spreadsheet or other software you use to plot it.

Analog sampling on the Beaglebone Black only reads noise

1 Answers