ADC Common Ground


I am working with a sensor that requires a 10-30VDC source. I am not seeing the proper way to tie together the ground from my voltage source with the ground for the ADC on the board. I tried to connecting the ground pin on the voltage source to the ground pin on the board, but that did not give me the effect I was looking for. Is there any documentation on the proper way to read voltage from a device NOT being powered by the 5V source on the board?


That is- insofar as I’m aware- the only way to tie the grounds between your external circuit and an ADC. However:

  1. What board are you using?
  2. What results did you expect when tying grounds?
  3. What results did you get?

  1. I am using the automation hat
  2. I expect to get a potential difference between the data pin on the sensor and the ground of about 4.2V, this is the potential I measure from the ground to the data pin using my multimeter.
  3. I get a value of around 0.02V, which is why I think the ground is not being properly tied to the ADC ground even when I connect the ground from my 24V source (to power the sensor) to one of the three ground pins on the automation hat.


Measuring (successfully) with DMM:


Measuring (unsuccessfully) with ADC/Automation Hat:


The ADC inputs are protected by a voltage divider which is effectively a 940k Ohm resistor to ground from the perspective of the sensor. This small load on the sensor might be what’s affecting the voltage- if the sensor is only capable of supplying a tiny current then it would encounter voltage drop.