Enable i2c and install python libs for OPI zero

1) armbian-config   and set i2c-0 or i2c-1 (or both)
and reboot OPI

2) check the existence of /dev/i2c-0 or /dev/i2c-1

3) apt install gpiod  
    apt install python3-libgpiod

NOTE: Access to GPIOs is possible only via root (a solution might exists but I did not spend time on it)

As root:

4) edit ~/.bashrc and add
export PYTHONPATH=/usr/lib/python3/dist-packages

5) Create virtual env and activate
python -m venv ~/adsvenv/
source ~/adsvenv/bin/activate

6) Install packages 
pip3 install setuptools_scm
pip3 install adafruit-circuitpython-ads1x15
pip3 install adafruit-blinka


Access to configuration
-----------------------

import board
import busio
import adafruit_ads1x15.ads1115 as ADS
i2c = busio.I2C(board.SCL, board.SDA)
adc = ADS.ADS1115(i2c)

adc._read_register(0) - data register
adc._read_register(1) - configuration register

Gain
----
9:11 bit

e.g. output of config register
11000111 10100011 -> 011 (9:11 bit)

000 : FSR= ±6.144V
001 : FSR= ±4.096V            (gain = 1)
010 : FSR= ±2.048V (default)  (gain = 2)
011 : FSR= ±1.024V            (gain = 4)
100 : FSR= ±0.512V            (gain = 8)
101 : FSR= ±0.256V
110 : FSR= ±0.256V
111 : FSR= ±0.256V

Configuration:
 set gain
   adc.gain = 4

Mode

set mode