Arduino PWM and Analog Output (True Analog Output using Low Pass Filer and DAC) - Pro HP Halvorsen - South-Eastern Norway Uni
https://www.halvorsen.blog/documents/technology/resources/resources/Arduino/Arduino%20PWM.pdf

* Arduino UNO has no true built-in Analog Output Channels (only PWM)
* ***What if we need a real Analog Out Signal (0-5V)?***
* We will use a 2 different options:
  (1) Create a RC Lowpass Filter that converts PWM to Voltage
  (2) Use a DAC chip/IC (Digital to Analog Converter)
* Such a chip uses either the SPI bus or the I2C bus