Adding an ADC to the Raspberry Pi app note (PDF!) from FTDIChip:
This application note will show how to connect the RPI-Hub-Module to the Raspberry Pi and install FTDI drivers to control the module. The application note will then explain how the module may be used to read values from an external ADC device over the SPI interface.
FTDI drivers? No siree, never, never, never again.
ADC over SPI? Hmmm… I’ve seen the contortions some people did while attempting various kinds of interfacing. First I’d replace that board with a set-top box chip with something else.
Don’t flame me! I’m just tweaking people’s noses! ;-)
U2A output should have something like R4 to be stable
Ftdi-no-more
Instead one can use one of the following:
ADC Pi / MCP3424 / MCP3428
MCP3008
PWM/Comparator
ADS1115