Wondering what is the best way to interface both ratiometric (absolute) and non-ratiometric (relative) sensors to a single a/d ( an A/D with, say, a 8 channel mux,internal ref, external ref option)
I think the most obvious is
1) Disable the internal reference, connect the ratiometric excitation power to the external a/d reference pins (so the ratiometric sensor outputs track with the a/d reference). For the non-ratiometric sensor inputs read the internal reference and use that to calibrate the non-ratiometric (absolute) readings.
or maybe,
2) Enable the internal reference so you can correctly read the non-ratiometric reading without calibration, for the ratiometric sensor, read its excitation power with a spare channel and use that to calibrate the ratiometric readings.
and perhaps you can do this
3)Power the ratiometric sensor (they only require 1 mA) with an external precision reference and also connect that reference to the external a/d reference pins. No calibration needed for either.