Sorry I am not an expert at all in Analog nor work for the company... Nor know what ratiometric means or implies... Other than there is a ratio...
But if it were me, probably the first thing I would do is look at the data sheet for the different processors, which you can find up at:
https://www.pjrc.com/teensy/datasheets.html
But other alternatives if these don't work, is to find an external device that does the analog conversions for you and you can retrieve the values using something like SPI or I2C...
Example:
https://www.adafruit.com/product/1085
But again I don't know enough to know how well these will work for you...