E Series Reference Voltage on the VDDA Pad Issue

I have an E Series board that I’m trying to use to get an accurate voltage reading for a battery bank. The problem is that the ADC is giving me a wide margin error(±1.5 volts. I think this may be related to how the VDDA pin is powered. This is partly my fault because I need to measure over a wide voltage range, (0-24v and even small variations in the reference voltage are causing wide swings in the measurement.

Despite this, does anyone know of a good way to supply a stable 3v3 volt reference to the VDDA pad?

Edit: I’ve been reading a little bit about the issue and some people are saying that this IC has the ability to calibrate the ADC. Is this something that Particle OS has given us the ability to use?