This chart shows the frequency measured with respect to the input voltage. It is a linear relationship, except at the lower frequencies where the relationship becomes asymptotic. This is problematic becomes the voltage range at these low frequencies is quite large (~1.1 voltage differential) for a frequency range that is extremely small. There is a great potential for losing accuracy here.
We didn’t have time to experiment and figure out how we could get around this problem so we had to go with only the ADC0831.