STM32F3 ADC fully differential errata

Having some trouble with a fully-differential signal driven directly into STM32F303 setup for differential mode.  There is an errata about common-mode voltage having to be within 10% of center which may or may not apply.  Does ST consider the common-mode to be the average of the two signals -- which is always centered?  Or do they use the term to simply be the voltage you present to the negative ADC input?  (Would appreciate knowledge-based answer rather than options based on general knowledge and reading the datasheet)