1) Can V_neg be greater than V_pos (assuming both are within the legal 0-3.3V range)?

2) How many counts does the ADC report when V_pos = V_neg?

Suppose that I have a balanced differential input (so V_pos + V_neg = V_cm), where both inputs can range from 0.6-2.2V and V_cm is approximately constant at 1.4V. If (V_pos = V_neg) maps to zero counts, then this can only usefully measure when V_pos >= V_neg. If (V_pos = V_neg) maps to half scale, and V_neg can be larger than V_pos, then the full 3.2V swing of the difference can be measured.

LT has a nice summary of different kinds of differential ADCs:

http://cds.linear.com/docs/en/product-selector-card/2PB_sar_input_types.pdf

I am really hoping the STM32F303 has a "fully differential" (unipolar) ADC. I just can't determine that from the available documentation.

Does anyone have more information? I may just have to breadboard this and see what it does, but I'd like to see published specifications if they exist. Thanks.

(edited to correct an error and to generally tidy up)

Cheers, Hal