maybe like this , the msb of a var is the sign bit
to convert between unsigned an signed use abs() or vice versa var=~var+1

Code:
Vadc var word
Vref var word
vDif var word
aDif var word


vDif = Vref -  Vadc ;signed difference
adif= abs(Vref -  Vadc);absoloute unsigned difference


if vDif.15 then
do  negative stuff
else
do positive stuff
endif