I used this system in my Star Wars games, and I proposed it at one point for my DnD game, but they didn't go for it.
Basically, instead of using a base of 10 for AC, you make a d20 roll and that counts as your AC base for that attack, adding any armor, shield, dex, magic, etc bonuses to that roll.
It adds extra die rolls to the game... but if you're playing tabletop, there's not much difference, really. For an online game, it would work if the DM made the opposed roll instead of the player.
It worked well in Star Wars, especially when my players had converted from d6 and were complaining that they didn't really have any "active" defense against attacks in the mechanics anymore.