In 3e, they did this:
We've talked about the different AC philosophy in 5e, and I think the general feeling is that the Amulet shouldn't convert directly. So what could it do?This amulet, usually crafted from bone or beast scales, toughens the wearer’s body and flesh, giving him an enhancement bonus to his natural armor bonus of from +1 to +5, depending on the kind of amulet.
I think there are a few ways to go with a conversion. First idea is that it provides a base AC, like armor or the Mage Armor spell. If the Amulet provides a base AC, you can't use it with armor, since only the best base AC is considered.
The second option is that it provides a bonus to AC like Bracers of Defense or a Ring of Protection. This option can potentially lead to a lot higher ACs because you could wear plate mail AND then use an Amulet with it. I don't like this option as much but we could maybe give it a limited effect. I could see using a reaction to invoke the Amulet, which provides an AC bonus for that round only (like the Shield spell), and make it only usable on a limited basis (once or twice per long rest, or maybe uses equal to your con modifier, minimum 1 time). I think if you can use the amulet 1 or 2 times per long rest you probably don't need attunement. If you get uses based on your Con modifier, it should require attunement.
The third option is that it doesn't provide an AC bonus, but rather a different protective effect, like resistance. You could vary the effect depending on how powerful you want to make it. For example a single fixed, always-on resistance is just a varient on armor of resistance, but a limited use effect that you can use as a reaction a few times per rest is less powerful. Or you could allow the user to chose which resistance they want and then that resistance is in effect until after you finish a long rest.
What do you guys think?