Binds cyclic di-AMP (c-di-AMP) and is probably involved in c-di-AMP-mediated signaling pathways. In vitro, can also bind cyclic GMP-AMP (3'3'-cGAMP), with lower affinity, but not c-di-GMP or 2'3'-cGAMP. Homotrimer. Binds three molecules of c-di-AMP, each in a pocket located between two subunits. c-di-AMP binding is accompanied by conformational changes of both the fold and the position of the B-loop. Not essential.