Catalyzes the condensation of 2 ATP molecules into cyclic di-AMP (c-di-AMP) (PubMed:25605729). c-di-AMP is a signaling compound secreted into the host's cytosol where it triggers the cytosolic surveillance pathway (CSP), a host pathway of innate immunity characterized by expression of beta interferon (IFN-beta) and coregulated genes (PubMed:20508090). Expression of truncated proteins (missing first 80 or 100 residues) in E.coli leads to c-di-AMP synthesis (PubMed:25605729). 2 ATP = 3',3'-c-di-AMP + 2 diphosphate Optimal activity at 0.5-1 mM CoCl(2), also functions with Mn(2+), for a construct missing residues 1-80. Homodimer, required for active site formation (Probable). Essential, it cannot be deleted. Belongs to the adenylate cyclase family. DacA/CdaA subfamily.