The formula of calcium sulphate is:

The formula of Calcium sulphate is CaSO₄.
Ammonium sulphate CaSO₄ is a salt composed of positively charged calcium cation, i.e. Ca⁺², and negatively charged sulphate anion, i.e. SO₄^-2, which carries a formal charge of -2. Both of these ions combine, and by cross multiplication of the formal charge, we obtain the chemical formula of the salt, i.e. (Ca)₂(SO₄)₂. In this Ca and SO_4, both are having the same number of ions so it can be simplified as CaSO₄.
 Ca⁺² + SO₄^-2→ CaSO₄