1. A loop filter, which receives first and second currents whose current ratio is n (where n is a natural number), the loop filter comprising: a first-order filter path including an operational amplifier generating an output impedance by increasing by as much as n times an impedance of a second input node to which the second current is applied, the first-order filter path performing a first-order filtering on the first current applied to a first input node by using the operational amplifier; a second-order filter path performing a second-order filtering on the first current applied to the first input node; and a third-order filter path performing a third-order filtering on the first current applied to the first input node.