1. A loop filter receiving first and second current respectively via first and second input nodes, the first and second current having a current ratio of n, n being a natural number, comprising: a first-order filter path including an operational amplifier, the first-order filter path performing a first-order filtering on the first current using the operational amplifier; a second-order filter path performing a second-order filtering on the first current; and a third-order filter path performing a third-order filtering on the first current, wherein the current ratio n of the first current to the second current is variable.