Digital filter and filtering method

The digital filter has a first-stage decimation filter and a second-stage decimation filter. The second-stage decimation filter has a shifter that performs shift operation on a filter coefficient, a complementer that performs complementary operation on the filter coefficient, a reset circuit that resetting the filter coefficient, and an adder that adds values selected from the filter coefficient, a shift operation result from the shifter, and a complementary operation result from the complementer.