Patent ID: 7705667

Claim:
A filter adjusting circuit, comprising: a filter circuit which has a circuit configuration operating as an n-th order filter including n (n≧1) integrators and can switch a connection of the circuit configuration to operate as a circuit equivalent to the n integrators; a signal generating circuit that outputs a first signal having a predetermined reference frequency to the filter circuit, and outputs a second signal having the reference frequency; a phase comparator that compares a phase of a third signal and a phase of the second signal and determines a phase shift between the signals, the third signal being obtained by processing the first signal in the filter circuit and outputted from the filter circuit; and a control circuit that switches, in an adjustment mode for adjusting characteristics of the filter circuit, the connection of the circuit configuration to operate the filter circuit as a circuit equivalent to the integrators and adjusts the integrators composing the filter based on a determination result of the phase comparator such that a phase rotation of the first signal is 90° in each of the integrators, and switches, in a normal mode for normally operating the filter circuit after the adjustment mode, the connection of the circuit configuration in a state in which the phase rotation of the first signal is 90° in each of the integrators and controls the filter circuit to operate the filter circuit as the filter.