Patent ID: 7245178

Claim:
An analog filter circuit, comprising: a band pass filter; a low pass filter and a high pass filter, to which an input signal with a reference frequency is input, the low pass filter and the high pass filter having correlation to the band pass filter in filter characteristics and wherein a frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while the corresponding line of the high pass filter linearly rises, both the lines crossing at the reference frequency; a pair of DC converters that convert respective filter signals from the low pass filter and the high pass filter to respective DC signals; and a comparison section that feeds back an adjustment signal corresponding to a signal level difference between the respective DC signals to the band pass filter, the low pass filter, and the high pass filter, and wherein the pair of DC converters comprise; an inverter section that inverts the filter signal; a first differential input pair to which the filter signal and a reference signal are input; a second differential input pair to which an inversion signal output from the inverter section and the reference signal are input; and a converter section that outputs the DC signal according to a signal level difference that the filter signal has with respect to the reference signal, obtained by the first differential input pair, and a signal level difference that the inversion signal has with respect to the reference signal, obtained by the second differential input pair, and wherein the adjustment signal compensates a gain difference between the filter signals from the low pass filter and the high pass filter, and accordingly, the filter characteristics in the band pass filter are adjusted.