Patent ID: 6344773
Filing Date: 2002-02-05
Classification: H03H

Abstract:
A low-pass filter circuit having an input for receiving an analog input signal Vin and having an output for producing a filtered analog output signal Vout, wherein the filter circuit has a gain, defined by a ratio of Vout to Vin, having a magnitude and a phase dependent upon frequency w, the filter circuit coupled between the input and the output for providing the magnitude, such that the magnitude as a function of frequency includes a passband to pass low frequencies and a stopband to stop high frequencies, and a cutoff frequency w0 in a transition region therebetween, the filter circuit causing the gain to conform to a second-order frequency-dependent function expressed as: H0&it;w02s2+s&it;w0Q+w02wherein s=j.w, where j represents a complex number equal to the square root of negative one, H0 represents the DC gain of the filter circuit, and Q represents the Q factor or pole quality factor of the filter circuit, the filter circuit comprising:a first and second resistors coupled between the input and the output; an operational amplifier having a negative input, a positive input, and an output; a third resistor connected between the first resistor and the negative input of the operational amplifier; a first capacitor connected between the negative input of the operational amplifier and the output of the filter circuit; a feedback element having an input, an output, and a negative gain forming a positive feedback path to achieve a very flexible choice of filter parameters w0, H0, and Q; and a second capacitor connected between the output of the feedback element and the first resistor.