In any digitizing system, antialiasing and anti-imaging filters are used to prevent the signal frequencies from “folding back” around the sample frequency and causing false (or alias) signals from appearing in the signal we are attempting to digitize. Very often, these filters must be very complex, high order analog filters in order to do their job effectively. As sampling rates of converter systems have increased, however, oversampling may be used to reduce the filters’ stopband attenuation requirements(1)(2). In digital audio systems, 4x oversampling may be used, and it can be shown(3) that for an antialiasing filter (which precedes the ADC), a
simple sixth order filter may be used. For the output side, after the DAC, a simple third order filter may be used. Realizing these filters in a way that maintains extremely low noise and low distortion then becomes a challenge.

Compact disk player manufacturers began using a filter topology that was described many years ago—the Generalized Immittance Converter (GIC)(4). This topology allows one to easily realize active filters beginning from a passive filter design. In addition, the GIC filter provides extremely low distortion and noise, at a reasonable cost. Compared with more familiar feedback filter techniques, such as Sallen & Key filter topologies, the GIC filter can be shown to have superior noise gain characteristics, making it particularly
suitable for audio and DSP type applications(5)