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Integrated solutions make analog filter design easy

Posted: 11 Nov 2009 ?? ?Print Version ?Bookmark and Share

Keywords:analog filter? filter design? switched capacitor?

Analog filters play an important part in electronic signal-synthesis subsystems, providing functions such as anti-aliasing and noise filtering for ADCs, and reconstruction post-filtering for DACs. Different design specifications and requirements dictate the use of the particular filter configurations. The most popular filters include Bessel, Butterworth and elliptic.

Bessel low-pass filters provide a linear phase response with a ripple-free passband and monotonic roll-off. These characteristics make Bessel filters ideal for time domain applications. Butterworth low-pass filters have a maximally flat frequency response in the passband with a monotonic roll-off that is much sharper than the Bessel filter. Their phase response varies nonlinearly with frequency. These characteristics present no problems for amplitude-based applications. Finally, elliptic low-pass filters have a nearly flat passband response with an extremely sharp roll-off characteristic. Elliptic filters are ideally suited to amplitude-based anti-aliasing applications.

Design and construction of continuous-time active filters pose substantial design challenges and require the use of multiple high-performance op amps surrounded by many matched tight-tolerance passive components. Design challenges include selection of the optimal filter configuration, as well as the use of specialized software for filter synthesis. A simpler approach might include the use of highly integrated switched-capacitor filters (SCFs)such circuits can reduce the number of components, make it easier to tune the filter and trim system power.

This article examines a filter implementation, first using a continuous-time filter and then with an SCF approach to show the differences in performance and complexity. As mentioned previously, due to their characteristics, Bessel filters are ideal for time domain applications, as they have no distortion in the oscilloscope/analyzer type measurements. However, the cost for the perfect characteristic filter is that the designer must construct a higher order Bessel filter (having more poles than a Butterworth or elliptic filter) to achieve a suitable level of stopband attenuation.

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