In “Filter DC Voltages Outside Your Supply Rails” we saw how a popular active-bandpass filter topology could be re-imagined into a building block for a form of higher-order lowpass filter that doesn’t “see” the DC potential on the signal it is filtering. This lowpass filter is a form of RDC filter, as introduced previously in “Bruton Charisma”. Its values are derived from those of an LCR passive filter. The last article begged some questions:
(1) Why can’t we just use an old-fashioned passive filter, completely getting around the problem of providing a supply voltage for active components?
(2) What response characteristics do we need from a lowpass filter in order that it has some benefits for filtering practical bias and reference voltages?
(3) Are there any hidden catches in the use of this “DC-free” filter?
Elaborating on a previous article, Kendall Castor-Perry (“The Filter Wizard”) shows how higher-order lowpass filters made using a “DC-free” technique can be much more effective in removing ripple from high-voltage sources, while responding rapidly to step-voltage changes and requiring far-lower capacitor values. The Figure shows the Excel plot of the fitted prototype filter response for the approach which the article discusses.
To read the entire article, click here.
About the author
Kendall Castor-Perry is a Principal Architect at Cypress Semiconductor Corp., doing mixed-signal system analysis and design for the new PSoC platform. Kendall uses decades of experience in analog engineering, filtering and signal processing to capture signals across many domains, extract the information from them and do something useful with it.