Departmental Papers (BE)

Document Type

Journal Article

Date of this Version

January 2005


We introduce a new approach to synthesizing Class AB log-domain filters that satisfy dynamic differential-mode and common-mode constraints simultaneously. Whereas the dynamic differential-mode constraint imposes the desired filtering behavior, the dynamic common-mode constraint solves the zero-dc-gain problem, a shortcoming of previous approaches. Also, we introduce a novel push–pull circuit that serves as a current-splitter; it rectifies a differential signal into the ON and OFF paths in our log-domain filter. As an example, we synthesize a first-order low-pass filter, and, to demonstrate the rejection of dc signals, we implement an adaptive filter by placing this low-pass circuit in a variable-gain negative-feedback path. Feedback gain is controlled by signal energy, which is extracted simply by summing complementary ON and OFF signals—dc signals do not contribute to the signal energy nor are they amplified by the feedback. We implement this adaptive filter design in a silicon chip that draws biological inspiration from visual processing in the mammalian retina. It may also be useful in other applications that require dynamic time-constant adaptation.


Copyright 2005 IEEE. Reprinted from IEEE Transactions on Circuits and Systems--I: Regular Papers, Volume 52, Issue 1, January 2005, pages 99-107.

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Adaptive filtering, artificial vision, class AB circuits, neuromorphic engineering



Date Posted: 22 November 2005

This document has been peer reviewed.