Implementing early visual processing in analog VLSI: light adaptation

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Abstract
A circuit which emulates the functioning of cone photoreceptors in the vertebrate retina has been designed and tested. Cone photoreceptors exhibit a local adaptation to background illumination over many orders of magnitude while retaining a high degree of instantaneous contrast sensitivity. This behavior permits visual discrimination of objects against difficult lighting situations such as bright backgrounds. This effort includes an examination of the trade-offs in various photodetection techniques available to the designer in silicon. Photodetection is followed by separate filter and gain stages which provide the appropriate temporal behavior. The filters and gain are independently tunable to permit extensions in operation to those environments which may fall outside the capability of human vision. The circuit also includes a UV writable floating gate which uses a locally generated error signal to provide cancellation of circuit offsets due to process variability.

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