Analog VLSI Phototransduction by continuous-time, adaptive, logarithmic photoreceptor circuits

Abstract

Over the last few years, we and others have built a number of interesting neuromorphic analog vision chips that do focal-plane time-domain computation. These chips do local, continuous-time, spatiotemporal processing that takes place before any sampling or long-range communication, for example, motion processing, change detection, neuromorphic retinal preprocessing, stereo image matching, and synthesis of auditory images from visual scenes. This processing requires photoreceptor circuits that transduce from light falling on the chip to an electrical signal. If we want to build analog vision chips that do high-quality focal plane processing, then we need good photoreceptors. It's not enough to just demonstrate a concept; ultimate usefulness will be determined by market forces, which, among other factors, depend a lot on raw performance. The receptor circuits we discuss here have not been used in any commercial product, so they have not yet passed that most crucial test, but by every performance metric we can come up with, including successful fabrication and test of demonstration systems, they match performance criteria met by other phototransduction techniques that are used in end-product consumer electronic devices. We hope that this article will serve several purposes: We want people to have a reference where they can look to see the functioning and practical problems of phototransducers built in a typical CMOS or BiCMOS process. We want to inspire people to build low-power, integrated commercial vision devices for practical purposes. We want to provide a photoreceptor that can be used as a front end transducer in more advanced research on neuromorphic systems. The transduction process seems mundane, but it is important --GIGO comes to mind. Subsequent computation relies on the information. We don't know of any contemporary (VLSI-era) literature that comprehensively explore the subject. Previous results are lacking in some aspect, either in the circuit itself, or in the understanding of the physics, or in the realistic measurement of limitations on behavior. We'll focus on one highly-evolved adaptive receptor circuit to understand how it operates, what are the limitations on its dynamic range, and what is the physics of the noise behavior. The receptor has new and previously unpublished technical improvements, and we understand the noise properties and illumination limits much better than we did before. We'll also discuss the practical aspects of the interaction of light with silicon: What are the spectral responses of various devices? How far do light-generated minority carriers diffuse and how do they affect circuit operation? How effective are guard bars to protect against them? Finally, we'll talk about biological receptors: How do their functional characteristics inspire the electronic model? How are the mechanisms of gain and adaptation related?

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