Browse Prior Art Database

Preventing Overload in Optical Scanners

IP.com Disclosure Number: IPCOM000083345D
Original Publication Date: 1975-May-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Gaffney, DP: AUTHOR [+2]

Abstract

Optical area scanners using charge-coupled or bucket-brigade shift registers are prone to bloom or smearing, if for any reason excessive charge is accumulated on individual sense nodes. The excess charge may be due to an intense spot of light or a localized high-leakage area in the semiconductor substrate. The resultant effect may overload the support circuits or the charge may climb over potential barriers separating the various sense nodes, both contributing to smearing.

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Preventing Overload in Optical Scanners

Optical area scanners using charge-coupled or bucket-brigade shift registers are prone to bloom or smearing, if for any reason excessive charge is accumulated on individual sense nodes. The excess charge may be due to an intense spot of light or a localized high-leakage area in the semiconductor substrate. The resultant effect may overload the support circuits or the charge may climb over potential barriers separating the various sense nodes, both contributing to smearing.

The figure shows an embodiment of an improved optical scanner using bucket-brigade techniques. Within substrate 10 are a plurality of isolated diffusion pockets 12 acting as sense nodes for radiation 14. A transfer gate 16, responsive to transfer pulses VTR, allows accumulated charge to be gated into a shift register (aligned perpendicular to the plane of the paper). The shift register includes electrode 18 which is driven by a shift register clock SR.

On the opposite side of the sense node there is a long narrow diffusion 20 which runs the length of the scanner. Diffusion 20 is biased at a positive potential for an N-channel device and acts as a sink for excess charge picked up at any of the sense nodes. Diffusion 20 and sense node 12 act as drain and source for a field-effect transistor (FET) having a gate electrode 22.

The gate is biased at a reference potential, Vref, such that this transistor serves as a diode to limit the level which a sense node ca...