Browse Prior Art Database

SELF-CALIBRATING DC RESTORE CIRCUIT

IP.com Disclosure Number: IPCOM000026704D
Original Publication Date: 1993-Apr-30
Included in the Prior Art Database: 2004-Apr-06
Document File: 4 page(s) / 219K

Publishing Venue

Xerox Disclosure Journal

Abstract

The video output of linear photodiode arrays typically have an array of imaging pixels which are used to scan an original document in an electrophotographic system. In addition to these pixels, there are a number of pixels which are covered with a light shield and thus do not respond to light.

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XEROX DISCLOSURE JOURNAL

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SELF-CALIBRATING DC RESTORE Proposed Classification CIRCUIT U.S. C1.358/241 David J. Metcalfe Int. C1. H04n 3/14

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XEROX DISCLOSURE JOURNAL - Vol. 18, No. 2 MarchlAprill993 211

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SELF-CALIBRATING DC RESTORE CIRCUIT (Cont'd)

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212 XEROX DISCLOSURE JOURNAL - Vol. 18, No. 2 MarcWApril 1993

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SELF-CALIBRATING DC RESTORE CIRCUIT (Cont'd)

The video output of linear photodiode arrays typically have an array of imaging pixels which are used to scan an original document in an electrophotographic system. In addition to these pixels, there are a number of pixels which are covered with a light shield and thus do not respond to light.

The analog output of a linear photodiode array is normally superimposed on a DC biased voltage level, which may not be compatible with the input voltage range of a corresponding analog to digital (MI)) converter. Thus, the video output of the linear photodiode array must be changed by a DC restore circuit so that the video output voltage, of the linear photodiode array, is compatible with the conversion range (the range of voltages between VREF-H and VREF-L) of the A/D converter.

During the dark reference pixel time, while scanning an original document, the output voltage levels from the linear photodiode array are the input voltages to a DC restore circuit, by the actuation of a switch. The DC output voltage of the linear photodiode array, is then shifted by an amount equal to the voltage difference between the voltage levels of: 1) the DC restore circuit, and 2) the DC voltage level of the dark reference pixels.

With an ideal linear photodiode array, for example a diode array characterized by a zero peak to peak voltage nonuniformity and offsets, the DC restore circuit voltage would be equal to the upper voltage limit (VREF-H of the A/D
conversion range. However, due to the particular nonuniformities, of any given linear photodiode array, the DC restore circuit voltage has to be adjusted so that all pixels fall within the A/D range, in order to accomplish a correction in gain and offset voltage.

It is desirable to adjust the DC restore circuit voltage level so that the uppermost imaging pixel is equal to or just below the VREF-H reference voltage level of the ALD converter, so that the dynamic range, and the number of gray levels, and the system signal to noise (SN) ratio can be optimized. Therefore, for every linear photodiode or imaging array, there exists an optimum DC restore circuit voltage level (VDCR)...