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Photodiode Array Noise Compensation Circuit

IP.com Disclosure Number: IPCOM000083218D
Original Publication Date: 1975-Apr-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Baker, BB: AUTHOR

Abstract

Due to the shift register phase lines coupling with the video output signal, the output of a photodiode array, Fig. 1, exhibits variable output levels when uniformly illuminated. The circuit described minimizes the effect of this noise on the thresholding of the photodiode array.

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Photodiode Array Noise Compensation Circuit

Due to the shift register phase lines coupling with the video output signal, the output of a photodiode array, Fig. 1, exhibits variable output levels when uniformly illuminated. The circuit described minimizes the effect of this noise on the thresholding of the photodiode array.

The noise generated by the phase lines coupling with the photodiode output is periodic and has a frequency equal to a fraction, 1/n, of the scan rate of the array. For example, with a RETICON linear array, n=4. Thus, the same amount of noise is added to every fourth diode output, Fig. 2.

Before undergoing any processing, the output of each photodiode of the array is integrated. The integrator is reset to zero volts just prior to the accessing of the next photodiode. When the photodiode array is uniformly illuminated, the noise due to coupling of phase lines and video signal results in a stairstep pattern out of the integrator, Fig. 3, rather than the flat pattern that would be expected. This stairstep pattern is repeated every n/th/ diode.

The magnitude of the stairstep pattern can be greatly reduced by injecting a current spike into the integrator. Since the noise pattern is repeated every n/th diode, the appropriate selection of magnitude and duration of n current spikes would cancel the effect of the noise, thus producing integrated video output that is a constant level prior to integrator reset when the array is uniformly illuminated, Fig. 4.

The circuits needed to achi...