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Signal Processor for Photodiode Line Scanners

IP.com Disclosure Number: IPCOM000086507D
Original Publication Date: 1976-Sep-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Hattersley, JR: AUTHOR

Abstract

Photodiode line scanners are high density monolithic linear arrays of silicon photodiodes with integrated scanning circuits for serial readout. Each element of the array includes a photodiode in parallel with a capacitor. In operation, the capacitors are first charged. Then the photodiodes are exposed to light, which allows a reverse current in the diode, thus discharging the capacitor proportional to the intensity of the light on the diode. The amount of light on each element is then determined by measuring the current required to recharge that element's capacitor. The array of elements is scanned during which time the capacitor recharging takes place. The output signal obtained from each scan of the array of N elements is a series of N charge pulses each proportional to the light intensity on the corresponding diode.

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Signal Processor for Photodiode Line Scanners

Photodiode line scanners are high density monolithic linear arrays of silicon photodiodes with integrated scanning circuits for serial readout. Each element of the array includes a photodiode in parallel with a capacitor. In operation, the capacitors are first charged. Then the photodiodes are exposed to light, which allows a reverse current in the diode, thus discharging the capacitor proportional to the intensity of the light on the diode. The amount of light on each element is then determined by measuring the current required to recharge that element's capacitor. The array of elements is scanned during which time the capacitor recharging takes place. The output signal obtained from each scan of the array of N elements is a series of N charge pulses each proportional to the light intensity on the corresponding diode. An example of such a pulse for one element is shown as the diode current in Fig. 2.

Fig. 1 shows a system which includes a self-scanning diode array 1 of the type described above. The array is actuated by driving and timing circuits 2 and provides an output signal that is amplified at 3 and then fed to integrating and sample-and-hold circuits 4. Such circuits are shown in more detail in Fig. 3. Array 1 may be of the type having a series of even-and-odd elements so that every other diode has a different output level and gain. Level adjustment 5, e.g., adjustable potentiometers, can be used to adjust the levels to provide the same light intensity and the s...