Digital automatic gain control circuit for image system

Details Digital automatic gain control circuit for image system Digital automatic gain control circuit for image system

1998-02-02

2001-08-14

Garber, Wendy R. (Department: 2712)

Television

Camera, system and detail

Combined image signal generator and general image signal...

C348S230100, C348S362000, C348S673000, C341S138000, C341S139000

Reexamination Certificate

active

06275259

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to automatic gain control circuits. More specifically, the present invention relates to apparatus and methods for digital automatic gain control for image systems.
2. Description of the Prior Art
Due to the advent of multimedia communications, the need has arisen for low-cost solid state imagers to complement communication devices and computers. The image input device is an integral part of any teleconferencing and multimedia application. An important advantage of complementary metal-oxide semiconductor (CMOS) image sensors is that signal processing circuits can be readily integrated on the same chip as the imager, thus enabling the design of smart single-chip camera systems. CMOS imagers are inherently lower cost than conventional charge-coupled devices (CCDs) because the CMOS imagers can be manufactured in conventional, widespread CMOS fabrication lines without any process modification.
Present day electronic cameras employ automatic gain control (AGC) circuits that dynamically adjust the amplification gain (i.e., dynamic range) of the light-induced electrical signal so that a relatively “dark” picture appears “bright” to the user. The automatic gain control circuits are implemented with analog circuits because the light-induced electrical signal is analog. With a CMOS imager sensor, it is possible to incorporate the analog-to-digital conversion function on the same chip as the image sensor. Accordingly, the light-induced electrical signal can be converted into the corresponding digital signal early in the signal processing chain. This technique is disclosed in co-pending U.S. patent application Ser. No. 08/876,694, entitled “Image Sensor with Direct Digital Correlated Double Sampling”, filed on Jun. 12, 1997, assigned to the assignee herein, incorporated by reference herein. Therefore, it is desirable to implement automatic gain control circuits using entirely digital means. It is to be appreciated that eliminating analog circuits from the system will make circuit design easier, and more compatible with conventional digital CMOS fabrication technologies.
SUMMARY OF THE INVENTION
The present invention relates to apparatus and methods for automatic gain control in which the automatic gain control function is performed entirely in the digital domain. In an illustrative embodiment, the digital automatic gain control circuit for an image sensor having associated therewith an analog-to digital converter (ADC) for converting analog electrical signals from the image sensor to corresponding digital codes, includes a min/max detector for determining minimum and maximum electrical signal values from the digital codes of the ADC for each frame of image. A digital filter, e.g., a low pass filter, is coupled to the min/max detector and dampens instantaneous changes of the minimum and maximum values to provide filtered minimum and maximum values. A digital-to-analog converter DAC coupled to the filter generates minimum and maximum analog reference voltages corresponding to the respective minimum and maximum filtered values, the reference voltages being applied to the ADC to control associated amplitudes of the digital codes provided thereby.
Optionally, the image sensor may be a CMOS sensor with ADC circuitry disposed in proximity to the image sensor array. Also, an analog low pass filter may be employed between the DAC and the ADC to further dampen instantaneous changes of the reference voltages. Additionally, the MIN/MAX detector may have encoded therein algorithms. The functions embodied in the algorithms may include averaging (low pass filtering) and/or detection and correction of excessively high or low maximum and minimum electrical signal values, respectively. Specifically, if excessively high or low signal values are detected, the MIN/MAX detector provides special codes to the DAC to increment or decrement the reference voltages supplied to the ADC in order to influence the instantaneous change in the reference voltages supplied to the ADC.


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