Patent ID: 7782496

Claim:
An image signal processing circuit, comprising: an input terminal configured to input an image signal obtained by optically reading image information contained in an original; a sample hold circuit coupled to the input terminal and configured to sample and hold a specified range of the image signal to output a sampled image signal; a variable gain amplifier coupled to the sample hold circuit and configured to amplify the sampled image signal with a gain to output an amplified image signal; an analog digital converter coupled to the variable gain amplifier and configured to convert the amplified image signal to image data; and a gain control circuit coupled to the analog digital converter and the variable gain amplifier and configured to control the gain of the variable gain amplifier based on the image data, the gain control circuit comprising: a signal level detector configured to detect a signal level of the image data output from the analog digital converter; and a gain calculating circuit configured to determine whether the detected signal level is substantially equal to a target level to generate a determination result, obtain an adjusted value of the gain of the variable gain amplifier based on the determination result, and sets the gain of the variable gain amplifier equal to the adjusted value such that the signal level of the image data is made substantially equal to the target level.