Patent ID: 7301678

Claim:
An image reading device, comprising: a charge coupled device which photoelectrically converts optical information and outputs an image signal in an analog form; a programmable gain amplifier which receives an output from said charge coupled device, amplifies the output with a programmable gain, and outputs the output; an analog digital converter which receives an output from said programmable gain amplifier, analog/digital-converts the output, and outputs an image signal in a digital form; and a central processing unit which calculates a gain which makes an output from said analog digital converter match a reference value by using an output from said analog digital converter and the reference value, and setting a gain for said programmable gain amplifier, wherein said programmable gain amplifier, comprises: an input terminal to which an output from said charge coupled device is supplied; a first switch and first resistor connected in series with each other between the input terminal and a node; a second switch and second resistor which are connected in series with each other and connected in parallel with the first switch and first resistor between the input terminal and the node; an nth switch and nth resistor which are connected in series with each other and connected in parallel with the first switch and first resistor, the second switch and second resistor, . . . , and the (n−1)th switch and (n−1)th resistor between the input terminal and the node; an operational amplifier whose inverting input terminal is connected to the node, non-inverting input terminal is grounded, and output terminal is connected to an input terminal of said analog digital converter; and an (n+1)th resistor connected in series between the output terminal of the operational amplifier and the node, and wherein said Central Processing Unit sets a gain by ON/OFF-controlling the first switch, the second switch, . . . , and the nth switch.