1. Field of the Invention
The present invention relates to a front-end signal processing circuit that processes an analog signal outputted from an image sensor, and an imaging device including such the front-end signal processing circuit.
2. Related Art
The analog signal outputted from an image sensor such as a CCD (Charge Coupled Device) is first processed by the front-end signal processing circuit, and is outputted to a DSP (Digital Signal Processor) at the following stage. JP-A-2001-189892 discloses an imaging device having the image sensor and the front-end signal processing circuit. The image sensor has an effective pixel region in which light from an object is received and photoelectric conversion of the light is performed, and an optical black region (hereinafter referred to as an “OB region”) being an optical black area in which the photoelectric conversion is not performed, and the imaging device outputs an output signal corresponding to the effective pixel region and the OB region. The front-end signal processing circuit includes a luminance detecting/digitizing section that samples the output signal of the image sensor and converts the sampled signal into a digital luminance signal, and a black level clamp section that makes up a feedback loop together with the luminance detecting/digitizing section. The black level clamp section supplies a feedback voltage to a CDS so that the digital luminance signal corresponding to the OB region is coincident with a prescribed set level, and controls the CDS so that the black level of the digital luminance signal corresponding to the effective pixel region is clamped to the prescribed set level. The clamped digital luminance signal corresponding to the effective pixel region is amplified and outputted to the DSP at the following stage. The luminance detecting/digitizing section includes circuit elements such as a correlated double sampling section (hereinafter referred to as a “CDS”) and an A/D converter (hereinafter referred to as an “ADC”), and the black level clamp section includes a circuit element such as a D/A converter (hereinafter referred to as a “DAC”).
If a DC offset component is not included in each circuit element such as CDS, ADC, and DAC included in the luminance detecting/digitizing section and the black level clamp section, the black level of the digital luminance signal corresponding to the effective pixel region and clamped based on the digital luminance signal corresponding to the OB region becomes equal to the set level. However, since the DC offset component is usually included in these circuit elements, an error is generated by an influence of the DC offset component, and the black level of the output signal corresponding to the effective pixel region is not equal to the set level. Therefore, there is a problem that an output corresponding to an original signal level of the effective pixel region can not be obtained. For example, when a difference between the signal level of the effective pixel region and the black level becomes small by the DC offset component, there is a problem that an image becomes darker than an original image. Particularly, when the output signal corresponding to the effective pixel region is amplified and outputted to the DSP at the following stage, the error by the DC offset component becomes large in accordance with an amplification factor. Therefore, there is a problem that an output obtained in the DSP becomes far removed from the original signal level of the effective pixel region.