Front-end signal processing circuit and imaging device
View Patent ↗A front-end signal processing circuit that stabilizes a black level of an output signal of an image sensor in a prescribed set level, without being influenced by a DC offset component of circuit elements making up a feedback loop, and an imaging device including such the front-end signal processing circuit, are provided. The front-end signal processing circuit includes a feedback loop made up of a luminance detecting/digitizing section and a black level clamp section, and clamps a black level of an output signal of an image sensor to a prescribed set level. The front-end signal processing circuit further includes an offset correction section. The offset correction section stores an offset value being a difference between a signal level of an OB region of the image sensor and the prescribed level, subtracts the offset value from a digital luminance signal corresponding to an effective pixel region of the image sensor, and outputs the obtained signal.
1. A front-end signal processing circuit, comprising:
a luminance detecting/digitizing section that detects luminance information included in an output signal of an image sensor having an optical black region and an effective pixel region, clamps a black level of the luminance information to a prescribed set level based on a feedback voltage, and converts the luminance information into a digital luminance signal;
a black level clamp section that generates the feedback voltage based on a digital luminance signal corresponding to the optical black region and supplies the feedback voltage to the luminance detecting/digitizing section; and
an offset correction section that corrects a difference between a black level of the digital luminance signal corresponding to the effective pixel region after clamping and the prescribed set level, based on the digital luminance signal corresponding to the optical black region.
2. The front-end signal processing circuit according to claim 1 , wherein the offset correction section comprises:
an offset storage section that stores the difference between a signal level of the digital luminance signal corresponding to the optical black region and the set level as an offset value; and
a subtracter that subtracts the offset value stored in the offset storage section, from the clamped digital luminance signal which is output from the luminance detecting/digitizing section and corresponds to the effective pixel region.
3. The front-end signal processing circuit according to claim 2 ,
wherein the offset storage section calculates an average value of the digital luminance signal corresponding to the optical black region on a plurality of horizontal lines of the image sensor, and stores a difference between the average value thus obtained and the set level as the offset value.
4. The front-end signal processing circuit according to claim 2 , wherein the offset storage section is a low-pass digital filter.
5. The front-end signal processing circuit according to claim 1 , further comprising:
a digital programmable gain amplifier that amplifies the digital luminance signal which is output from the offset correction section and corresponds to the effective pixel region based on a gain control signal.
6. The front-end signal processing circuit according to claim 1 , further comprising:
an addition section that adds a prescribed value being a reference of the black level to the digital luminance signal corresponding to the effective pixel region and outputted from the offset correction section.
7. An imaging device, comprising:
an image sensor having an optical black region and an effective pixel region; and
the front-end signal processing circuit according to claim 1 , that processes an output signal of the image sensor.