Pixel-by-pixel digital control of gain and offset correction for video imaging
View Patent ↗A method and apparatus for adjusting, on a pixel-by-pixel basis, the gain and offset in an AFE as the pixels are sequentially processed. Although the method can be used for any purpose, it is directed in particular to light source non-linearity, such as edge effects of a scanner. A unique clocking method clocks the gain and offset values into the register at a higher clock rate than the image sampling rate.
1. An image digitizer comprising:
(a) an image input;
(b) an analog front end comprising
(1) a correlated double sampler (CDS) coupled to said image input;
(2) a programmable gain amplifier (PGA) coupled to an output of said correlated double sampler;
(3) a multi-bit digital to analog converter coupled to an offset input of said CDS;
(4) a multi-bit digital gain bus coupled to a gain input of said PGA;
(c) a multi-bit gain/offset bus for providing multiple multi-bit gain and offset values in series;
(d) a gain register and an offset register connected in parallel to said multi-bit gain/offset bus; and
(e) a clocking circuit configured to clock data into said gain and offset registers at a higher clock rate than the clocking of image data through said image input.
2. The image digitizer of claim 1 wherein said offset input of said CDS is connected to an input stage of said CDS.
3. The image digitizer of claim 1 wherein said PGA is a switched capacitor, split digital to analog converter amplifier having:
two banks of capacitors;
a serial capacitor connecting said two banks of capacitors; and
an operational amplifier having a virtual ground input connected to said two banks of capacitors, wherein said multi-bit digital gain bus connects to switches which determine which of said capacitors in said banks of capacitors are connected to said virtual ground input.
4. The image digitizer of claim 1 wherein said multi-bit is 10 bits.
5. The image digitizer of claim 1 wherein:
said gain and offset registers comprise, for each of said gain and offset values, a pipeline of three registers including an input register, a holding register and a final register; and
wherein said clocking circuit is configured to
serially clock said multiple multi-bit gain and offset values into said input registers during a first sampling period;
clocking the contents of all of said input registers, in parallel, into said holding registers during a first part of a second sampling period;
clocking the contents of all of said holding registers, in parallel, into said final registers during a second part of said second sampling period;
such that the contents of said final registers are available to said offset inputs of said CDS and said gain inputs of said PGA during a third sampling period.
6. The image digitizer of claim 1 further comprising:
a processor configured to determine which pixels in an image sensor are defective, and to provide said gain and offset values to compensate for said defective pixels.
7. The image digitizer of claim 1 further comprising:
a processor configured to determine lighting and lens nonlinearities, and to provide said gain and offset values to compensate for said nonlinearities.
8. The image digitizer of claim 1 further comprising multiple analog front ends, gain and offset registers for multiple colors.
9. An image digitizer comprising:
(a) an image input;
(b) a plurality of analog front ends for each of a plurality of colors, each analog front end comprising
(1) a correlated double sampler (CDS) coupled to said image input;
(2) a programmable gain amplifier (PGA) coupled to an output of said correlated double sampler;
(3) a multi-bit digital to analog converter coupled to an offset input of said CDS;
(4) a multi-bit digital gain bus coupled to a gain input of said PGA;
(c) a multi-bit gain/offset bus for providing multiple multi-bit gain and offset values in series;
(d) a gain register and an offset register, for each of said colors, connected in parallel to said multi-bit gain/offset bus, said gain and offset registers comprising, for each of said gain and offset values, a pipeline of three registers including an input register, a holding register and a final register; and
(e) a clocking circuit configured to clock data into said gain and offset registers at a higher clock rate than the clocking of image data through said image input, wherein said clocking circuit is configured to
serially clock said multiple multi-bit gain and offset values into said input registers during a first sampling period;
clocking the contents of all of said input registers, in parallel, into said holding registers during a first part of a second sampling period;
clocking the contents of all of said holding registers, in parallel, into said final registers during a second part of said second sampling period;
such that the contents of said final registers are available to said offset inputs of said CDS and said gain inputs of said PGA during a third sampling period.
10. An image digitizer comprising:
(a) an image input;
(b) an analog front end comprising
(1) a correlated double sampler (CDS) coupled to said image input;
(2) a programmable gain amplifier (PGA) coupled to an output of said correlated double sampler;
(3) a multi-bit digital to analog converter coupled to an offset input of said CDS;
(4) a multi-bit digital gain bus coupled to a gain input of said PGA;
(c) a multi-bit gain/offset bus for providing multiple multi-bit gain and offset values in series;
(d) a gain register and an offset register connected in parallel to said multi-bit gain/offset bus; and
(e) a clocking circuit configured to clock data into said gain and offset registers at a higher clock rate than the clocking of image data through said image input;
(f) wherein said PGA is a switched capacitor, split digital to analog converter amplifier having:
two banks of capacitors;
a serial capacitor connecting said two banks of capacitors; and
an operational amplifier having a virtual ground input connected to said two banks of capacitors, wherein said multi-bit digital gain bus connects to switches which determine which of said capacitors in said banks of capacitors are connected to said virtual ground input.