The PGA and black level auto-calibration are controlled through a simple serial interface. The timing circuitry is designed to enable users to select a wide variety of available CCD and image sensors for their applications. Read back of the serial data registers is available from the digital output bus.
Tuesday, September 14, 2010
Digital Camera System
A digital camera data flow likes
Light -> Lenses -> CCD -> AFE -> Image Processing -> Memory
In camera front end (AFE) includes CDS, PGA and ADC. The main functions of the AFE in dataflow order:
Sampling: sample the analog data from the CCD. Many samplers offer Correlated Double Sampling (CDS) which sample twice: once a reference level and once the data. Those samples are subtracted to eliminate noise (dark current and die inconsistency). The Correlated Double Sampler (CDS) subtracts the CCD output signal black level from the video level. Common mode and power supply noise are rejected by the differential CDS input stage.
Amplification: most AFE’s have a Programmable Gain Amplifier (PGA), which lets the user adjust the gain according to the expected lighting of the scene. The PGA is digitally controlled with 10-bit/12-bit resolution on a dB scale, resulting in a some gain range. The PGA can be programmed to switch gain every pixel, in a user defined pattern of up to some different gains. A control logic allows a camera system to set the desired gain ratios for color balance. The system gain can then be changed by writing to a single register, and the color balance will be maintained.
ADC: Analog to Digital Converter that converts the analog input to digital output.
A quick CCD Image Processing Prototyping Platform paper is in
TI digital camera solutions using DMVA1 and DM368:
TI solutions using DM365/DM355:
A CCD vertical driver:
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