High-speed CMOS image sensors are available in several types for a variety of markets, for use in general purpose, high end or custom high-speed cameras. Such cameras can be used for applications like scientific research, crash tests, high-speed scanning, machine vision and military research; all requiring high frame rate motion capturing.
The sensors have resolutions from VGA up to 10Mpixels and some can run at 10,000 full frames per second. The sensor architecture can consist of two halves, quadrants or one pixel array. The outputs can be parallel analog outputs, one digital 10 bit output or digital serial LVDS outputs. The outputs operate at speeds up to 50Msamples/s each, thus realizing a 5.5Gpix/sec pixel throughput. This is the highest reported continuous pixel throughput for an image sensor to date. Image quality is at least 10bit, so after digitization in the camera the data throughput can be 55Gbit/sec. The target applications always require a 6T snapshot pixel with a high sensitivity and high dynamic range. The sensitivity of these image sensors depends greatly on the pixel size. This results in very big pixels and thus very big custom image sensors for some specific applications. Internal multiplexing schemes allow random windowing with increased frame rate. When reducing the window size to a small ROI, the frame rate rises up to 170,000 frames/sec. Most sensors are realized in a 0.25 process.
Recent trends in high speed image sensors
Today CMOS is the technology preferred for high-speed imaging. In today's market we can clearly see three trends in high-speed image sensors; very high speed, feature integration on-chip and generic high-speed imagers.
Pixel rate is the product of resolution and frame rate, and it is advancing by huge proportions. Today we can announce an image sensor of 1024x1024 pixels, working at more than 5000 full frames per second. Taking 10-bit quality into account, this means a total data rate on camera level of 55Gbit/sec. To achieve this extremely high data rate on the sensor level, combined with high image quality and (typically for this type of application) a very high sensitivity, it is important to not only focus on the design with respect to correct schematics, but also making sure the whole layout is well balanced. This means that power lines need to be well-distributed and all parasitic effects, electrical and optical, on each node in the layout should be well controlled. The power budget requires the designs of low power modules to keep within the total power requirement.
Examples of applications for high speed image sensors.
A different trend, in high-speed imaging, is the integration of high-speed ADC's, sequencers, LVDS transmitters and correction algorithms on-chip. These imagers are generally inferior with respect to speed and sensitivity to the imagers above but compensate this with ease-of-use and system integration capabilities. A third type of imagers we see emerging in the market today are generic high-speed imagers. Older (simple) generic imagers with analog outputs or without on-board timing generation are being replaced by faster and more complex image sensors. These imagers allow generic high-speed cameras to be built in a short amount of time.
Next: The pixel
