Various Types and Applications of High Speed CMOS Image Sensors

In the broad market, there are several types of high-speed CMOS image sensors, namely general-purpose, high-end or customized high-speed cameras. These cameras are used in scientific research, impact testing, high-speed scanning, machine vision and military research. All applications require high frame rate motion capture.

The resolution of these sensors ranges from VGA to 10m pixels, and some can reach 10000 full frames per second. The sensor architecture has two and a half points, a quadrant or a pixel array. The output can be a parallel analog output, a digitized 10 bit output, or a digitized serial LVDS output. The working rate of each output is up to 50m samples / s, and the throughput of 5.5G pixels / s can be realized. This is the highest continuous pixel throughput reported so far. The image quality is at least 10 bits, so the digital data throughput in the camera is as high as 55gb / s. Target applications always require a 6T snapshot pixel with high sensitivity and high dynamic range. The sensitivity of these image sensors mainly depends on the size of pixels. Therefore, for some specific applications, it leads to a very large number of pixels, so that the image sensor is also very large. Internal multiplexing technology allows the implementation of random windows with increased frame rate. When the window size is reduced to a small ROI, the frame rate is increased to 170000 frames / second. At present, most sensors adopt 0.25 micron process.

The latest trend of high speed image sensing

Today, CMOS is the preferred technology for high-speed image sensors. In today's market, we can clearly see three development trends of high-speed image sensors, namely ultra-high speed, on-chip function integration and ordinary high-speed imaging.

Pixel rate is the product of resolution and frame rate, which has been improved a lot. At present, the published image sensor is 1024x1024 pixels, and the full frame rate per second is more than 5000. If the image quality is 10 bits, this means that the total data rate in the camera is as high as 55gb / s. In order to achieve such a high rate, high-quality image and very high sensitivity (usually used for high-quality image) in the camera, it is important not only to focus on the design of the circuit, but also to ensure a good balance of the whole wiring. This means that the required power lines must be well distributed. On each node of the circuit layout, all parasitic parameter effects, electrical and optical parts must be well controlled. The power budget requires a low-power module design to ensure that the total power demand can be met.

Another different trend in high-speed imaging is to integrate high-speed ADC, sequencer, LVDS transmitter and calibration algorithm on chip. These imagers are generally inferior to the above imagers in speed and sensitivity, but they have the advantages of high integration and easy to use. Now the third kind of imager emerging on the market is an ordinary high-speed imager. The old (simple) ordinary imagers with analog output or without timing function are being replaced by faster and more complex image imagers. This kind of imager can ensure the design of ordinary high-speed camera in a short time.

Pixel and pixel rate

Fig. 1 shows the implementation circuit diagram of pixels used in common high-speed image sensors, which is a so-called 6 transistor (6T) pixel structure. The pipelined spherical shutter function is important in this kind of image sensor.

Figure 1: pixel circuit unit

All pixels open and terminate the optically integrated spherical shutter at the same time, which is very important for high-speed applications. It can better realize motion fuzzy control and make all pixel pairs accurately consistent. The spherical shutter enables the high-speed motion sensing to be held by the imager.

A typical high-speed capture sequencer is shown in Figure 2 (like a small projectile hitting a matchstick). The pipelining function means that the optical integration in the pixels for the next frame is in progress during the reading of the pixel array. This requires ensuring that the frame rate is independent of the integration time.

Figure 2: typical high speed capture sequence (projectile hitting matchstick)

In order to obtain possibly higher sensitivity, the photodiode responsible for collecting image charge and converting it into voltage should be as small as possible in order to minimize its parasitic capacitance. In addition, the filling coefficient of the pixel, that is, the open area contributing to the optical sensing area, should be as large as possible. By using the N-well pixel patent and collecting the p-well open around the photodiode, the above two functions of small photodiode and large filling coefficient can be realized. In addition to high sensitivity, it is also important to use a pixel storage capacitor, which does not contribute any noise. It will shield light well and have little leakage. This pixel structure has a good effect in storing pixel signals in the reading process. However, the main disadvantage of this structure is that there is no fixed graphics noise correction in the pixel, so it must be realized outside the image sensor.

The rate of an image sensor is the product of resolution and frame rate, which determines the pixel rate of the sensor. In the ultra-high-end high-speed imaging market, this parameter is not high enough. Users want to design a very complex camera under the condition that the desired full frame rate can be achieved. Figure 3 shows an image of a typical high-speed application (car crash test).

Figure 3: high speed imaging application: vehicle impact test

This high speed can only be achieved by parallel analog output (up to 128 outputs), which poses a challenge for the integration of camera system. The structure of this imager is quite simple, including 6T pixel circuit in the pixel array randomly divided into quadrants, several parallel high-speed analog buses, and some parallel output amplifiers used to drive the output.

On this kind of chip, there is no ADC, sequencer and other on-chip image processing. The chip wide analog bus ensures that all parallel outputs can be used regardless of the size of the half frame image in the X direction being read. This improves the frame rate when reading a half frame image.

Ghost elimination

An important problem in ultra-high speed image sensor is ghosting in X direction. This is caused by the relatively large RC constant of the chip wide analog bus. For the signal on the bus, because it takes a long time to process the accuracy of 10 bits, part of the information of the previous pixel may remain on the current pixel. This results in ghosting in the X direction of the image. This ghosting is difficult to correct in subsequent image processing.

One technique to solve this problem is to precharge the bus briefly before each new signal. This ensures that all information about the previous pixels is washed away. This technology requires the generation of short-term precharge pulses. These pulses are used to short circuit the analog bus to ground. The vast majority of imagers are manufactured according to the needs of customer products, because there is no real demand to turn this kind of ultra-high speed imager products into general products. Customers' applications range from VGA to 10mpixel, frame rate from 500fps to 10000fps, and data throughput up to 5.5gpix/s. Figure 4 shows a typical ultra-high speed image sensor architecture. The two hemispheres are read in parallel, and the parallel analog output of each hemisphere is 64 channels. The total is 128 high-speed parallel analog outputs.

Figure 4: a typical ultra high speed image sensor architecture

Application requirements: small size and easy to design

In contrast to the previous very complex (and large) camera systems designed around sensors, the market demand for small and easy to implement high-speed image sensors is increasing.

High speed imagers have been used in several quasi consumer applications, such as scanning, vision systems and holographic data storage. The following figure shows a typical holographic data storage application, in which an imager is used.

Figure 5: holographic data storage and its high-speed imager

These applications require the image sensor on the board to have many system functions. This is why ADC, timing generation, image processing and some output circuits are integrated into the chip. For this kind of imager, the integrated function is as important as sensitivity and speed. Most of these imagers are customized according to the special functions of customers to simplify their high-speed camera design. The following figure shows a typical architecture of this kind of high-speed imager. This kind of imager usually has only clock input, some power supply and some synchronization pins. All other signals that need to be read and sent to the imager will be generated in the chip.

Figure 6: architecture of a typical high-speed image sensor with many on-chip logic and additional functions

Universal high speed image sensor

The third type of high-speed image sensor we see in the market is a general-purpose high-speed image sensor. Its applications range from machine vision camera to traffic monitoring, scientific test motion capture and impact test inspection. In the early days, this kind of general-purpose high-speed image sensor only included parallel ultra-high-speed imagers). Recently, however, we have seen many functions implemented on chip to enable the imager to be used in various applications (such as multi slope, undersampling, bin feeding, flipping, mirroring, gain, compensation, etc.).

Today, a high-speed star shutter image sensor capable of providing a 1.3mpxl rate and a frame rate of 1000fps is being developed. In particular, this kind of sensor has pipeline snapshot shutter function and multi slope function. These on-chip functions also vary with different sensors.

Figure 7: various applications for universal high-speed imager

There are several different types of high-speed image sensors to meet the different needs of the current market. Ultra high speed image sensor is a real analog image sensor with very high frame rate and data throughput, which is suitable for complex - therefore, it is mainly customized camera design. The high-speed image sensor with many on-board functions provides many special on-board functions. These functions help designers implant the imager into their high-speed camera design, which is more suitable for customer-oriented applications. These functions are implemented according to customer requirements, so they are also mainly used for customized design.

Finally, the general-purpose high-speed image sensor combines most of the general functions of the above image sensor, making it a general-purpose image sensor, which can be used in widely used cameras. Such image sensors are now commercially available from stock. The market trend is that the integration, data rate and resolution of on-board functions will continue to improve. The real challenge for future designers will be how to achieve the perfect combination of ultra-high data rate of sensors and on-board functions such as LVDS and image processing.

Figure 8: some examples of high-speed image sensor applications

Various Types and Applications of High Speed CMOS Image Sensors 1

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