VV6801 Ver la hoja de datos (PDF) - Vision
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VV6801 Datasheet PDF : 23 Pages
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VISION VV6801/5801 PRELIMINARY CUSTOMER DATASHEET Rev 1.1
4. Removing Noise
There are many possible ways achieve FPN cancellation in order to produce the highest quality stills images
from the VV6801 sensor. The exact method chosen will depend on the intended use of the imager system,
and the ancillary devices available in the system, such as the frame buffer and mechanical shutter typical of
a Digital Stills Camera. A number of schemes are discussed.
In order to obtain high quality, low noise images from the VV6801 sensor pixel to pixel offset variations, or
Fixed Pattern Noise (FPN), must be removed. This can be done by reading the image array more than once,
for example reading in the dark to establish a reference for each pixel, then reading the exposed array to
collect ‘image plus offset’ data, then subtracting to remove the offsets. To obtain the lowest noise operation
the random pixel ‘reset’ noise must also be removed.
4.1 Sources of Fixed Pattern Noise
The major sources of Fixed Pattern Noise in the sensor that can be cancelled are:
y • Transistor Threshold Offsets
r • Dark Current
Each of the above can be effectively cancelled to a much lower residual random noise level by using the
a techniques described below. The residual noise sources in the sensor, such as flicker noise, dark current shot
noise, thermal noise and ADC Quantisation noise, that cannot be cancelled, or are a function of the
in cancellation techniques, define the overall camera noise performance.
4.2 Methods of Removing Fixed Pattern Noise
4.2.1 Transistor Threshold Offsets
lim Each pixel amplifier, each column source follower and each output channel multiplexer, has a unique offset
caused by process variations in the threshold voltage of the transistors. This offset is independent of
exposure, and will be relatively stable with respect to temperature and operating conditions.To remove
Transistor Threshold FPN, the VV6801 is used in conjunction with an ADC and either a frame buffer or a line
e buffer:
• Pixel offset removal frame by frame with a shutter: A frame buffer is used to obtain the pixel to pixel
r DC offsets for the whole image. The offsets are obtained by capturing a dark (FPN) frame with the
shutter closed, and an ‘image’ frame with the shutter open. The ‘clean’ image data can then be extracted
P by subtraction. (This technique can only be used with a physical shutter, and with at least one extra dark
frame acquisition period.)
• Pixel offset removal frame by frame with a reference frame: A non-volatile frame buffer is used to
obtain the pixel to pixel DC offsets for the whole image at camera build. These offsets are then
subtracted from the exposed ‘image’ as it is read to obtain the ‘clean’ image data. (This technique gives
the fastest frame acquisition time at the expense of accuracy.)
• Pixel offset removal line by line: A line of pixel information is read and stored in a line buffer. The line
is then reset to black using the CDSR signal, before being re-read to obtain the pixel to pixel DC offsets
for that line. As the line is re-read the offset data for each pixel is subtracted from the value stored in the
line buffer, the result being the ‘image’ data. (The COLsam signal must be used to ensure that samples
in the same line have the same integration period.)
With line by line offset removal the time for reading out a complete frame is doubled, since each line has to
be read twice. It is also not possible to remove pixel reset noise or dark current, thus there is a trade off
between the frame readout rate and image quality, and the amount of memory required.
Full frame offset removal can be achieved in many ways, depending on what ancillary devices are available
in the camera system, and constraints such as image quality required and acceptable minimum frame
readout rate.
4.2.2 Dark Current
The ‘dark current’ in a pixel photodiode is the inherent leakage that discharges the integrating capacitance
in the same way as incident light. Hence, Dark Current FPN builds up on the array whenever the array is
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VISION VV6801/5801 PRELIMINARY CUSTOMER DATASHEET Rev 1.1
released from reset, that is when FI is high. This means that the amount of dark signal depends on exposure
time, and varies from pixel to pixel.
The same degree of dark current charge build-up occurs in the array whether or not the array is exposed to
light. Therefore, if the array is allowed to integrate (FI high) with no incident light for the same length of time
as for the image exposure, the dark current element of the exposed image data can be ascertained and
removed from the image data by subtraction, leaving behind the dark current shot noise.
Since dark current also depends on temperature the dark frame should be taken close in time to the image
frame, in order to avoid ambient temperature variations.
4.2.3 ‘Reset Noise’ Cancellation
One random noise source that can be cancelled is ‘reset noise’ (or ‘kTC’ noise), which is due to the switching
of the photodiode capacitance when the pixel is released from reset. This is present in all subsequent reads
of the array (without reset) to the same extent. These can therefore be extracted by reading the array
immediately after reset (when FI goes high) and subtracting the value obtained from the ‘exposed’ array data.
y This operation also cancels Pixel Threshold Offsets.
r To achieve reset noise cancellation, FR should be taken high for two LCK periods when FI goes high, and
1306 lines read before the array is exposed to the required image. The pixel data from this pass of FR through
a the VSRs must be stored in a frame buffer, and subtracted from the exposed image data. The exposed image
is obtained when FR is pulsed high again, coincident with the last two LCK periods of FI being high after the
exposure period.
in It is not possible to describe all of the many operating schemes that can be devised for image capture and
FPN reduction. The basic recommended modes for camera operation are described in Section 5., with
Prelim detailed timing requirements in Section 7.
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