Pixel Size vs Sensor Efficiency

Started May 18, 2012 | Discussions thread
DSPographer
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Re: You have ignored read-out speed
In reply to bobn2, Jul 10, 2012

Bob,

To simultaneously read out more than two rows of pixels through the first stage pixel source follower, we also need to route more column output wires in parallel. This is why more than two row read-out can be difficult when using front illumination.

bobn2 wrote:

DSPographer wrote:

Great Bustard wrote:

DSPographer wrote:

Your statement that "perfect scaling" results in lower effective read noise is flawed. The reason is that the time available to read each pixel changes when the pixel count is increased while the frame rate is held constant.

Well, how 'bout we say that we simply scale the number of readout channels in proportion to the pixel count so we read out the pixels at the same rate in a "perfect scaling" scenario?

That would be difficult to do.

It is easy to do

Currently all the columns of one or two rows are transferred in parallel to read-out circuitry above and/or below the sensor array.

But not one set of readout circuitry. In the column parallel ADC systems (Sony, Panasonic GH) there is a separate set of readout circuitry for each column, often two for each column (top and bottom of the array). For others there is one analog channel leading to each ADC, arranged in banks of pixels. The ADC channels do not share a common analog channel (that would negate the point).

From there either all columns continue to be processed in parallel, as in Sony's sensors, or they are processed partially serially through several high-speed outputs and A-D converters, like Canon's sensors.

They are processed in parallel, just not as parallel as the column ADC model.

Either way the number of parallel first stage amplifiers active at one time in CMOS imaging sensors is normally limited to two times the number of columns in the array.

So, the number of column amplifiers scales only be the square root of the pixel count. If the column amplifier noise is a major noise source, then you need to scale the number of channels by the square of the pixel rate factor. That is if the column amplifier is a major noise source. If it isn't the amount of extra read channels is fewer.

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Bob

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