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# Datasheet question for CCD linear light sensor TCD2711DG

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#### Johnny Kang

##### Newbie level 4
At the header of the datasheet(you can google it, I wasn't sure if a datasheet counts as an ebook), it says 600DPI sensor.
However, it has 7500 pixels per line(7500*3 for RGB technically), and sensors are almost 70mm long. How does this translates to 600DPI?
By my calculation, 7500pix/70mm = 107DPmm = 2721.4 DPI.

I've never worked with linear sensors-although one of the uni assignment involved a ccd area sensor-.
Looking at the timing scheme, everything looks in order with 2 shift registers per color and really nothing that indicates certain pixels are left out.

Far as I can see from photos in the web, the glass element doesn't seem to do much other than keeping the dust out.

So, what's going on?

John

Correct, its not a 600dpi sensor and I cant see any mention of this in the data sheet...

Generally the DPI output of a scan is done by image scaling, though I presume you could somehow alter the number of sensors used during scanning. The thing is you can achieve a true 600dpi image with this sensor, the high pixel count will allow for downscaling or combining groups of pixels during scanning to achieve this level of output. In a way it gets a bit academic with not only scanning but any raster image (photographs for instance, my cameras sensor has approx. 4000 pixels per inch):
https://en.wikipedia.org/wiki/Image_scaling
https://entropymine.com/resamplescope/notes/photoshop/
https://www.cambridgeincolour.com/tutorials/image-resize-for-web.htm

Johnny Kang

### Johnny Kang

Points: 2
Thanks, this is both a good thing and a bad thing: The number of lines that I need to scan increased significantly as my DPI of the line itself increased. 2700DPI is pretty high given that it is a tri-linear ccd sensor not bayers style sensor witch each pixel having all three RGB pixels.

One good news I found is this: TCD2712DG It has 12 shift registers, twice of 6 from TCD2711DG, so I can scan it twice as faster...with twice the amount of ADCs...
Another question I have is consequence of using CCD with higher clock speed. I've read few papers where tests were done at 30MHz and the datasheet says the maximum clock speed is 30MHz, I have no idea what the difference would be compared to running it at say 16MHz.
Also, with 12 signal outs per CCD and I'm going to use 6 sensors in total, so the amount of ADC IC that I need is huge unless I use multi-channel ADCs. I have little experience with ADC, but is there a way to get around this? From the look of it, I may have to make modules for each CCD sensor with a 16 channel 14bit ADC.

John

Having worked on a board for something similar I believe you can multiplex the ADCs, or multiplex the analogue data fed to the ADCs reducing the circuitry, increasing the system complexity.
Texas and Analog sites would be the place to stqart, especially Ti, if I remember correctly they have some papers covering this sort of thing.

Johnny Kang

Points: 2