View Full Version : Strange DPI indicated in datasheet for TCD2711DG

29-Jun-2015, 21:36
Hi, I'm new, I've joined this wonderful community as I like doing low budget large format photography.

So, I did CCD area sensor based machine camera last year and decided to make my own scanner back using a CCD linear sensor.
One feature of my design is using multiple linear sensors, so if there is 60mm that I need to scan, using 6 linear sensors(these things are thick, and cannot really fit more than that, not to mention each unit means additional stitching time and more IC for ADC) each units would only have to scan 10mm, reducing the scan time.

Exciting part:
I saw this 7500pixels long color linear CCD sensor that can do upto 30MHz, and since output from shift register is separate for each colors, number of clock cycles per line readout is virtually halfed to 3800~, so optimistically around 7894lines per second. This is very optimistic, but even a fraction of that would be useful. So assuming lower bound read out time of 2000lines per second, we could scan 10mm at 600DPI, so 24DPmm, 240lines, and we can scan it in almost 1/10th of a second. The difficult part is 6 outputs per sensor, so...36 signal outs from the whole set up. It's not surprising faster scan time means larger burst of ADC operations...

Strange part:
Looking at the header of the datasheet, since it's 7500pixels per line(actually 7500*3 for each RGB, but let's think of 40+bit color pixel), 600DPI, so I assumed 7500/600 =12.5inches long sensor. It would be painful to find lens to fit and I would have to build the camera, but it is do-able. I continued to down to the actual dimension and the sensor is only 70mm long!

So I've went back to the time chart and circuit diagram and noticed that 7500pixels are arranged in single line, but with double shift registers. So, really, there are 7500pixels stuffed in 70mm line, so what is going on? I find it a bit odd how they managed to cram two shift registers per sensor array, but I don't see any pixels that are not sent out.

Why would they mark it off as a 600DPI scanner? 7500 pixels/ 70mm = 107DPmm = 2717.8DPI.

30-Jun-2015, 07:08
Not sure where you got the 600 dpi. Perhaps in the datasheet? 24 lines/mm in A3 format? Around 600 dpi?

That's what happens when marketing gets their hands in the IC datasheet.

2700/600 = around 4.5X. That's the magnification of the lens if the sensor were/was used in a particular case: the design of a scanner covering the A3 size . Can also be used to estimate the width of the scanning path. 4.5 x 70mm = 315mm = 12.4"