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Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs

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Counting 01101 01110 01111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001 00000 00001 00010 00011 00100 00101 00110 00111 01000 01001 01010 01011 01100 13 14 15 16 17 18 19 20 21 22 23 24 25 0 1 2 3 4 5 6 7 8 9 10 11 12

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Decimal Number

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Binary Number 2n2n 2727 2626 2525 2424 23232 2121 2020 subtract1286432168420 Left over 50 1822220 binary10110010 =10110010 Binary Decimal

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Binary voltage pulse and reference pulse. 0 1 1 0 1 1 1 0 Reference pulse

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The Compact Disk (CD)

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Distance between tracks 1.6μm 150nm 0.83μm 0.5μm Use the dimensions of the bumps and flats to estimate the storage space of a CD.

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Example: The laser of a typical DVD player has a frequency of 4.70 x 10 14 Hz. Calculate the minimum height of the bumps (depth of pits) that must be etched onto the CD in order that the stored data can be read. d Receiver/emitter

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Advantages of digital storage over analogue storage Quality and Corruption Reproducibility (accuracy) Portability and high capacity Manipulation

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Data Capture; Digital imaging using CCDs A charge-coupled device (CCD) is a type of complimentary metal oxide semiconductor (CMOS) used in digital imaging. When light (photons) are focused on the surface of a CCD, electron-hole pairs are produced in each pixel. The number of electron-hole pairs produced is proportional to the intensity of the incident light (photons). The free electrons migrate to relevant electrodes resulting in a change in potential across the pixel. The magnitude and position of the potential is converted to a digital signal. At a simple level each pixel acts as a capacitor storing specific charge, resulting in a specific voltage (pd).

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Things to remember. C = Capacitance (Farads F) Q = Charge (Coulombs C) V = Voltage or Potential Difference (volts = J/C = V) Energy of a photon E = Energy (Joules J) f = frequency (hertz = 1/s) c = speed of light 3.0 x 10 8 m/s λ = wavelength (meters m) h = planks constant 6.63 x 10 -34 Js

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silicon pixels _ _ + _ _ + _ _ + - - - - - - - - - pd

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Example: Suppose that a pixel has a capacitance of 40pF as a result of light incident on the pixel for a period of 30ms, the change in potential across the pixel is 0.24 mV. Calculate the rate at which photons are incident on the pixel.

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Quantum efficiency Magnification Resolution

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Quantum efficiency: The percentage of photons in the incident light that produce electron-hole pairs. Typical values are 70-80%

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Magnification

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Resolution: The total number of pixels in the image collecting area of the CCD. 2500x2000 pixels = 5000000 = 5Megapixels (Mp) Resolution is also a function of the spacing between individual pixels

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Quality: The quality of the image is a function of the magnification and the resolution:

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Example: The collection area of CCD used in a particular digital camera has an area of 30mm x 30mm. Each pixel has an area of 2.2 x 10 -10 m 2. Estimate the resolution of the digital camera.

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Example: Light of wavelength 430nm and intensity 1.4MWm -2 is incident on a pixel of area 2.2 x 10 -10 m 2 for 20ms. The capacitance of the pixel is 25pF. Calculate the change in potential difference across the pixel if the quantum efficiency of the CCD is 70%

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