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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

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Decimal Number

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Binary Number 2n2n subtract Left over binary = Binary Decimal

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Binary voltage pulse and reference pulse 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 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 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 = = 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 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 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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