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

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

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

Page 1: Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs.

Digital Technology

14.1 Analogue and digital signals

14.2 Data capture; digital imaging using CCDs

Page 2: Digital Technology 14.1 Analogue and digital signals 14.2 Data capture; digital imaging using CCDs.

Counting01101011100111110000100011001010011101001010110110101111100011001

00000000010001000011001000010100110001110100001001010100101101100

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Decimal Number

012 108107101 178

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Binary Number0

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21 178

2n 27 26 25 24 23 22 21 20

subtract 128 64 32 16 8 4 2 0

Left over

50 50 18 2 2 2 2 0

binary 1 0 1 1 0 0 1 0

=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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-8.0

-6.0

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sample

pd/V

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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 1014 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.

V

QC

C = Capacitance (Farads F)

Q = Charge (Coulombs C)

V = Voltage or Potential Difference (volts = J/C = V)

Energy of a photon

hc

hfE

E = Energy (Joules J)

f = frequency (hertz = 1/s)

c = speed of light 3.0 x 108 m/s

λ = wavelength (meters m)

h = planks constant 6.63 x 10-34Js

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silicon

pixels

_ _+ _ _+_ _+

- - - - - -- - -

pd pdpd pdpd 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 efficiencyMagnification

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

object of size

CCD on theobject of sizeM

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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:

QR

QM

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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 m2. 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 m2 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%