Lecture on Analog to Digital Conversion

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• Lecture on Analog to Digital Conversion

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Definitions

• Digital source produces a finite set of possible
  values.
• Analog source produces a infinite set of possible
  values.
• Signal is a measurable quantity (e.g., voltage)
  which bears information.
• Noise is a measurable quantity which carries
  undesired interference.

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Why Digital?

• Less expensive circuits
• Privacy and security
• Small signals (less power)
• Converged multimedia
• Error correction and reduction

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Why Not Digital?

• More bandwidth
• Synchronization in electrical circuits
• Approximated information

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Analog to Digital Conversion

• Sampling Obtaining values at discrete points in
  time.
• Quantization Non-linear transformation that maps
  continuous values to discrete values.

Analog Signal - Continuous time - Continuous
value
Digital Signal - Discrete time - Discrete
value
Sample
Quantize
- Discrete time - Continuous value
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Sampling

w(t)
t
t1
t4
t3
t2
t5
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sampler
w(t)
ws(t)
ws(t)
w(t)
t
t
t1
t4
t3
t2
t1
t4
t3
t2
t5
t5
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Gated Samples
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Gated Sample and Hold
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Impulse Samples
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Sample and Hold
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Quantization
output w2(t)
V
-V
V
input w1(t)
-V
Region of operation
For M2n levels, step size ? ? 2V /2n
V(2-n1)
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Quantization Error, e
output w2(t)
V
-V
V
input w1(t)
-V
Error, e
?/2
input w1(t)
-?/2
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?
?
Error is symmetric around zero.
0
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Noise

• Types of Noise
• Quantizing noise (during A/D conversion)
• Environment noise (e.g., EM interference)
• Filtering noise (low pass filtering at decoder)
• Types of Quantization Noise
• Overload noise (input too large)
• Random noise (input too small)
• Granular noise (non uniform error jump)
• Hunting noise (too long of quite time)

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Non-uniform Quantizer
Used to reduce quantization error and increase
the dynamic range when input signal is not
uniformly distributed over its allowed range of
values.
allowed values
input
values for most of time
time
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Digital to Analog Converter Circuit
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Analog to Digital Converter Circuit
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