Analog to Digital (A/D) Conversion

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Analog to Digital (A/D) Conversion
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A/D fundamentals

• A / D conversion is a process where an analog
  signal is converted into a numeric
  representation.
• Analog input is normally classified into one of
  two types and a voltage range
• Types and typical ranges
• Uni-polar (0 -1 V, 0 - 5V, 0 - 10V)
• Bi-polar (-1 to 1V, -5 to 5V, -10 to 10V)
• Digital output is normally binary at the hardware
  level and typically decimal at the software
  interface level
• Primary critical specifications for A/D
  conversions
• Resolution
• Conversion rate
• Elements contributing to inaccuracy
  (non-linearity, offset/bias, missing codes,
  non-monotonicity)

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A/D fundamentals

• Resolution
• Normally specified in terms of the number of
  binary digits that the analog value is converted
  to
• eg. 8 bit conversion, 12 bit conversion, 16 bit
  conversion
• Analog resolution can be computed from the
  specified resolution
• analog resolution analog range / 2(binary
  digits in result)
• example a bi-polar A/D converter set to input a
  range of -10 to 10 V with a 12 bit conversion
• analog resolution 20V / 212 20/4096 V /
  bit 4.9 mV/bit
• Conversion Rate
• The rate at which an A/D converter can make a
  conversion is critical. In general higher
  resolutions require greater time of greater cost
  or both.
• Typically specified in samples per second

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Transfer Function

• The ideal output from an A/D converter is a
  stair-step function (see right)
• Ideal worst case error in conversion is ? 1/2
  bit.
• Missing codes or the imperfections where
  increasing voltage does not result in the next
  step being output are described as monotonicity.
• Errors in A/D conversion may be significant
  particularly if the full range of the analog
  signal is significantly less than the range of
  the analog input of the A/D.

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A/D Converter Types

• Dual Slope Integrating

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A/D Converter Types

• Operation
• Input signal is integrated for a fixed time
• Input is switched to the negative reference and
  the negative reference is then integrated until
  the integrator output is zero
• The time required to integrate the signal back to
  zero is used to compute the value of the signal
• Accuracy dependent on Vref and timing
• Characteristics
• Noise tolerant (Integrates variations in the
  input signal during the T1 phase)
• Typically slow conversion rates (Hz to few kHz)

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A/D Converter Types

• Successive Approximation (Digital to Analog
  Conversion null balancing)
• 4 bit D/A using a summing amplifier and switch
• 4 bit D/A using R-2R ladder
• Digital value (D1, D2, D3 etc.) is converted to
  an analog value

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A/D Converter Types

• Successive Approximation

Converter Schematic
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• Conversion
• At start of conversion, the clock is used to
  cycle a counter that drives the D/A converter.
• When the D/A output is larger than the input then
  the count is reduced otherwise it is increased
  using an algorithm to home in on the matching
  value.
• When the counter step size is within the
  tolerance desired (usually 1 count) then
  conversion is stopped and the digital value being
  output to the D/A is output

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Sample and hold devices

• Some A/D converters require the input analog
  signal to be held constant during conversion,
  (eg. successive approximation devices)
• In other cases, peak capture or sampling at a
  specific point in time necessitates a sampling
  device.
• This function is accomplished by a sample and
  hold device as shown to the right
• These devices are incorporated into some A/D
  converters

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A/D Converter Types

• Flash Conversion
• A multi-level voltage divider is used to set
  voltage levels over the complete range of
  conversion.
• A comparator is used at each level to determine
  whether the voltage is lower or higher than the
  level.
• The series of comparator outputs are encoded to a
  binary number in digital logic (an encoder)

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A/D Converter Types

• Sigma / Delta

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A/D Converter Types

• Operation
• Comparator feedback signal is subtracted from
  analog input and the difference is integrated.
• The average value of VF is forced to equal Va.
• VF is a digital pulse stream whose duty cycle is
  proportional to Va This is known as Delta
  modulation
• This pulse stream is sampled digitally and
  averaged numerically (decimation) Giving a
  numerical representation of the voltage in.

• The error in the average or mean is
• The greater the number of samples averaged, the
  greater the accuracy
• The greater the number of samples averaged, the
  greater the time between the start of gathering
  samples and the output of the mean (group delay)
• This A/D does not work well if switched from
  channel to channel because of the delay till
  valid result