Interfacing to the Analog World

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Chapter 15

• Interfacing to the Analog World

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Objectives

• You should be able to
• Perform the basic calculations involved in the
  analysis of operational amplifier circuits.
• Explain the operation of binary-weighted and R/2R
  digital-to-analog converters.
• Make the external connections to a
  digital-to-analog IC to convert a numeric binary
  string into a proportional analog voltage.

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Objectives

• (Continued)
• Discuss the meaning of the specifications for
  converter ICs as given in a manufacturers data
  manual.
• Explain the operation of parallel-encoded
  counter-ramp, and successive-approximation
  analog-digital converters.

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Objectives

• (Continued)
• Make the external connections to an
  analog-to-digital converters IC to convert an
  analog voltage to a corresponding binary string.
• Discuss the operation of a typical data
  acquisition system.

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Digital and Analog Representations

• An analog signal can be represented with digital
  values at some time interval.

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Digital and Analog Representations

• Four binary positions 4-bit resolution
• 16 different representations
• Eight binary positions 8-bit resolution
• 256 different representations

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Operational Amplifier Basics

• Very high input impedance
• Very high voltage gain
• Very low output impedance

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Discussion Point

• Determine Vout

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Binary-Weighted Digital-to-Analog Converters

• Sum of the currents from the input resistors
• Binary weighting factor

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Binary-Weighted Digital-to-Analog Converters

• Accurate resistance over a wide range is
  difficult
• Not practical for conversions greater than 4-bit

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R/2R Ladder Digital-to-Analog Converters

• Only two resistor values
• 8, 10, 12, 14, and 16 bit resolutions are common

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R/2R Ladder Digital-to-Analog Converters

• Current division and analog output versus digital
  input

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R/2R Ladder Digital-to-Analog Converters

• Current division and analog output versus digital
  input

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Integrated-Circuit Digital-to-Analog Converters

• DAC0808 block diagram and pin configuration

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Integrated-Circuit Digital-to-Analog Converters

• DAC0808 Application

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Integrated-Circuit Digital-to-Analog Converters

• Testing the 256-step output of a DAC with an 8
  bit counter

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Integrated-Circuit Digital-to-Analog Converters

• Multisim DAC simulation

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IC Data Converter Specifications

• Differential nonlinearity
• Gain error
• Missing codes

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IC Data Converter Specifications

• Nonmonotonic, offset error, relative accuracy,
  settling time, and 3-bit ADC transfer
  characteristic

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Parallel-Encoded Analog-to-Digital Converters

• Parallel encoding
• Also called simultaneous, multiple comparator, or
  flash converting
• Several comparators with different reference
  voltages drive a priority encoder

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Parallel-Encoded Analog-to-Digital Converters

• Three-bit parallel encoded ADC
• priority encoder
• Analog range of 0-7 V
• 3 bit (8 level) resolution

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Counter-Ramp Analog-to-Digital Converters

• Counter used in conjunction with a D/A converter
• To change for continuous conversions
  end-of-conversion line is tied back to clear
  input
• Disadvantage is slow conversion time

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Counter-Ramp Analog-to-Digital Converters (Figure
15-12)
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Successive-Approximation Analog-to-Digital
Conversion

• Most used in modern ADC ICs
• Converter circuit is similar to counter-ramp
• Uses successive approximation register to quickly
  narrow in on the analog value
• Result is a much faster conversion when compared
  to the counter-ramp method

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Successive-Approximation Analog-to-Digital
Conversion

• Simplified SAR A/D converter

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Integrated-Circuit Analog-to-Digital Converters

• NE5034 similar to the SAR ADC just presented
  but uses a three-state output buffer instead of a
  D flip-flop
• Conversion speeds up to 17 ?s
• Compatible with bus oriented microprocessors

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Integrated-Circuit Analog-to-Digital Converters

• NE5034 block diagram and pin configuration

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Integrated-Circuit Analog-to-Digital Converters

• ADC 0804
• Successive-approximation
• Two analog inputs for differential measurements
• Internal clock (determined by external R and C)
• Operation similar to NE5034
• Analog and digital ground are both provided

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Integrated-Circuit Analog-to-Digital Converters

• ADC 0804 block diagram and pin configuration

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Data Acquisition System Application

• Data bus
• Control bus
• Analog Multiplexer Switch (AM3705)
• Sample-and-Hold Circuit (LF198)
• Programmable-Gain Instrumentation Amplifier
  (LH0084)
• Analog-to-Digital Converter (ADC0804)

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Data Acquisition System Application
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Transducers and Signal Conditioning

• Physical quantities to electrical quantities
• Must be conditioned due to different output
  ranges and signals
• Manufacturers specifications must be studied
• Analog output of transducer is converted to
  binary by ADC
• Data can then be manipulated by software

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Transducers and Signal Conditioning

• Thermistor resistance is dependent on temperature
  and response is nonlinear

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Transducers and Signal Conditioning

• Thermistors Example conversion circuit

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Transducers and Signal Conditioning

• Linear IC Temperature Sensors
• Simplify process of converting a nonlinear
  response

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Transducers and Signal Conditioning

• The Strain Gage
• Resistance changes when stretched
• Example of signal conditioning for a strain gage

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Summary

• Any analog quantity can be represented by a
  binary number. Longer binary numbers provide
  higher resolution, which gives a more accurate
  representation of the analog quantity.
• The binary-weighted D/A converter is the simplest
  to construct, but it has practical limitations in
  resolution (number of input bits).

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Summary

• Operational amplifiers are important building
  blocks in analog-to-digital (A/D) and
  digital-to-analog (D/A) converters. They provide
  a means for summing currents at the input and
  converting a current to a voltage at the output
  of converter circuits.
• The R/2R ladder D/A converter uses only two
  different resistor values, no matter how many
  binary input bits are included. This allows for
  very high resolution and ease of fabrication in
  integrated-circuit form.

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Summary

• The DAC0808 (or MC1408) IC is an 8-bit D/A
  converter that uses the R/2R ladder method of
  conversion. It accepts 8 binary input bits and
  outputs an equivalent analog current. Having 8
  input bits means that it can resolve up to 256
  unique binary values into equivalent analog
  values.

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Summary

• Applying an 8-bit counter to the input of an
  8-bit D/A converter will produce a 256-step
  sawtooth waveform at its output.
• The simplest way to build an analog-to-digital
  (A/D) converter is to use the parallel encoding
  method. The disadvantage is that it is practical
  only for low-resolution applications.

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Summary

• The counter-ramp A/D converter employs a counter,
  a D/A converter, and a comparator to make its
  conversion. The counter counts from zero up to a
  value that causes the D/A output to exceed the
  analog input value slightly. That binary count
  is then output as the equivalent to the analog
  input.

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Summary

• The method of A/D conversion used most often is
  called successive approximation. In this method,
  successive bits are tested to see if they
  contribute an equivalent analog value that is
  greater than the analog input to be converted.
  If they do, they are returned to zero. After all
  bits are tested, the ones that are left ON are
  used as the final digital equivalent to the
  analog input.

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Summary

• The NE5034 and the ADC0802 are examples of A/D
  converter ICs. To make a conversion, the
  start-conversion pin is made LOW. When the
  conversion is completed the end-of-conversion pin
  goes LOW. Then to read the digital output, the
  output enable pin is made LOW.

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Summary

• Data acquisition systems are used to read several
  different analog inputs, respond to the values
  read, store the results, and generate reports on
  the information gathered.
• Transducers are devices that convert physical
  quantities such as heat, light, or force into
  electrical quantities. Those electrical
  quantities must then be conditioned (or modified)
  before they can be interpreted by a digital
  computer.

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