Analog to Digital Converters

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Analog to Digital Converters

• Charlie Hagadorn
• Ben Hopwood
• Stewart Skiles
• Jay Upchurch

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What is ADC?

• The real world is analog, but computers are
  digital
• ADC converts analog information to digital
  information
• Analog signals contain an infinite amount of data
• ADC samples the data and splits it into finite
  information.

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Basic ADC Conversion
7 6 5 4 3 2 1 0
Amplitude
1 2 3 4 5 6 7 8
Time
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ADC Constraints

• Resolution
• Speed
• Cost
• As with any engineering system these constraints
  are dependant on each other. Trade-offs must be
  made.

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Types of A/D Converters

• Dual slope integrating converters
• Successive-approximation converters
• Flash (Parallel) converters
• Voltage to frequency converters
• Ramp converters

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What makes a good ADC?

• High resolution
• High speed
• Low cost

Generally, in order to improve one of these
aspects, you must degrade one or more of the
others.
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Dual Slope Integrating
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Advantages

• High resolution (10-20 bits)
• Excellent accuracy (0.01 or better)
• Excellent noise immunity
• Low cost
• Excellent stability (time and temperature)

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Disadvantages

• Slow (1-10/s)

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Uses

• Voltmeters
• Digital panel meters
• Digital multimeters
• Etc.

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Successive-Approximation
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Advantages

• Higher speed (10-105/s)
• Good accuracy (0.1 typ)
• Reliable conversion technique
• Medium resolution (8-12 bits)

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Disadvantages

• Highly susceptible to noise (must prefilter)
• Higher cost than dual slope integrating
  converters
• Accuracy limited by D/A converter

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Uses

• Signal processing
• Applications where the input signal is
  continuously varying at relatively high speed
• Applications where several inputs entered in
  rapid succession

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Flash (Parallel)
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Advantages

• Very fast (4-1000ns), useful for high speed
  applications

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Disadvantages

• Poor resolution (4-10 bits)
• High cost (150 for 6 bits, 3000 for 10 bits)
• Cost becomes a major factor over 6 bits

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Voltage to Frequency
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Advantages

• Excellent noise reduction

Disadvantages

• Slow
• Generally limited to 10 bits or less

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Uses

• Digital voltmeters
• Digital multimeters
• Applications requiring good noise reduction but
  not good speed

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Ramp Converters
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Advantages

• Fast
• Medium resolution (8-12 bits)

Disadvantages

• Requires highly accurate ramp source

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Comparison
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A/D Subsystem
4 Modes of Operation
Single Channel Single Channel 4-Channel 4-Chann
el
4 Consecutive Conversions Continuous
Conversions 1 Conversion Each Continuous
Conversions
Each conversion is stored in 1 of 4 registers.
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Single Channel, 4 Consecutive Conversions
Set MULT 0 (for single channel) and SCAN 0
(for consecutive)
Single Channel, Continuous Conversions
Set MULT 0 (for single channel) and SCAN 1
(for continuous)
Overwrite ADRs
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Multi-Channel, 1 Conversion Each
Set MULT 1 (for multi-channel) and SCAN 0
(for consecutive)
then
then
then
Stop
Multi-Channel, Continuous Conversions
Set MULT 1 (for multi-channel) and SCAN 1
(for continuous)
Overwrite ADRs
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A/D Converter Registers
1031 - 1034
4, 8 Bit A/D Result Registers
Option Register
1039
ADPU
CSEL
DLY
Startup Delay Bit
0 for E Clock 1 for Internal R-C Clock
A/D Powerup
Use Internal R-C Clock when E Clock Frequency is
below 750 kHz
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Single Channel Selection
ADCTL Bits 3-0
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Multi-Channel Selection
ADCTL Bits 3-2
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A/D Wait and Stop Modes

• During Wait Mode
• All A/D conversion stops
• Immediately After Wait Mode
• Re-sampling begins
• A/D circuits are stable and conversions are
  accurate

• During Stop Mode
• All A/D conversion stops
• Immediately After Stop Mode
• Re-sampling begins
• A/D circuits are unstable
• If DLY is of Option Register is set, conversion
  is stable
• Otherwise, conversion stabilizes after 100
  microseconds

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A/D Converter Characteristics

• Linearity to ½ LSB for VRL 0V to VRH VDD

• Monotonicity Guaranteed (conversion result never
  decreases
• with an increasing input voltage)
• Maximum Difference VRH VRL 5V
• A/D system stabilization delay after power-up or
  stop is
• is about 100 microseconds.
• Sample and hold capability

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Example of ADC

• Using Successive Approximation

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Example Problem

• Problem Statement
• Successive Approximation ADC
• 10-bit resolution or 0.0009765625 of Vref
• Vin0.6 Volt and Vref 1 Volt
• Find digital value of Vin

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Example Problem

• Solution
• Divide Vref by 2 and Compare Vref/2 with Vin
• If Vin is greater, turn MSB ON
• If Vin is less than Vref/2, turn MSB OFF

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Example Problem

• Calculate the state of MSB
• Compare Vin0.6 V and V Vref/2 0.5V
• 0.6 gt 0.5, thus
• MSB 1
• Calculate the state of MSB-1
• Compare Vin and (Vref/2 Vref/4)
• 0.6V and (0.5 0.25) 0.75V
• Because 0.6 lt 0.75, MSB-1 is turned off
• MSB-1 0

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Example Problem

• Calculate the state of MSB-2
• Go back to last voltage value that cause an ON
  (0.5V) and add Vref/8 and compare with Vin.
• Vin and (.5 Vref/8) 0.625
• Vin lt 0.625
• MSB-2 is turned off
• MSB-2 O

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Example Problem

• Calculate the state of MSB-3
• Go back to last voltage value that cause an ON
  (0.5V) and add Vref/16 and compare with Vin.
• Vin and (.5 Vref/16) 0.5625
• Vin gt 0.5625
• MSB-3 is turned ON
• MSB-3 1

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Examples of ADC Applications

• Thermocouples
• Strain Gage
• Pressure Transducers
• Digital Music Recording
• Digital Speedometer