Analog-Digital

Analog-Digital
ELCTEC-131
Analog-to-Digital Conversion

Uses a circuit that converts an analog
signal at its input to a digital code.

Called an A-to-D converter, A/D
converter, or ADC.
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© 2009 Richard Lokken
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Unipolar ADC

Converts positive input voltages.

Generates a 2n-bit binary code for any
given input voltage.
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© 2009 Richard Lokken
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Unipolar ADC Code Equation
 Va
= analog input voltage to be sampled.

FS = Full scale range of input voltage.

n = number of bits in the output code.
Va
n
×2
code =
FS
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© 2009 Richard Lokken
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Unipolar ADC Code Equation
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© 2009 Richard Lokken
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Unipolar ADC Output Codes
Nominal Voltage of Input Step (volts) Range (volts) Output Code
0.0
0.0 - 0.5
000
1.0
0.5 - 1.5
001
2.0
1.5 - 2.5
010
3.0
2.5 - 3.5
011
4.0
3.5 - 4.5
100
5.0
4.5 - 5.5
101
6.0
5.5 - 6.5
110
7.0
6.5 - 8.0
111
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© 2009 Richard Lokken
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Bipolar ADC (Offset Binary Coding)

Used to represent positive and negative
input voltages.

Output code an unsigned binary number.
◦ Numbers below 0 V are negative.
◦ Numbers above 0 V are positive.
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© 2009 Richard Lokken
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Bipolar ADC (Offset Binary Coding)
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Bipolar ADC Code Equation
 Va
n
× 2  + offset
code = 

 FS
n
2
 Va
n
=
×2 +
 FS
 2
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© 2009 Richard Lokken
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Bipolar ADC Output Codes
- 4.0
- 3.0
- 4.0 to - 3.5
- 3.5 to - 2.5
000
001
- 2.0
- 1.0
0
+ 1.0
- 2.5 to - 1.5
- 1.5 to - 0.5
- 0.5 to + 0.5
+ 0.5 to + 1.5
010
011
100
101
+ 2.0
+ 1.5 to + 2.5
110
+ 3.0
+ 2.5 to + 4.0
111
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© 2009 Richard Lokken
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Bipolar ADC
(2’s Complement Coding)

Uses a 2’s complement number system.

Most significant bit (MSB) is the sign bit.
◦ MSB = ‘0’ sign positive.
◦ MSB = ‘1’ sign negative.
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© 2009 Richard Lokken
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2’s Complement Output Codes
Nominal Voltage of Input Step (volts) Range (volts) Output Code
- 4.0
- 4.0 to - 3.5
100
- 3.0
- 3.5 to - 2.5
101
- 2.0
- 2.5 to - 1.5
110
- 1.0
- 1.5 to - 0.5
111
0
- 0.5 to + 0.5
000
+ 1.0
+ 0.5 to + 1.5
001
+ 2.0
+ 1.5 to + 2.5
010
+ 3.0
+ 2.5 to + 4.0
011
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© 2009 Richard Lokken
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Flash ADC

Uses a resistive voltage divider,
comparators, and a priority encoder to
produce a digital code.

Conversion occurs in one clock cycle
(fastest conversion time).
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© 2009 Richard Lokken
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Flash ADC
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Flash ADC
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Disadvantage of Flash ADC

Requires 2n resistors and 2n – 1
comparators for an n-bit output.

For any large number of bits, the
circuit becomes overly complex.
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© 2009 Richard Lokken
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Successive Approximation ADC

The most widely used ADC.

Finds the digital representation using a
“binary search.”

Also called a SAR.
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© 2009 Richard Lokken
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Binary Search
1.
Set the MSB of the digital representation
to 1, all other bits to 0.
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Binary Search
2.
Compare the analog value produced in the
first step to the voltage being converted.
a) If the test voltage is higher than the voltage
being converted, reset the MSB and set the
second MSB.
b)
3.
If the test voltage is less than the voltage being
converted, leave the MSB set and set the second MSB.
Repeat Steps 2, 2A, and 2B until all the bits have
been tested.
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Binary Search
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Binary Search – 4
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Binary Search
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SAR - ADC Characteristics

Final answer is always less than the input
voltage.

Conversion always requires a fixed
number of clock cycles.

Conversion requires n clock cycles where
n is the number of bits in the digital
representation.
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© 2009 Richard Lokken
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Dual Slope ADC

Based on an integrator, a circuit whose
output is the accumulated sum of all
previous input values.

Circuit relies on storing charge
representing current flow in a capacitor.
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© 2009 Richard Lokken
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Dual Slope ADC Characteristics

High accuracy.

Relatively slow conversion time.
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© 2009 Richard Lokken
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Dual Slope ADC Characteristics
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Dual Slope ADC Characteristics
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ADC0808 IC ADC

Successive approximation ADC.

Able to convert analog information from
up to 8 (multiplexed) channels.

Can form the basis of a data acquisition
network.
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© 2009 Richard Lokken
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ADC0808 IC ADC

START conversion with HIGH pulse.

Conversion process driven by the clock.

End-of-conversion indicated by a HIGH
on EOC.
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© 2009 Richard Lokken
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ADC0808 IC ADC

Making OE HIGH allows the digital
output to be read.

When OE inactive, outputs in Hi-Z state.
ADC0808 IC ADC
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© 2009 Richard Lokken
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