Transcript Analog to Digital & DAC

Data-Converter Circuits
A/D and D/A
Chapter 9
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Analog Signals every where
Examples of A/D
• Microphones - take your voice varying pressure waves in the air
and convert them into varying electrical signals
• Seat Belt• Thermocouple – temperature measuring device converts thermal
energy to electric energy
• Voltmeters
• Digital Multimeters
• ADSL
Need to Sample an
analog signal
Most signals are
analog
Then convert to digital
by A/D converter
Are sensor outputs Analog ?
Eg. Seatbelt ? EEG, oil temp
Figure 9.36 The process of periodically sampling an analog signal. (a) Sample-and-hold (S/H) circuit.
The switch closes for a small part (t seconds) of every clock period (T). (b) Input signal waveform. (c)
Sampling signal (control signal for the switch). (d) Output signal (to be fed to A/D converter).
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A/D converter and D/A Converters
Analog to Digital
What parts of your iPhone
operation are Analog ? / Digital
Digital to Analog
Your internet access: Analog ?
Digital ?
Figure 9.37 The A/D and D/A converters as circuit blocks.
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A/D converter
Converts analog signals into binary words
D/A Conversion
Normal Output from digital domain is staircase
Filtered to produce smooth Analog output
Figure 9.38 The analog samples at the output of a D/A converter are usually fed to a sample-and-hold
circuit to obtain the staircase waveform shown. This waveform can then be filtered to obtain the smooth
waveform, shown in color. The time delay usually introduced by the filter is not shown.
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Conversion accuracy: eg 2-bits
Blue line ?
• Analog is continuous
• But digital is discrete
• Limited by number
of bits
Red ?
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3-bit conversion example
Quantization levels
Eg 5V divided into 8
levels – each 0.625
Each binary representation is a “range”
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D/A conversion implementation
Weighted Summing Circuit
in = ?
i =?
vo = - [(Rf / R1) * v1 + (Rf / R2) * v2 +….+ (Rf / Rn) * vn]
Figure 2.10 A weighted summer.
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N-bit D/A Converter Implementation
Binary weighted resistive ladder
digital values control switches S1 - Sn
Figure 9.39 An N-bit D/A converter using a binary-weighted resistive ladder network.
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Analog  Digital Conversion
2-Step Process:
• Quantizing - breaking down analog value to
set of finite states
• Encoding - assigning a digital word or number
to each state
Step 1: Quantizing
Example: a 3 bit A/D , N=23=8 (no. of steps)
0-10V signals.
Separated into discrete
states with 1.25V
increments.
Analog quantization
size:
Q=(Vmax-Vmin)/N =
(10V – 0V)/8 = 1.25V
Output
States
Discrete Voltage
Ranges (V)
0
0.00-1.25
1
1.25-2.50
2
2.50-3.75
3
3.75-5.00
4
5.00-6.25
5
6.25-7.50
6
7.50-8.75
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8.75-10.0
Encoding
give value to each state
Output
States
Output Binary Equivalent
0
000
1
001
2
010
3
011
4
100
5
101
6
110
7
111
Accuracy of A/D Conversion
two ways to improve accuracy:
• Increase resolution: improves accuracy in measuring
analog signal amplitude
• Increase sampling rate: increases max frequency that
can be measured. Eg high pitch audio
A/D Converter Types
–
–
–
–
Flash ADC
Delta-Sigma ADC
Dual Slope (integrating) ADC
Successive Approximation ADC
ADC Types Comparison
ADC Resolution Comparison
Dual Slope
Flash
Successive Approx
Sigma-Delta
0
5
10
15
Resolution (Bits)
20
25
Type
Speed (relative)
Cost (relative)
Dual Slope
Slow
Med
Flash
Very Fast
High
Successive Appox
Medium – Fast
Low
Sigma-Delta
Slow
Low
Analog to Digital Converter
Simple, Cheap but slow : (SAR)
Increment counter  D/A  compare
A
N
A
L
O
G
D
I
G
I
T
A
L
Figure 9.43 A simple feedback-type A/D converter.
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PIC microcontroller A/D
10-bit resolution
8 Analog channels
controlled by
program. registers
If 0-5V range
What is pic resolution
What is 3.65V
In digital domain ?
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Flash ADC
• series of comparators, each one compares
input to a unique reference voltage.
• comparator outputs connect to a priority
encoder circuit  produces binary output
Flash Analog to Digital Converter
Fast – but more expensive :
Single cycle - Uses many Comparators in parallel with
different reference voltages
Analog
• 2N-1 comparators for N-bits
• Each reference voltage
equivalent to a quantization
level
• Encoding logic produces
word
Digital
Figure 9.45 Parallel, simultaneous, or flash A/D conversion.
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How Flash Works
• As the analog input voltage exceeds the
reference voltage at each comparator, the
comparator outputs will sequentially saturate
to a high state.
• The priority encoder generates a binary
number based on the highest-order active
input, ignoring all other active inputs.
Flash
Advantages
• Simplest in terms of
operational theory
• Most efficient in terms of
speed, very fast
• limited only in terms of
comparator and gate
propagation delays
Disadvantages
• Lower resolution
• Expensive
• For each additional output
bit, the number of
comparators is doubled
• i.e. for 8 bits, 256
comparators needed
A / D Converter
– CMOS
Implementation
Charge-redistribution
A/D
FYI
Figure 9.46 Charge-redistribution A/D converter suitable for CMOS implementation: (a) sample phase, (b) hold
phase, and (c) charge-redistribution phase.
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