1. Analog to digital converters (ADCs) sample analog signals and convert them into digital words. This allows analog signals from sensors to be processed digitally. 2. The conversion process has two steps - quantization breaks down the analog value into discrete levels, and encoding assigns a digital code to each level. For example, a 3-bit ADC of a 0-10V signal quantizes it into 8 levels separated by 1.25V and encodes each with a 3-bit binary code. 3. There are several types of ADCs including flash, successive approximation, delta-sigma, and dual slope. Flash ADCs are fastest but most expensive, while successive approximation and dual slope ADCs are slower

1. 1
Data-Converter Circuits
A/D and D/A
Chapter 9
1

2. 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

3. 3
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).
Need to Sample an
analog signal
Then convert to digital
by A/D converter
Most signals are
analog
Are sensor outputs Analog ?
Eg. Seatbelt ? EEG, oil temp

4. 4
Figure 9.37 The A/D and D/A converters as circuit blocks.
A/D converter and D/A Converters
Analog to Digital Digital to Analog
What parts of your iPhone
operation are Analog ? / Digital
Your internet access: Analog ?
Digital ?

5. A/D converter
Converts analog signals into binary words

6. 6
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.
D/A Conversion
Normal Output from digital domain is staircase
Filtered to produce smooth Analog output

7. Conversion accuracy: eg 2-bits
7
Blue line ?
Red ?
• Analog is continuous
• But digital is discrete
• Limited by number
of bits

8. 3-bit conversion example
8
Each binary representation is a “range”
Quantization levels
Eg 5V divided into 8
levels – each 0.625

9. 9
Figure 2.10 A weighted summer.
D/A conversion implementation
Weighted Summing Circuit
vo = - [(Rf / R1) * v1 + (Rf / R2) * v2 +….+ (Rf / Rn) * vn]
in = ?
i = ?

10. 10
Figure 9.39 An N-bit D/A converter using a binary-weighted resistive ladder network.
N-bit D/A Converter Implementation
Binary weighted resistive ladder
digital values control switches S1 - Sn

11. 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

12. 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
7 8.75-10.0

13. 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

14. 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

15. A/D Converter Types
– Flash ADC
– Delta-Sigma ADC
– Dual Slope (integrating) ADC
– Successive Approximation ADC

16. ADC Resolution Comparison
0 5 10 15 20 25
Sigma-Delta
Successive Approx
Flash
Dual Slope
Resolution (Bits)
Type Speed (relative) Cost (relative)
Dual Slope Slow Med
Flash Very Fast High
Successive Appox Medium – Fast Low
Sigma-Delta Slow Low
ADC Types Comparison

17. 17
Figure 9.43 A simple feedback-type A/D converter.
Analog to Digital Converter
Simple, Cheap but slow : (SAR)
Increment counter  D/A  compare
D
I
G
I
T
A
L
A
N
A
L
O
G

18. PIC microcontroller A/D
10-bit resolution
controlled by
program. registers
18
If 0-5V range
What is pic resolution
What is 3.65V
In digital domain ?
8 Analog channels

19. 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

20. 20
Figure 9.45 Parallel, simultaneous, or flash A/D conversion.
Flash Analog to Digital Converter
Fast – but more expensive :
Single cycle - Uses many Comparators in parallel with
different reference voltages
Analog
Digital
• 2N-1 comparators for N-bits
• Each reference voltage
equivalent to a quantization
level
• Encoding logic produces
word

21. 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.

22. 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

23. 23
Figure 9.46 Charge-redistribution A/D converter suitable for CMOS implementation: (a) sample phase, (b) hold
phase, and (c) charge-redistribution phase.
A / D Converter
– CMOS
Implementation
Charge-redistribution
A/D
FYI