The document discusses different types of analog to digital converters (ADCs). It begins by defining analog and digital signals and the basic principle of an ADC which uses a comparator to determine binary output bits. It then discusses three main ADC types: flash ADCs which use multiple comparators, dual slope/counter ADCs which use a capacitor and counter, and successive approximation ADCs which use feedback to iteratively approximate the analog value. It compares the resolution, speed and cost of different ADC types and gives examples of ADC applications.

2. Presented By:
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3. Presentation Topic:
Analog to Digital Conversion
(ADC)

4. Analog-to-Digital Conversion
Terminology
analog: continuously valued signal, such as
temperature or speed, with infinite possible
values in between
digital: discretely valued signal, such as integers,
encoded in binary
analog-to-digital converter: ADC, A/D, A2D;
converts an analog signal to a digital signal

5. Analog Signals
Analog signals – directly measurable quantities
in terms of some other quantity
Examples:
• Thermometer – mercury height rises as
temperature rises
• Car Speedometer – Needle moves farther
right as you accelerate

6. Digital Signals
Digital Signals – have only two states. For digital
computers, we refer to binary states, 0 and 1.
“1” can be on, “0” can be off.
Examples:
• Light switch can be either on or off
• Door to a room is either open or closed

7. ADC Basic Principle:
• The basic principle of operation is to use the
comparator principle to determine whether or
not to turn on a particular bit of the binary
number output.
• It is typical for an ADC to use a digital-toanalog converter (DAC) to determine one of
the inputs to the comparator.

8. Quantization
• Quantization is the process of converting the sampled continuousValued signals into discrete-valued data
©Alex Doboli 2006

9. Quantizing
The number of possible states that the
converter can output is:
N=2n
where n is the number of bits in the AD
converter
Example: For a 3 bit A/D converter, N=23=8.
Analog quantization size:
Q=(V max -V min)/N = (10V – 0V)/8 = 1.25V

10. Analog  Digital Conversion
2-Step Process:
• Quantizing - breaking down analog value is a
set of finite states
• Encoding - assigning a digital word or number to
each state and matching it to the input signal

11. Step 1: Quantizing
Example:
You have 0-10V signals.
Separate them into a set
of discrete states with
1.25V increments. (How
did we get 1.25V?
(Discussed in previous slide)
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

12. Step 2. Encoding
• Here we assign the
digital value (binary
number) to each state
for the computer to
read.
Output
States
Output Binary Equivalent
0
000
1
001
2
010
3
011
4
100
5
101
6
110
7
111

13. Sampling
• It is a process of taking a sufficient number of
discrete values at point on a waveform that
will define the shape of waveform.
• The more samples you take, the more
accurately you will define the waveform.
• It converts analog signal into series of
impulses, each representing amplitude of the
signal at given point…….

14. Sampling
Collect sufficient data for correctly representing a
continuous-time signal
©Alex Doboli 2006

15. 3 Basic Types
• Flash ADC
• Digital-Ramp/Dual slope/Counter slope ADC
• Successive Approximation ADC

16. 1->
Flash ADC
• Consists of a series of comparators, each one
comparing the input signal to a unique
reference voltage.
• The comparator outputs connect to the inputs
of a priority encoder circuit, which produces a
binary output

17. 3 bit Flash ADC Circuit

18. 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.

19. ADC Output

20. 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

21. 2->
Dual Slope ADC
• Also known as Counter-Ramp or Digital Ramp ADC
• A dual slope ADC is commonly used in
measurement instruments (such as DVM’s).
ADC 1.21

22. Dual Slope ADC circuit
Input
Oscillator
Switch
Control Logic
Counter
VReference
Registers
Digital Output
ADC 1.22

23. Dual Slope Function
• The Dual Slope ADC functions in this manner:
– When an analog value is applied the capacitor begins to
charge in a linear manner and the oscillator passes to
the counter.
– The counter continues to count until it reaches a
predetermined value. Once this value is reached the
count stops and the counter is reset. The control logic
switches the input to the first comparator to a reference
voltage, providing a discharge path for the capacitor.
– As the capacitor discharges the counter counts.
– When the capacitor voltage reaches the reference
voltage the count stops and the value is stored in the
register.
ADC 1.23

24. Successive approximation ADC
• Much faster than the
digital ramp ADC
because it uses digital
logic to converge on
the value closest to the
input voltage.
• A comparator and a
DAC are used in the
process.

25. Successive Approximation ADC
• A Successive Approximation Register (SAR) is
added to the circuit
• Instead of counting up in binary sequence,
this register counts by trying all values of bits
starting with the MSB and finishing at the LSB.
• The register monitors the comparators output
to see if the binary count is greater or less
than the analog signal input and adjusts the
bits accordingly

26. Successive Approximation ADC
Circuit

27. Output

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

29. Examples of A/D Applications
• Microphones - take your voice varying pressure waves in the air
and convert them into varying electrical signals
• Strain Gages - determines the amount of strain (change in
dimensions) when a stress is applied
• Thermocouple – temperature measuring device converts
thermal energy to electric energy
• Voltmeters
• Digital Multimeters