Syed Umair

3. Analog to Digital Conversion (ADC)

4. Analog-to-Digital Conversion
An analog signal is a continuous signal that contains time-varying
quantities, such as temperature or speed, with infinite possible values
in between
An analog signal can be used to measure changes in some physical
phenomena such as light, sound, pressure, or temperature.
For example, an analog microphone can convert sound waves into an
analog signal.
Even in digital devices, there is typically some analog component that
is used to take in information from the external world, which will then
get translated into digital form
(using an analog-to-digital converter).

5. Analog-to-Digital Conversion
A digital signal refers to an electrical signal that is
converted into a pattern of bits. A digital signal has a discrete
value at each sampling point. The precision of the signal is
determined by how many samples are recorded per unit of
time.
For example an analog pattern (represented as the curve)
alongside a digital pattern (represented as the discrete lines).
A digital signal is easily represented by a computer because
each sample can be defined with a series of bits that are
either in the state 1 (on) or 0 (off).
Digital signals can be compressed
and can include additional
information for error correction.

6. An analog-to-digital converter
Abbreviated ADC, A/D or A to D,A2D
ADC is a device that converts a continuous
quantity to a discrete digital number. The
reverse operation is performed by a digital-to-
analog converter (DAC).

7. ©Alex Doboli 2006
Quantization
• Quantization is the process of converting the sampled continuous-
Valued signals into discrete-valued data
In mathematics and digital signal processing, is the process of mapping a
large set of input values to a (countable) smaller set – such as rounding values
to some unit of precision.

8. 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

9. 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

10. 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

11. 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

12. 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…….

13. 3 Basic Types
• Flash ADC
• Digital-Ramp/Dual slope/Counter slope ADC
• Successive Approximation ADC

14. Flash ADC
• Flash ADC
• Also known as a Direct conversion ADC is
a type of analog-to-digital converter that
uses a linear voltage ladder with a
comparator at each "rung" of the ladder
to compare the input voltage to
successive reference voltages.

15. 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.

16. 3 bit Flash ADC Circuit

17. Flash ADC
• Very fast for high quality audio and video
• Very expensive for wide bits conversion
• Sample and hold circuit usually NOT required
• The number of comparators needed is 2n-1
which grows rapidly with the number of bits
i.e. 4-bit,15 comparators; for 6-bit,63
comparators

18. Flash ADC
• Flash ADC is one of the fastest ADC
• Complexity is less when compared to other
ADC’s
• As # bits increases we need more comparators
so required more space on chip

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
Digital Output
Oscillator
Control Logic
Registers
Switch
Counter
VReference
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. Successive Approximation
Advantages
• Capable of high speed and
reliable
• Medium accuracy compared
to other ADC types
• Good tradeoff between speed
and cost
• Capable of outputting the
binary number in serial (one
bit at a time) format.
Disadvantages
• Higher resolution successive
approximation ADC’s will be
slower
• Speed limited to ~5Msps