digital design for diploma students

1. DIGITAL ELECTRONICS
CHAPTER 6
DEE 204

2. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D and D-A
converter
Principle of operation of a single and
dual ramp
A continuously balanced and successive
approximation A-D converter

3. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D and D-A
converter
Basic on digital signal processing
Digital signal processing:
- converts signals that naturally occur in
analogue form to digital form using digital
techniques to enhance and modify analogue
signal data for applications

4. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation
of a A-D & D-A
converter
Basic on digital signal
processing
Digital signal processing:
- First translates
continuously varying
analogue signal into
series of discrete levels
resembling a staircase
as shown

5. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of
a A-D & D-A converter
Basic on digital signal
processing
Digital signal processing:
- Changing the analogue
signal to staircase form by
a sample and hold circuit

6. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D & D-A converter
Basic on digital signal processing
Digital signal processing:
- The stairstep approximation is quantized into binary
codes by a circuit called analogue-to-digital
converter (ADC)
- Binary coded form is then fed into DSP, signal
converted back into original analogue signal by
digital-to-analogue converter (DAC)

7. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D & D-A converter
Basic on digital signal processing
Digital signal processing:
- The basic block diagram of a typical digital signal
processing system is shown

8. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D converter
Sampling and filtering
- The process of taking sufficient number of
discrete values at points on a waveform
- Converts an analogue signal into a series of
impulses
- Must be passed through a low-pass filter (anti-
aliasing filter to eliminate harmonic
frequencies above Nyquist frequency)

9. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a A-D converter
Sampling and filtering: Nyquist frequency
Or in other words, the highest analogue frequency should
be less than half the sampling frequency
( )
( ) frequencyNyquistis
2
max
max
a
asample
f
ff ≥

10. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a
A-D converter
Sample and hold:
- The sampled level must
be held constant until
the next sample occurs
- Results in a stairstep
waveform

11. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of
operation of a A-
D converter
Quantization:
- Process of
converting an
analogue value to a
code
- Each sampled value
of the analogue
signal is converted
to a binary code

12. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a D-A converter (DAC)
- Could be either parallel transfer or series transfer
of data
- Parallel transfer: 4-bit codes are coupled out in
sequence as they are processed by digital
processor and accepted by DAC at the same time
- Series transfer: 4-bit codes are sent out one bit at a
time, following the other in sequence, accepted by
DAC in sequence

13. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a D-A converter
Basic diagram of digital-to-analogue conversion is shown

14. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of
operation of a
single ramp
- Consist of ramp
generator and BCD or
binary counter
- A single slope ADC is
shown

15. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a
single ramp
- At first reset signal is provided to ramp
generators
- Counter reset to 0
- Analogue input voltage is applied to
positive terminal of comparator
- Comparator output goes HIGH, enabling
AND gate and ramp starts until exceeds
input voltage
- Comparator goes LOW, disables AND
gate, resets counter and ramp generator

16. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a single
ramp
- e.g. for the ramp given the ramp is generated
at 1V/ms
- The ramp is given as:
timeramptotaltheisand
x
Vat voltagetimeramptheiswhere
21
1
21
1
tt
t
V
V
tt
t
in
X
+
=
+
VX
t1
Vin
t1+t2

17. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
• Principle of operation of a single ramp
Example:
A single ramp ADC is fed with Vin = 5V and
gives a ramp for 1.2ms. At 0.7ms,
determine the output voltage of the
ramp and the pulses produced if the
frequency is 3MHz.

18. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
• Principle of operation of a single ramp
Solution:
( )
( )( ) 210037.0
9.25
2.1
7.0
;7.0
5,2.1
1
1
21
===
=





=
=
==+
MHzmsftN
VV
ms
ms
V
mst
VVmstt
x
in

19. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of
operation of a
dual ramp
- A dual slope ADC is
shown
- Consist of integrator
(ramp generator),
comparator, binary
counter, output
latch and reference
voltage

20. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation of a dual ramp
- Assuming the counter is reset and
output at integrator is ‘0’
- Positive input voltage applied,
assuming Vin is constant, capacitor
charges linearly
- Counter reaches specified count and
resets, switches to negative
reference voltage, capacitor charges
negative voltage, discharges linearly
- As capacitor discharges, counter
advances from RESET state

21. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
Principle of operation
of a dual ramp
( )
( )
( ) ( )1223
23
12
givingThus,
1
asgivenisslopedecreasingfor theequationThe
1
asgivenissloperisingfor theequationThe
3
2
2
1
ttVttV
tt
RC
V
dtV
RC
V
tt
RC
V
dtV
RC
V
inref
ref
t
t
refr
in
t
t
inr
−=−
−==
−==
∫
∫
Vr
t1 t2 t3

22. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
• Principle of operation of a dual ramp
Example:
A dual slope ADC has a 100kΩ resistor and 1µF
capacitor. If the input voltage is 2V, calculate
the output voltage after 1s. If a 5V reference
voltage is fed to the integrator, determine
the time range from t2 to t3

23. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
• Principle of operation of a dual ramp
Solution:
( )
( )
( )( )
( )
( ) ( )
( )( ) ( )( )
( ) stt
ttVsV
ttVttV
VV
Vs
Fk
V
V
tt
RC
V
V
sttFCkRVV
refin
ref
r
in
ramp
in
4.0
512
;5
201
1100
2
;1,1,100,2
23
23
2312
12
12
=−
−=
−=−
=
=
Ω
=
−=
=−=Ω==
µ
µ

24. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
A continuously balanced
and successive
approximation A-D
converter
- A successive
approximation ADC
block diagram is shown
- Consisting DAC,
successive
approximation register,
and comparator

25. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
A continuously balanced and successive approximation
A-D converter
- For a 4-bit successive approximation ADC as shown,
input bits are enabled one at a time, starting with
the most significant bit (MSB)
- Comparator produces LOW when input signal
voltage is lower than DAC output, register will RESET
the bit
- Comparator produces HIGH when input signal
voltage is higher than DAC output, register retains bit
- System does this from the MSB to the LSB, and once

26. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
A continuously balanced and successive approximation
A-D converter
Output of DAC is 8V, greater than
5.1V input, output is LOW, MSB in
SAR is reset
Output of DAC is 4V, less than
5.1V input, output is HIGH, bit is
retained in SAR

27. CONVERSATION BETWEEN ANALOG AND
SYSTEM FUNCTION OF AD AND DA
CONVERTERS
A continuously balanced and successive approximation
A-D converter
Output of DAC is 6V, greater
than5.1V input, output is LOW, bit
is reset to ‘0’
Output of DAC is 5V, binary code
in register is 0101, approximate to
5.1V