Digital electronics power point Presentaion

ppt

1. DEPARTMENT OF TECHNICAL EDUCATION
ANDHRA PRADESH
Name : Amarnath Katta
Designation : Lecturer
Branch : Electronics and Communication
Institute : GMR Polytechnic, Paderu,Vizag Dist.
Year/Semester : III-Semester
Subject : DIGITAL ELECTRONICS
Subject code: EE505
Topic : A/D and D/A converters
Duration : 100 Mts
Sub Topic : Weighted Resistor DAC
Teaching Aids: PPT, Block Diagrams, Model graphs
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OBJECTIVES
On completion of this period ,you would be able to
know
•Types of DACs
•Weighted resistor DAC
a) Working
b) Disadvantage

3. Recap
D/A converters are used for conversion of digital data to
analog data .

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D/A Converters

5. TYPES OF DAC
•Weighted resistor DAC.
•R-2R Ladder Network DAC.
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6. Block diagram of DAC
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RESISTOR
NETWORK
OP-AMP
DIGITAL
INPUT
ANALOG
OUTPUT
FIG(1)

7. WEIGHTED RESISTOR DAC
• The weighted resistor DAC requires
a) Resistor Network
b) Switches
c) Reference Voltage
d) OP-AMP
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8. 4-Bit Weighted resistor DAC
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FIG(2)

9. WORKIN
G
•Open switch stands for 0 bit
Closed switch stands for 1 bit
• When all switches are closed
The input currents are given by
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R
V
ref
8 R
V
ref
4 R
V
ref
2 R
V
ref
I
0
= I
1
= I
2 = I
3 =

10. OUTPUT CURRENT
•The output current when all switches are closed is given
by,
I = I
0 + I
1 + I
2 + I
3
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R
V
ref
I = ( 1 + 0.5 + 0.25 +0.125 )

11. •If “ 0 ” stands for open switch and “ 1 ” stands for closed
switch.
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R
V
ref
• Hence the output current is the sum of binary weighted
input currents.
I = ( D3 + 0.5D2 + 0.25D1 + 0.125D0 )

12. •For instance

V
ref = 5 V and R = 5 KΩ
•Then the total output current varies from 0 to 1.875 mA.
•The output current can be can be converted to voltage by
using an OP-AMP.
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13. 4-Bit Weighted Resistor DAC
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DIGITAL INPUTS OUTPUT
CURRENT(MA)
0 0 0 0 0
0 0 0 1 0.125
0 0 1 0 0.25
0 0 1 1 0.375
0 1 0 0 0.5
0 1 0 1 0.625
0 1 1 0 0.75
0 1 1 1 0.875
1 0 0 0 1.0
1 0 0 1 1.125
1 0 1 0 1.25
D
3
D
2 D
1 D
0

14. DIGITAL INPUTS OUTPUT CURRENT (MA)
1 0 1 1 1.375
1 1 0 0 1.5
1 1 0 1 1.625
1 1 1 0 1.75
1 1 1 1 1.875
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Table 1
D
3
D
2
D
1 D
0

15. DISADVANTAGES OF WEIGTED RESISTOR
DAC
•The settling time is dependent on response time
of Switches and OP-AMP.
•It require wide range of resistor values

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•For Example,
8-bit DAC needs resistors R,2R,4R…128R
12-bit DAC needs resistors R,2R,4R…2048R

•The tolerance of resistors should be less than % resolution
to get monotonic output, which is practically impossible

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The following points were discussed
• Types of DAC’s
• Weighted resistor DAC
• Working
•Disadvantages
Summary

17. QUIZ
1. The weighted resistor values for a 4-bit DAC starting from
LSB are
a)R,2R,3R,4R

b) R,2R,4R,8R
c)8R,4R,2R,R

d) 4R,3R,2R,R
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18. Question asked in previous examinations
1.Explain the working of weighted resistor DAC.
(MAR/APRIL-04)
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19. Frequently asked questions
1.What are the disadvantages of weighted resistor
DAC.
2. Explain the working of weighted resistor DAC.
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20. ASSIGNMENT
1.What are the disadvantages of weighted resistor DAC.
2. Explain the working of weighted resistor DAC.
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