Dac 386ax

EXPERIMENT NO:

Aim:

To design 1) 4 bit weighted resistor DAC using OP AMP

2) 4 bit R-2R ladder Network DAC using OP AMP Components / Equipments required: S.No

Equipment/Component name

1

741 IC OPAMP

2

Resistors

3

Regulated Power supply

4

Multimeter(DMM)

5

connecting wires

6

Breadboard Board

Specifications/Value

Quantity 1

1KΩ,2KΩ,4KΩ, 8KΩ 0

Each one

To ± 15V

1

3 ½ digit display

1

1

Theory: Digital systems are used in ever more applications, because of their increasingly efficient, reliable, and economical operation with the development of the microprocessor, data processing has become an integral part of various systems Data processing involves transfer of data to and from the microcomputer via input/output devices. Since digital systems such as microcomputers use a binary system of ones and zeros, the data to be put into the microcomputer must be converted from analog to digital form. On the other hand, a digital-to-analog converter is used when a binary output from a digital system must be converted to some equivalent analog voltage or current. The function of DAC is exactly opposite to that of an ADC.

A DAC in its simplest form uses an op-amp and either binary weighted resistors or R-2R ladder resistors. In binary-weighted resistor op-amp is connected in the inverting mode, it can also be connected in the non-inverting mode. Since the number of inputs used is four, the converter is called a 4-bit binary digital converter.

Circuit Diagrams: Fig 1: Binary weighted resistor DAC

Fig 2: R – 2R Ladder DAC

Design: 1. Weighted Resistor DAC

b

b

b

b

Vo = -Rf  A  B  c  D  8R 4 R 2 R R



For input 1111, Rf = R = 4.7KΩ Vo = -

Rf 1 1 1     1 x5  8 4 2 R

Vo = - 9.375 V

2.R-2R Ladder Network:

b

b

b

b

Vo = -Rf  A  B  c  D 16 R 8R 4 R 2 R

 X5

For input 1111, Rf = R= 1KΩ

Procedure: 1. Connect the circuit as shown in Fig 1. 2. Vary the inputs A, B, C, D from the digital trainer board and note down the output at pin 6. For logic ‘1’, 5 V is applied and for logic ‘0’, 0 V is applied. 3. Repeat the above two steps for R – 2R ladder DAC shown in Fig 2.

Observations: Weighted resistor DAC

S.No

D

C

B

A

Theoretical Voltage(V)

Practical Voltage(V)

1

0

0

0

0

0

0

2

0

0

0

1

-0.62

3

0

0

1

0

-1.25

4

0

0

1

1

-1.87

5

0

1

0

0

-2.5

6

0

1

0

1

-3.12

7

0

1

1

0

-3.75

8

0

1

1

1

-4.37

9

1

0

0

0

-5

10

1

0

0

1

-5.62

11

1

0

1

0

-6.25

12

1

0

1

1

-6.87

13

1

1

0

0

-7.5

14

1

1

0

1

-8.12

15

1

1

1

0

-8.75

16

1

1

1

1

-9.37

R-2R Ladder Network S.No

D

C

B

A

Theoretical Voltage(V)

1

0

0

0

0

-0.31

2

0

0

0

1

-0.62

3

0

0

1

0

-0.93

4

0

0

1

1

-1.25

5

0

1

0

0

-1.56

6

0

1

0

1

-1.87

7

0

1

1

0

-2.18

8

0

1

1

1

-2.5

9

1

0

0

0

-2.81

10

1

0

0

1

-3.12

11

1

0

1

0

-3.41

12

1

0

1

1

-3.75

13

1

1

0

0

-4.06

14

1

1

0

1

-4.2

15

1

1

1

0

-4.37

16

1

1

1

1

-4.68

Practical Voltage(V)

Model Graph: Decimal Equivalent of Binary inputs

Precautions: Check the connections before giving the power supply. Readings should be taken carefully.

Results: Outputs of binary weighted resistor DAC and R-2R ladder DAC are observed.

Inferences: Different types of digital-to-analog converters are designed

Questions & Answers: 1. How do you obtain a positive staircase waveform? Ans: By giving negative reference voltage. 2. What are the drawbacks of binary weighted resistor DAC? Ans: Wide range of resistors is required in binary weighted resistor DAC. 3. What is the effect of number of bits on output ? Ans: Accuracy degenerates as the number of binary inputs is increased beyond four