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# Important Questions and Answers: Linear Integrated Circuits : Analog to Digital (ADC) And Digital to Analog (DAC) Converters

Linear Integrated Circuits ( LIC ) Subject : Analog to Digital And Digital to Analog Converters (ADC and DCA)

ANALOG TO DIGITAL AND DIGITAL TO ANALOG CONVERTERS

PART-A

1. Name the essential parts of a DAC.

• Drive motors

• Analog devices

• Deglitcher

• Filter

2. Write down the drawback of weighted D/A converter.

The main disadvantage of binary weighted D/A converter is the requirement of wide range of resister values. As the length of the binary word is increased .the range of resister values needed also increases.

4. List out the direct type ADCs.

1. Flash (comparator) type converter

2. Counter type converter

3. Tracking or servo converter

4. Successive approximation type converter

5. List out some integrating type converters.

6. What is integrating type converter?

An ADC converter that perform conversion in an indirect manner by first changing the analog I/P signal to a linear function of time or frequency and then to a digital code is known as integrating type A/D converter.

7 .Explain in brief the principle of operation of successive Approximation ADC.

The circuit of successive approximation ADC consists of a successive approximation register (SAR), to find the required value of each bit by trial & error. With the arrival of START command, SAR sets the MSB bit to 1. The O/P is converted into an analog signal & it is compared with I/P signal. This O/P is low or High. This process continues until all bits are checked.

a. The integrating type of ADC’s do not need a sample/Hold circuit at the input.

b. It is possible to transmit frequency even in noisy environment or in an isolated form.

9. Where are the successive approximation type ADC‟s used?

The Successive approximation ADCs are used in applications such as data loggers & instrumentation where conversion speed is important.

10. What is the main drawback of a dual-slop ADC?

The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC

It provides excellent noise rejection of ac signals whose periods are integral multiples of the integration time T.

12. Define conversion time.

It is defined as the total time required to convert an analog signal into its digital output. It depends on the conversion technique used & the propagation delay of circuit components. The conversion time of a successive approximation type ADC is given by T(n+1) where T---clock period Tc---conversion time n-- --no. of bits

13. Where are the successive approximation type ADC‟s used?

The Successive approximation ADCs are used in applications such as data loggers & instrumentation where conversion speed is important.

14. What is the main drawback of a dual-slop ADC?

The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC

It provides excellent noise rejection of ac signals whose periods are integral multiples of the integration time T.

16. Define conversion time.

It is defined as the total time required to convert an analog signal into its digital output. It depends on the conversion technique used & the propagation delay of circuit components. The conversion time of a successive approximation type ADC is given by T(n+1)

where T---clock period

Tc---conversion time

n----no.of bits

17. Define resolution of a data converter.

The resolution of a converter is the smallest change in voltage which may be produced at the output or input of the converter. Resolution (in volts)= VFS/2n-1=1 LSB increment. The resolution of an ADC is defined as the smallest change in analog input for a one bit change at the output.

18. Define accuracy of converter.

Absolute accuracy:

It is the maximum deviation between the actual converter output & the ideal converter output.

Relative accuracy:

It is the maximum deviation after gain & offset errors have been removed. The accuracy of a converter is also specified in form of LSB increments or % of full scale voltage.

19. What is settling time?

It represents the time it takes for the output to settle within a specified band ±½LSB of its final value following a code change at the input (usually a full scale change). It depends upon the switching time of the logic circuitry due to internal parasitic capacitance & inductances. Settling time ranges from 100ns. 10Ws depending on word length & type circuit used.

20. Explain in brief stability of a converter:

The performance of converter changes with temperature age & power supply variation. So all the relevant parameters such as offset, gain, linearity error & monotonicity must be specified over the full temperature & power supply ranges to have better stability performances.

21. What is meant by linearity?

The linearity of an ADC/DAC is an important measure of its accuracy & tells us how close the converter output is to its ideal transfer characteristics. The linearity error is usually expressed as a fraction of LSB increment or percentage of full-scale voltage. A good converter exhibits a linearity error of less than ±½LSB.

22. What is monotonic DAC?

A monotonic DAC is one whose analog output increases for an increase in digital input.

23. What are the specifications of D/A converter?

The specifications are accuracy, offset voltage, monotonicity, resolution, and settling time.

24. What is a sample and hold circuit? Where it is used?

A sample and hold circuit is one which samples an input signal and holds on to its last sampled value until the input is sampled again. This circuit is mainly used in digital interfacing, analog to digital systems, and pulse code modulation systems.

25 .Define sample period and hold period.

The time during which the voltage across the capacitor in sample and hold circuit is equal to the input voltage is called sample period. The time period during which the voltage across the capacitor is held constant is called hold period.

26. Which is the fastest ADC and why?

Simultaneous type A/D converter (flash type A/D converter) is the fastest because A/D conversion is performed simultaneously through a set of comparators.

• Easier to build

• Number of bits can be expanded by adding more sections.

• More power dissipation makes heating, which in turns develops non-linearties in DAC.

28. Give the disadvantages of flash type A/D converter.

The simultaneous type A/D converter is not suitable for A/D conversion with more than 3 or 4 digital output bits. Then (2n- 1) comparators are required for an n-bit A/D converter and the number of comparators required doubles for each added bit.

29. Define quantization error.

In A/D converter the smallest digital step is due to the LSB and it can be made smaller only by increasing the number of bits in the digital representation. This error is called quantization error.

30. Define Dither.

It is a very small amount of random noise (white noise) which is added to the input before A/D conversion to improve the performance of A/D converter.

31. Define Delta modulation.

Delta modulation is a technique in which derivative of the signal is Quantized. The delta modulation shows slope overload for fast input signals and their performance is dependent on input signal frequency.

32. Define slope overload noise and granular noise.

Slope overload noise is introduced due to the use of a step size delta is too small to follow some portions of the waveform with a step size. Granular noise results from using a step size that is too large in parts of the Waveform having a small slope.

33. Define resolution of a data converter.

Resolution of a converter is a smallest change in voltage which may be produced at the output.

Integrating type ADC perform conversion in an indirect manner by first changing the analog input signal to a linear function of time or frequency and then to digital code. Here accuracy is more.

Requires wide range of resistor values. R-2R ladder DAC:

Only two resistor values are required.

36. Define conversion time of DAC.

It is the total time required to convert digital signal into analog signal.

37. Define following performance parameters of D/A converters:

Accuracy:

It is the maximum deviation between the actual converter output and the ideal converter output.

Monotonicity:

Monotonic DAC is the one whose analog output increases for an increase in

digital input.

38. Which is the fastest ADC? State the reason.

Flash type ADC is the fastest ADC as the conversion takes place simultaneously rather than sequentially.

39. Define settling time of D/A converter.

Time taken for the output to settle within specified band + ½ LSB of its final value.

40. What is the main drawback of dual slope ADC

Long conversion time

41. Mention any two specifications of a D/A converter.

Accuracy & Resolution.

42. For an n-bit flash type A/D converter, how many comparators are required? State the disadvantage of that type of converter.

2n– 1.

43. State the principle of single slope A/D Converter

It uses a integrator to generate a sawtoothwafeform which is then compared against the analog input by a comparator

Efficient

Conversion speed is more

45. What is over sampling?

Oversampling converters sample the analog signal at a rate much higher than the sampling rates normally required with nyquist converters.

46. What would be produced by a DAC whose output range is 0-10v and whose input

binary number is 10111100 (for a 8 bit DAC)?

V0 = 10V [1/21+0/22+1/23+1/24+1/25+1/26+0/27+0/28]

= 10[47/64]

= 7.34 V

47. Determine the number of comparators and resistors required for 8 bit flash type ADC.

48. Mention two advantages of R-2R ladder type Digital to Analog converter when compared to weighted resistor type Digital to Analog Converter.

49. Why is an interval R-2R ladder network DAC better than R-2R ladder DAC?

50. Which is the fastest ADC and Why?

51. Explain in brief the principle of operation of successive Approximation ADC.

The circuit of successive approximation ADC consists of a successive approximation register (SAR), to find the required value of each bit by trial & error. With the arrival of START command, SAR se s the MSB bit to 1. The O/P is converted into an analog signal & it is compared with I/P signal. This O/P is low or High. This process continues until all bits are checked.

52. Where are the successive approximation type ADC’s used?

The Successive approximation ADCs are used in applications such as data loggers & instrumentation where conversion speed is important.

53. What is the main drawback of a dual-slop ADC?

The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC.

It provides excellent noise rejection of ac signals whose periods are integral multiples of the integration time T.

55. Define conversion time.

It is defined as the total time required to convert an analog signal into its digital output. It depends on the conversion technique used & the propagation delay of circuit components. The conversion time of a successive approximation type ADC is given by

T(n+1) where T---clock period

Tc---conversion time n----no. of bits

56.. Define resolution of a data converter.

The resolution of a converter is the smallest change in voltage which may be produced at the output or input of the converter.

Resolution (in volts)= VFS/2n-1=1 LSB increment . The resolution of an ADC is defined as the smallest change in analog input for a one bit change at the output.

57. Define accuracy of converter.

Absolute accuracy:

It is the maximum deviation between the actual converter output & the ideal converter output.

Relative accuracy:

It is the maximum deviation fter gain & offset errors have been removed. The accuracy of a converter is also specified in form of LSB increments or % of full scale voltage.

58. What is settling time?

It represents the time it takes for the output to settle with n a specified band }.LSB of its final value following a code change at the input (usually a full scale cha ge). It depends upon the switching time of the logic circuitry due to internal parasitic capacitance & i ductances. Settling time ranges from 100ns. 10μ s depending on word length & type circuit used.

59. Explain in brief stability of a converter:

The performance of converter changes with temperature age & power supply variation . So all the relevant parameters such as offset, gain, linearity error & monotonicity must be specified ov r the full temperature & power supply ranges to have better stability performances.

60. What is meant by linearity?

The linearity of an ADC/DAC is an important measure of its accuracy & tells us how close the converter output is to its ideal transfer characteristics. The linearity error is usually expressed as a fraction of LSB increment or percentage of full-scale voltage. A good converter exhibits a linearity error of less than }.LSB.

61. What is monotonic DAC?

A monotonic DAC is one whose analog output increases for an increase in digital input.

62. Find the resolution of an 8-bit DAC.

Resolution =1/( 28 -1) =1/ 255

63. Which is the fastest A/D converter and why?

`Flash type A/D converter is the fastest ADC, because the fast conversion speed is accomplished by providing 2n -1 comparators and simultaneously comparing the input signal with unique reference levels spaced 1 LSB apart.

64. What are limitations of Flash type ADC?

Flash type ADC employs 2n-1 comparators for conversion which makes it costlier which tradeoffs in the speed of conversion.

65. Name the essentials parts of a DAC.

Easier to build accurately as only two precision mental film resistors are required

Number of bits can be expanded by adding more sections of same R/2R values.

In inverted R/2R ladder DAC, node voltages remain constant with changing input binary

This avoids any slo down effects by stray capacitances.

With increasing output bits the circuit becomes larger

The switches used are noted for the sources of errors.

67. What is a sample and hold circuit? Where it is used?

A sample and hold circuit is one which samples an input s gnal and holds on to its last sampled value until the input is sampled again. This circuit is mainly used in digital interfacing, analog to digital systems, and pulse code modulation systems.

68. Define sample period and hold period.

The time during which the voltage across the capacitor in sample and hold circuit is equal to the input voltage is called sample period. The time period during which the voltage across the capacitor is held constant is called hold period.

69. What are the specifications of data convertors?

conversion time, settling time, accuracy, linearity, monotonic etc.

PART-B

1. Draw and explain the functional diagram of the successive approximation ADC converter.

2. Explain the operation of R-2R ladder type DAC and the weighted resistor type DAC.

3. a) Draw and explain the operation of sample and hold circuits.

b) Explain the operation of voltage to time converter.

4. Explain the principle of operations.

5. Explain the working of flash ADC.

6. An 8 bit DAC produces an output voltage „0v‟ corresponds to an input sequence‟00000000‟.if the DAC is connected for a basic increment of 10mv. What is the output when input is „11111111‟?

7. a) A dual slope ADC uses a 16 bit counter and a 4 MHz clock rate. The Maximum input voltage is =10V. The maximum integrator output voltage should be -8V when the counter has cycled through 2n counts. The capacitor used in the integrator is 0.1μF. Find the value of the resistor R of the integrator. If the analog signal is = 4. 129 V, find the corresponding binary number.

8. How many bits are required to design a DAC that can have a resolution of 5mV? The ladder has =8V full scale.

9. Explain delta sigma modulation with required diagram.

10. What are the limitations in weighted resistor type D/A converters and explain how this problem can solve in R-2R ladder type D/A converters.

11. (i) Explain the working of R-2R ladder DAC.

(ii) Explain the working of success approximation ADC.

12. (i) A dual slope ABC uses a 16-bit counter and a 4 MHz clock rate. The maximum input voltage is +10V. The maximum integrator output voltage should be -8V when the counter has recycled through n 2 counts. The capacitor used in the integrator is0.1μF.

12. Find the value of resistor R of the integrator.

(ii) What is a sample and hold circuit? Briefly explain its construction and application.

13. Describe the operation of dual slope and successive approximation type ADC. What are the advantages of dual slope ADC?

14.(i) Explain voltage mode and current mode operations of R-2R ladder type DAC

(ii) Discuss the operation of sample and hold circuit with circuit diagram.

15.(i) Explain the following type of electronic switches used in D/A converter with suitable diagrams (1) Totem pole MOSFET switch (2) CMOS inverter as switch

(ii) Explain the working of R-2R ladder DAC, by taking example of a 3 bit DAC circuit. Sketch the corresponding equivalent circuits and hence obtain the equation for output .   16.(i)With neat circuit diagram and wave form of output, explain the working of dual slope A/D converter.

(ii) Give a table of comparison of Flash, Dual slope and successive approximation ADC‟s in terms of parameters like Speed, Accuracy, resolution and input hold time.

(ii) Draw the circuit and explain the working of dual slope A/D converter.

(iii) For a particular dual slope ADC, t1 is 83.33 ms and the reference voltage is 100mV.

Calculate to if

1)  V1 is 100 mv and

2) 200 mv.

18. Draw the block diagram and explain the working of:

i) Charge Balancing VFCS

ii)Voltage to Time converter.

19. Explain the weighted resistor type and R-2R type DAC.

20. What are oversampling data convertors?

21. With a neat block diagram, explain the working of Successive Approximation type Analog to Digital Converter. Also determine the conversion time of 8bit and 16 bit Successive Approximation type Analog to digital Converter if its clock frequency is 50Hz.

22. With a neat block diagram, explain the working of two bit flash type analog to digital Converter.

23. Design a suitable D/A converter to convert 8-bit binary input in parallel form. Binary „0‟corresponds to 0V and binary „1‟ to 5V.Maximum output is +5V.Assume any other data that may be required. Explain its operation.

24. Write a note on high speed sample and hold circuits

25. With circuit diagram explain the operation of a flash type A/D Converter.

26. Compare the properties of Successive Approximation type and dual slope type converter

27. Explain the successive approximation type A/D converter.

28. Narrate the function of analog switches.

29. State the significance of using high speed sample and hold circuits. Explain its working principle.

30. Compare the performance of various DACS.

31. The basic step of a 9 bit DAC is 10.3 mV. If 000000000 represents 0V, what output is produced if the input is 101101111?

32. Calculate the values of LSB, MSB and full scale output for an 8 bit DAC for 0-10 V range.

33. What output voltage would be produced by a D/A converter whose output range is (0-10)V and whose binary input is i) 10(for 2 bit DAC) ii) 0110(4 bit DAC) iii) 10111100 (8 bit DAC).

34. A Dual slope ADC uses a 16 bit counter and a 4MHz clock rate. The maximum input voltage is 10 V. The maximum integrator output voltage should be -8V when the counter has cycled through 2n counts. The capacitor used in the integrator is 0.1µF.Find value of resister R of the integrator. Also find equivalent digital number for analog signal Va= 4.129V.

35. Compare the resolutions of 3 ½ DVM(digital voltmeter) and 4 ½ DVM which are commonly used in Laboratory.

36. Consider R-2R 4 bit converter and assume feedback resistance Rf of opamp is variable, the resistance R=10K and Vr=10V.Determine the value of Rf that should be connected to achieve the following output condition.

i). The value of 1 LSB at the output is 0.5 V

ii). The analog output of 6V for a binary input of 1000.

iii). The full scale output voltage of 12 v.

iv). The actual maximum output voltage of 10V.

37. A R-2R ladder network has 10 V reference voltage.The DAC produces a short circuit current of 1.875mA when a digital code 1111 is applied.Design a DAC for the above condition and Check the magnitude of short circuit current for an input code 1100.

38. Find step size and analog output for 4 bit R-2R ladder DAC when input is 0111 and 1111.Assume Vref = +5V.

39. A 10 bit A/D converter has an input voltage of -10V to +10 V. What is the resolution and percentage resolution.

40. An A/D converter has a conversion time of 1µs.Calculate the maximum frequency at which it can be used.

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