ADC and DAC Best Ever Pers

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ADC and DAC Best Ever Pers

  1. 1. UNIVERSITY OF HORMUUD Analog to digital converter And Digital to analog converter By: Ahmed Salad Osman 1 1
  2. 2. Part one And Part two 2 2
  3. 3. Part one analog to digital converter 3 3
  4. 4. Outline Definition Why we need ADC Types ADC and each basic operation Applications of analog to digital converter 4 4
  5. 5. Definition An electronic integrated circuit which transforms a signal from analog(continues) to digital(discrete) form Analog signals are directly measurable quantities Digital signals only have two states for digital computer we refer to binary states, 0 and 1 5 5
  6. 6. Continue The heart of computer-based data acquisition is usually the analog to digital converter Basically this device is digital volt meter Digital Systems require discrète digital data Analog ? Digital 6 Digital System 6
  7. 7. Continue Digital computers require signals to be in digital form whereas most instrumentation transducers have an output signal in analogue form. ADC conversion is therefore required at the interface between analogue transducers and the digital computer 7 7
  8. 8. Examples of use • Voltmeter 7.77 V ΔV • Cell phone (microphone) Wave Voice 8 8
  9. 9. Why we need ADC Microprocessors can only perform complex processing on digitized signals When signals are in digital form they are less susceptible to the deleterious effects of additive noise ADC Provides a link between the analog world of transducers and the digital world of signal processing and data handling. 9 9
  10. 10. Types of analog to digital converter There are many different types of analog to digital converters Each offers something in the way of Speed Cost Power dissipation complexity 10 10
  11. 11. Types of analog to digital converter Counter type Successive approximation There are many types such as flash type and sigma-delta but we will cover these two types 11 11
  12. 12. Counter type One of the simplest types of analog to digital converter is counter type ADC The input signal of ADC is connected to the signal input of its internal comparator The ADC then systematically increases the voltage of the reference input of the comparator until the reference becomes larger than the signal 12 12
  13. 13. Continue And the comparator output goes to 0 Ex: consider an input signal is 4.78 volts. The initial comparator’s input would be 2.5 volts The comparator compares the two value then the result this is less than 4.78 then the next higher voltage (5.00 volts) is applied The comparator compares the two value and says this is greater than 4.78 and switches 0 13 13
  14. 14. Continue The digital output of the ADC is the number of times the ADC increase the voltage after starting at the initial 2.5 volts This scheme is relatively simple , but as the number of ADC increases the time it takes to scan through all possible values lower than input will grow quickly 14 14
  15. 15. Components of counter type This type of converter uses some type of counter as part of its operation Counter type contains the following elements: Digital to analog converter Some type of counting mechanism Comparator clock 15 15
  16. 16. Features of counter type Use a clock to index the counter Use DAC to generate analog signal to compare against input Comparator is used to compare VIN and VDAC where VIN is the signal to be digitized The input to the DAC is from the counter 16 16
  17. 17. Operation of counter type START Comparator Vin Control Logic clock Counter DA C Digital Output 17 17
  18. 18. Operation of counter type START Comparator Vin Control Logic clock Counter DA C Digital Output 18 18
  19. 19. Successive approximation 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 19
  20. 20. Continue The SAR architecture mainly uses the binary search algorithm The SAR ADC consists of fewer blocks such as one comparator, one DAC (Digital to Analog Converter) and one control logic. The algorithm is very similar to like searching a number from telephone book 20 20
  21. 21. How Successive Approximation Works • Example : analog input = 6.428v, reference = 10.000v MSB 5.000V 2SB 2.500V 3SB 1.250V LSB 0.625V VIN > 5.000V VIN > 7.500V VIN > 6.250V VIN > 6.875V YES NO YES NO 0 1 0 1 21 21
  22. 22. Applications Scanner : when you scan a picture with a scanner , what scanner is doing is an analog to digital conversion : it is taking the analog information provided by the picture(light) and converting into digital Recording a voice : when u=you record your voice or use a VoIP solution on your computer you r using analog to digital converter to convert you voice , which is analog into digital information 22 22
  23. 23. Part two Digital to analog converter 23 23
  24. 24. Outline Definition Types of DAC and each operation DAC performance specifications Applications of ADC 24 24
  25. 25. Definition To convert digital values to analog voltage Performs inverse operation of analog to digital converter 100101… DAC 25
  26. 26. Analog output signal What is DAC 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 Digital input signal 26
  27. 27. Continue REFERENCE INPUT DIGITAL INPUT ANALOG OUTPUT RESOLUTION = N BITS Digital Input Analog Output = x Reference Input (2N - 1) 27 27
  28. 28. continue ADC is function that converts digital data(usually binary) into analog signal(current , voltage, or electric charge) digital-to-analog converter, a device (usually a single chip) that converts digital data into analog signals.  Modems require a DAC to convert data to analog signals that can be carried by telephone wires. Video adapters also require DACs, called RAMDACs, to convert digital data to analog signals that the monitor can process. 28 28
  29. 29. Types of DAC There are two types of ADC Weighted Resistor or Resistive Divider type And there is an other type of R -2R ladder N bit digital data 0 1 2 Digital to analog converter Analog data n-2 29 29
  30. 30. Weighted Resistors • In this type of DAC components used is – Operational amplifier – Switches – Resistors R – Voltage source MSB – Ground Rf = R Ii 2R 4R 8R LSB -VREF 30 30
  31. 31. Definition of weighted resistors Binary Weighted resistors are used to distinguish each bit from the most significant to the least significant Binary weighted resistors Reduces current by a factor of 2 for each bit 31 31
  32. 32. Continue Binary Weighted resistors is reliable, and simple to do The circuit shown is a digital to analog converter 4-bits weighted binary resistance network circuit types. Resistor values ​can be calculated using the weight of the binary number. 32 32
  33. 33. Circuit diagram of weighted resistors 33 33
  34. 34. Weighted Binary Resistance Network Weighted Binary Resistance Network Circuit D C B A 18.7K 37.5K 75K 150K 3V R4 R3 R2 RF R1 20K ++ 34 Vout Vout O VVV UT 34
  35. 35. Continue For example Referring to the circuit as shown, the highest value resistor (150KΩ) is a digital input resistor. The smallest bit (least significant bit), and the values of other resistor is 35 35
  36. 36. Circuit analysis to find Vout If binary input is 0001 R1 = 150KΩ, RF = 20KΩ, Vref = 3V Voltage Gain (AV) = RF = 20KΩ = 0.133 R1 150KΩ Vout = Vref X AV = 3V X 0.1333 = 0.4V 36 36
  37. 37. Continue  If binary input is 0110 R2 = 75KΩ, R3 = 37.5KΩ, RF = 20KΩ, Vref = 3V RT = R2//R3 = 25KΩ Voltage Gain (AV) = RF RT Vout = 20KΩ = 0.8 25KΩ = Vref X AV = 3V X 0.8 = 2.4V 37 37
  38. 38. Calculate If binary input is 1100 R3 = 37.5KΩ, R4=18.75 RF = 20KΩ, Vref = 3V RT = R3//R4 = 12.5KΩ Voltage Gain (AV) = RF RT Vout = 20KΩ = 1.6 12.5KΩ = Vref X AV = 3V X 1.6 = 4.8 38 38
  39. 39. Simply that we can see the resulting output is shown in the table below Decimal 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Digital input D 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 C 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 B 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 A 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 Vout (V) 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 39 39
  40. 40. Example Find output voltage and current for a binary weighted resistor DAC of 4 bits where : R = 10 k Ohms, Rf = 5 k Ohms and VR = 10 Volts. Applied binary word is 1001. 40 40
  41. 41. Solution Rf = (R/2) Ii 8R 4R 4-bit 2R 3-bit Vo R 2-bit 1-bit MSB VR 41 41
  42. 42. Solution Cont’d Io I0 10 V 1 0 4 2 *10 - 0.001125A 0 1 4 2 *10 0 2 4 2 *10 1 3 4 2 *10 V0 - R f I0 V0 3 (5 *10 )( 0.001125A) 5.625 V 42 42
  43. 43. Solution Cont’d Binary input = 1001 = 9 From example, V0 = 5.625V V0/VR = 5.625V/10V = 9/16 43 43
  44. 44. Binary Weighted Resistor  Advantages  Simple Construction/Analysis  Fast Conversion  Disadvantages  Requires large range of resistors (2000:1 for 12bit DAC) with necessary high precision for low resistors  Requires low switch resistances in transistors  Can be expensive. Therefore, usually limited to 8-bit resolution. 44 44
  45. 45. Limitations of binary weighted Has problems if bit length is longer than 8 bits For example, if R = 10 k Ohms R8 = 28-1(10 k Ohms) = 1280 k Ohms If VR = 10 Volts, I8 = 10V/1280 k Ohms = 7.8 A Op-amps to handle those currents are expensive because this is usually below the current noise threshold. 45 45
  46. 46. Limitations Cont’d If R = 10 Ohms and Vref = 10 V I = VR/R = 10V/10 Ohms = 1 A This current is more than a typical op-amp can handle. Large resistors more error 46 46
  47. 47. DAC performance specification Resolution Reference Voltages Settling Time Linearity Speed Errors 47 47
  48. 48. Resolution Resolution: is the amount of variance in output voltage for every change of the LSB in the digital input. How closely can we approximate the desired output signal(Higher Res. = finer detail=smaller Voltage divisions) A common DAC has a 8 - 12 bit Resolution VRef N = Number of bits Resolution VLSB N 2 48 48
  49. 49. Resolution continue Better Resolution(3 bit) Poor Resolution(1 bit) Vout Vout Desired Analog signal Desired Analog signal 111 110 8 Volt. Levels 2 Volt. Levels 1 101 110 101 100 100 011 011 010 010 001 0 Approximate output 0 001 000 000 Digital Input Approximate output 49 Digital Input 49
  50. 50. Reference voltage Reference Voltage: A specified voltage used to determine how each digital input will be assigned to each voltage division. Types: Non-multiplier: internal, fixed, and defined by manufacturer Multiplier: external, variable, user specified 50 50
  51. 51. Reference voltage types Multiplier: (Vref = Asin(wt)) Non-Multiplier: (Vref = C) Voltage Voltage 11 11 10 10 10 01 01 10 01 01 0 0 00 00 Digital Input 51 00 00 Digital Input 51
  52. 52. Settle time Settling Time: The time required for the input signal voltage to settle to the expected output voltage(within +/- VLSB). Any change in the input state will not be reflected in the output state immediately. There is a time lag, between the two events. 52 52
  53. 53. Settle time continue Analog Output Voltage Expected Voltage +VLSB -VLSB Settling time 53 Time 53
  54. 54. Linearity Linearity: is the difference between the desired analog output and the actual output over the full range of expected values. Ideally, a DAC should produce a linear relationship between a digital input and the analog output, this is not always the case. 54 54
  55. 55. Linearity continue NON-Linearity(Real World) Desired/Approximate Output Analog Output Voltage Analog Output Voltage Linearity(Ideal Case) Desired Output Approximate output Digital Input Digital Input Miss-alignment Perfect Agreement 55 55
  56. 56. Speed Speed: Rate of conversion of a single digital input to its analog equivalent Conversion Rate Depends on clock speed of input signal Depends on settling time of converter 56 56
  57. 57. Errors Non-linearity Differential Integral Gain Offset 57 57
  58. 58. Non linearity: differential Analog Output Voltage Differential Non-Linearity: Difference in voltage step size from the previous DAC output (Ideally All DLN’s = 1 VLSB) Ideal Output 2VLSB Diff. Non-Linearity = 2VLSB VLSB Digital Input 58 58
  59. 59. Non linearity: integral Integral Non-Linearity: Deviation of the actual DAC output from the ideal (Ideally all INL’s = 0) Analog Output Voltage Ideal Output Int. Non-Linearity = 1VLSB 1VLSB Digital Input 59 59
  60. 60. Gain error Gain Error: Difference in slope of the ideal curve and the actual DAC output High Gain High Gain Error: Actual slope greater than ideal Low Gain Error: Actual slope less than ideal Analog Output Voltage Desired/Ideal Output Low Gain Digital Input 60 60
  61. 61. Offset Offset Error: A constant voltage difference between the ideal DAC output and the actual. – The voltage axis intercept of the DAC output curve is different than the ideal. Output Voltage Desired/Ideal Output Positive Offset Digital Input Negative Offset 61 61
  62. 62. Applications of DAC Digital Motor Control Computer Printers Sound Equipment (e.g. CD/MP3 Players, etc.) Function Generators/Oscilloscopes Digital Audio 62 62
  63. 63. References • Callis, J. B. “The Digital to Analog Converter.” 2002. http://courses.washington.edu/jbcallis/lectures/C464_L ec5_Sp-02.pdf. 14 March 2006 • “DAC.” 2006. http://en.wikipedia.org/wiki/Digital-toanalog_converter#DAC_types. 14 March 2006. • Johns, David and Ken Martin. “Data Converter Fundamentals.” © 1997. http://www.eecg.toronto.edu/~kphang/ece1371/chap11_ slides.pdf. 14 March 2006 • Goericke, Fabian, Keunhan Park and Geoffrey Williams. “Digital to Analog Converter.” © 2005. http://www.me.gatech.edu/mechatronics_course/DAC_ F05.ppt. 14 March 2006 63
  64. 64. 64 64
  65. 65. Questions 65

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