1. UNIT-III- DATA CONVERTERS Data Converters: Introduction, Basic DAC techniques, Different types of DACs-Weighted resistor DAC, R-2R ladder DAC, Inverted R-2R DAC, Different Types of ADCs – Parallel Comparator Type ADC, Counter Type ADC, Successive Approximation ADC and Dual Slope ADC,DAC and ADC Specifications.

2. Introduction-Data Conversion  Basically we have two types of data in nature: Analog and Digital. Analog: continuously valued signal, such as temperature or speed, with infinite possible values in between. Digital: discretely valued signal, such as integers, encoded in binary  Data transmission – Electrical wires (for analog) and data bus (for digital).  Most of the real-world physical quantities are in analog form.  Data storage and processing – Analog versus Digital. (That’s the idea for data conversion!!!)  Digital data processors – Microprocessors, DSPs, FPGAs etc.  when a data conversion needed? – When digital devices interface with analogue devices and vice versa.

3. Data Conversion Circuits  Digital-to-Analog (D/A) and Analog-to-Digital (A/D) converters (DAC and ADC). Application of A/D and D/A converters  Today, both ADC and DAC are available in IC form.

4. ADC and DAC  Applications – digital audio recording and playback, computers, data acquisition, digital multimeter, digital control, digital signal processing, microprocessor based instrumentation.  Since most of the ADCs use DACs in their inside, we have to discuss DAC first.

5. What is a DAC? A digital to analog converter (DAC) converts a digital signal to an analog voltage or current output. DAC’s current or voltage analog output is proportional to the weighted sum of digital inputs (characteristic of a linear circuit).  For a voltage output DAC,

6. Contd… Each binary number sampled by the DAC corresponds to a different output level.

7. What a DAC Looks Like

8. Types of DACs Many types of DACs available. Usually switches, resistors, and op-amps used to implement conversion Three Types: Binary Weighted Resistor R-2R Ladder Inverted R-2R Ladder DAC

9. Binary Weighted Resistor Utilizes a summing op-amp circuit Weighted resistors are used to distinguish each bit from the most significant to the least significant Transistors are used to switch between V ref and ground (bit high or low)

10. Contd..

11. Binary Weighted Resistor Advantages Simple Construction/Analysis Fast Conversion Disadvantages Requires large range of resistors (2000:1 for 12-bit DAC) with necessary high precision for low resistors Requires low switch resistances in transistors Can be expensive. Therefore, usually limited to 8-bit resolution.

12. R-2R Ladder DAC – Equation 3-bit R-2R Ladder DAC (Voltage mode)

13. R-2R Ladder Each bit corresponds to a switch: If the bit is high, the corresponding switch is connected to the inverting input of the op-amp. If the bit is low, the corresponding switch is connected to ground.

14. R-2R Ladder V ref V2V2 V1V1 V3V3 2R V3V3 Ideal Op-amp

15. R-2R Ladder V ref V2V2 V1V1 V3V3 V out V2V2 V3V3 RR Likewise, I

16. V ref V2V2 V1V1 V3V3 R-2R Ladder Results: Where b 3 corresponds to bit 3, b 2 to bit 2, etc. If bit n is set, b n =1 If bit n is clear, b n =0

17. R-2R Ladder For general n-Bit R-2R Ladder or Binary Weighted Resister DAC For a 4-Bit R-2R Ladder

18. R-2R Ladder Summary Advantages ◦Only 2 resistor values ◦Does Not Require High Precision Resistors. Disadvantages Lower Conversion Speed Than Binary Weighted DAC Summary ◦Better than weighted resistor DAC

19. Comparision Binary Weighted Resistor vs. R-2R Ladder

20. Inverted R-2R Ladder DAC The inverting R/2R ladder DAC works on the principle of summing currents and it is also said to operate in the current mode. An important advantage of the current mode is that all ladder node voltages remain constant with changing input codes, thus avoiding any shutdown effects by stray capacitances. In inverted R/2R ladder DAC, Position of the MSB & LSB is interchanged, Current flowing in the resistances is constant and independent of switch position.

22. Advantages 1. The major advantage of Inverted R-2R ladder D/A converter is that the voltage change across each switch is minimal. So the charge injection is virtually eliminated and the switch driver design is made simpler. 2. In Current mode or inverted ladder type DACs, the stray capacitance do not affect the speed of response of the circuit due to constant ladder node voltages. So improved speed performance.

23. Applications Generic Use Circuit Components Audio and Video Oscilloscopes/Generators Motor Controllers

24. Applications Generic Use Used when a continuous analog signal is required. Signal from DAC can be smoothed by a Low pass filter Orlando Carreon

25. Applications Circuit Components Voltage controlled Amplifier Digital input, External Reference Voltage as Control Digitally operated attenuator External Reference Voltage as Input, Digital Control Programmable Filters Digitally Controlled Cutoff Frequencies

26. Applications Audio Most modern audio signals are stored in digital form and in order to be heard through speakers they must be converted into an analog signal. -CD Players - Digital Telephones -MP3 Player- Hi-Fi Systems Video Video signals from a digital source must be converted to analog form if they are to be displayed on an analog monitor - Computers - Digital Video Player

27. Applications Oscilloscopes/Generators Digital Oscilloscopes ◦ Digital Input ◦ Analog Output Signal Generators ◦ Sine wave generation ◦ Square wave generation ◦ Triangle wave generation ◦ Random noise generation

28. Applications Motor Controllers Cruise Control Valve Control Motor Control

32. Analog to Digital Converters (ADCs) An Analog to Digital Converter (ADC) converts an analog signal into a digital signal. The digital signal is represented with a binary code, which is a combination of bits 0 and 1. ADC Provides a link between the analog world of transducers and the digital world of signal processing and data handling. ADC are used virtually everywhere where an analog signal has to be processed, stored, or transported in digital form. Some examples of ADC usage are digital voltmeters, cell phone, thermocouples, and digital oscilloscope.

33. A/D Conversion process Converts analog signals into binary words

34. Analog  Digital Conversion 2-Step Process: Two main steps of process 1.Sampling and Holding 2.Quantization and Encoding Quantizing - breaking down analog value is a set of finite states Encoding - assigning a digital word or number to each state and matching it to the input signal t t Input: Analog Signal Sampling and Hold Quantizing and Encoding

35. ADC Process t Sampling & Hold Measuring analog signals at uniform time intervals Ideally twice as fast as what we are sampling Digital system works with discrete states Taking samples from each location Reflects sampled and hold signal Digital approximation

36. Step 1: Quantizing Example: You have 0-10V signals. Separate them into a set of discrete states with 1.25V increments. (How did we get 1.25V? See next slide…) Output States Discrete Voltage Ranges (V) 00.00-1.25 11.25-2.50 22.50-3.75 33.75-5.00 45.00-6.25 56.25-7.50 67.50-8.75 78.75-10.0

37. Quantizing The number of possible states that the converter can output is: N=2 n where n is the number of bits in the AD converter Example: For a 3 bit A/D converter, N=2 3 =8. Analog quantization size: Q=(V max -V min )/N = (10V – 0V)/8 = 1.25V

38. Encoding Here we assign the digital value (binary number) to each state for the computer to read. Output States Output Binary Equivalent 0000 1001 2010 3011 4100 5101 6110 7111

39. Types of ADC There are two types of ADCs: Direct type ADCs and Indirect type ADC. If the ADC performs the analog to digital conversion directly by utilizing the internally generated equivalent digital (binary) code for comparing with the analog input, then it is called as Direct type ADC. The following are the examples of Direct type ADCs − Flash type or Parallel type ADC Counter type ADC Successive Approximation ADC

40. Flash or Parallel type ADC It is the fastest ADC but very expensive. Typical conversion time is 100ns or less. Flash ADCs are ideal for applications requiring very large bandwidth, but they consume more power and much bigger in size than other ADC architectures. A Flash converter requires a huge number of comparators compared to other ADCs, especially as the resolution increases. A Flash converter requires 2 n -1 comparators for an n- bit conversion.  Elements Encoder – Converts output of comparators to binary Comparators

41. Flash ADC

42. 2 bit Flash ADC

43. How Flash Works As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. A Priority Encoder is used to transform the comparator outputs to the correct digital binary output.Priority Encoder Change the input voltage and observe the comparators and the priority encoder digital outputs. The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs.

44. Contd..

45. ADC Output

46. Flash Advantages Simplest in terms of operational theory Most efficient in terms of speed, very fast limited only in terms of comparator and gate propagation delays Disadvantages It is not suitable for higher number of bits Lower resolution Expensive For each additional output bit, the number of comparators is doubled i.e. for 8 bits, 256 comparators needed

47. Counter type A/D Converter The Counter type ADC is the basic type of ADC which is also called as digital ramp type ADC or stair case approximation ADC. This circuit consists of N bit counter, DAC and Op-amp comparator. The N bit counter generates an n bit digital output which is applied as an input to the DAC. The analog output corresponding to the digital input from DAC is compared with the input analog voltage using an opamp comparator.

48. Contd.. The opamp compares the two voltages and if the generated DAC voltage is less, it generates a high pulse to the N bit counter as a clock pulse to increment the counter. The same process will be repeated until the DAC output equals to the input analog voltage. If the DAC output voltage is equal to the input analog voltage, then it generates low clock pulse and it also generates a clear signal to the counter and load signal to the storage resistor to store the corresponding digital bits. These digital values are closely matched with the input analog values with small quantization error. For every sampling interval the DAC output follows a ramp fashion so that it is called as Digital ramp type ADC. And this ramp looks like stair cases for every sampling time so that it is also called as staircase approximation type ADC.

49. Contd..

50. Conversion time of Counter type ADC The maximum conversion of Counter type ADC is = (2 N -1) T Where, T is the time period of clock pulse. If N=2 bit then the T max = 3T. Ts >= (2 N -1) T

51. Contd.. Merits Simple to understand and operate. Cost is less because of less complexity in design. Demerits Speed is less because every time the counter has to start from ZERO. There may be clash or aliasing effect if the next input is sampled before completion of one operation.

52. Successive Approximation ADC Successive Approximation type ADC is the most widely used and popular ADC method. The conversion time is maintained constant in successive approximation type ADC, and is proportional to the number of bits in the digital output, unlike the counter and continuous type A/D converters. The basic principle of this type of A/D converter is that the unknown analog input voltage is approximated against an n-bit digital value by trying one bit at a time, beginning with the MSB.

53. BLOCK DIAGRAM OF SAR Uses a n-bit DAC to compare DAC and original analog results. Uses Successive Approximation Register (SAR) supplies an approximate digital code to DAC of Vin. Comparison changes digital output to bring it closer to the input value. Uses Closed-LoopFeedback Conversion

54. Contd.. This type of ADC operates by successively dividing the voltage range by half, as explained in the following steps. (1) The MSB is initially set to 1 with the remaining three bits set as 000. The digital equivalent voltage is compared with the unknown analog input voltage. (2) If the analog input voltage is higher than the digital equivalent voltage, the MSB is retained as 1 and the second MSB is set to 1. Otherwise, the MSB is set to 0 and the second MSB is set to 1. Comparison is made as given in step (1) to decide whether to retain or reset the second MSB.

55. Ex: 4 Bit S Successive Approximation ADC operates by successively dividing the voltage range by half, as explained in the following steps. (1) The MSB is initially set to 1 with the remaining three bits set as 000. The digital equivalent voltage is compared with the unknown analog input voltage. (2) If the analog input voltage is higher than the digital equivalent voltage, the MSB is retained as 1 and the second MSB is set to 1. Otherwise, the MSB is set to 0 and the second MSB is set to 1. Comparison is made as given in step (1) to decide whether to retain or reset the second MSB.

56. Output

57. Successive Approximation Advantages Capable of high speed and reliable Medium accuracy compared to other ADC types Good tradeoff between speed and cost Capable of outputting the binary number in serial (one bit at a time) format. Disadvantages Higher resolution successive approximation ADC’s will be slower Speed limited to ~5Msps Applications The SAR ADC will used widely data acquisition techniques at the sampling rates higher than 1 0KHz

58. TypeSpeed (relative)Cost (relative) Dual SlopeSlowMed FlashVery FastHigh Successive AppoxMedium – FastLow Sigma-DeltaSlowLow ADC Types Comparison

59. Integrating ADC An integrating ADC is a type of analog-to-digital converter that converts an unknown input voltage into a digital representation through the use of an integrator.analog-to-digital converterintegrator In its basic implementation, the dual-slope converter, the unknown input voltage is applied to the input of the integrator and allowed to ramp for a fixed time period (the run-up period). Then a known reference voltage of opposite polarity is applied to the integrator and is allowed to ramp until the integrator output returns to zero (the run- down period). The input voltage is computed as a function of the reference voltage, the constant run- up time period, and the measured run-down time period. The run-down time measurement is usually made in units of the converter's clock, so longer integration times allow for higher resolutions. Likewise, the speed of the converter can be improved by sacrificing resolution. Types: 1. Charge balancing ADC 2. Dual slope ADC

60. Dual Slope A/D Converter Fundamental components Integrator Electronically Controlled Switches Counter Clock Control Logic Comparator

61. How does itwork A dual-slope ADC (DS-ADC) integrates an unknown input voltage (V IN ) for a fixed amount of time (T INT ), then "de-integrates" (T DEINT ) using a known reference voltage (V REF ) for a variable amount of time. The key advantage of this architecture over the single-slope is that the final conversion result is insensitive to errors in the component values. That is, any error introduced by a component value during the integrate cycle will be cancelled out during the de-integrate phase.

62. Contd… At t<0, S 1 is set to ground, S 2 is closed, and counter=0. At t=0 a conversion begins and S 2 is open, and S 1 is set so the input to the integrator is V in. S 1 is held for T INT which is a constant predetermined time interval. When S 1 is set the counter begins to count clock pulses, the counter resets to zero after T INT V out of integrator at t=T INT is V IN T INT /RC is linearly proportional to V IN At t=T INT S 1 is set so -V ref is the input to the integrator which has the voltage V IN T INT /RC stored in it. The integrator voltage then drops linearly with a slop -V ref /RC. A compartor is used to determine when the output voltage of the integrator crosses zero When it is zero the digitized output value is the state of the counter.

63. Dual Slope Converter Advantages Input signal is averaged Greater noise immunity than other ADC types High accuracy Disadvantages Slow High precision external components required to achieve accuracy Cost

64. Applications of ADC ADC are used virtually everywhere where an analog signal has to be processed, stored, or transported in digital form. Some examples of ADC usage are digital volt meters, cell phone, thermocouples, and digital oscilloscope. Microcontrollers commonly use 8, 10, 12, or 16 bit ADCs, our micro controller uses an 8 or 10 bit ADC.