How To Understand The Theory Of Analog To Digital Converter (Dac)

Transcription

1 Chapter 20 DAC, DAC AND ANALOG- DIGITAL MIX INTERFACES

2 Lesson 2 DIGITAL TO ANALOG CONVERTER (DAC or D/A) Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

3 Outline DAC DAC Circuit DAC Characteristics Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

4 n-bit DAC Physical Output to motor, loudspeaker, furnace Actuator Signal Filter and Amplifier Analog current output DAC b 0 b1 b n 2 b n 1 Digital bits as output for logic circuits Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

5 DAC as current controller Case A Analog output to DAC = 94 ma for 94 d DAC input and = 108 ma for 108 d DAC input 94 ma 94 d b 110 ma 110 d b Resolution of DAC = 1 ma/bit Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

6 DAC Output Analog Output V + ref i + maz V ref = Analog output when input = 0000 V + ref = Analog output when input = 1111 V ref i min Digital Input Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

7 Outline DAC DAC Circuit DAC Characteristics Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

8 R-2R DAC circuit n R 2R R R Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

9 Principle of working R-2R DAC Operation amplifier as a summing amplifier Lower bit of DAC faces higher resistance at the input. Therefore, its contribution is one half of the next stage bit. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

10 Principle of working R-2R DAC Using summing amplifier formula, the DAC V = {(R )/R}. [ d0. {1/(2 n-1 )}R + out p dac d1. {1/(2 n-2 )}(R )2 + d. {1/(2 n dac n-1 1)}(R ) n-1 ]. R is DAC resolution, dac dac voltage output per bit increment at the DAC input. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

11 Principle of working R-2R DAC The successive sum terms is double that of the preceding term. This is because, the resistance in the path of (n-1) bit input = R, for (n -2)th bit, it is 2R, and so on. It is 2 n-2 time R in the path of lowest bit input.. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

12 Outline DAC DAC Circuit DAC Characteristics Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

13 DAC Characteristics The DAC analog output resolution R = β(v + ref V ref )/ (2n 1). β depends on amplifier and filter circuit Range of output signal = V + ref or i + max and V ref. or i min A suitable amplifier at the output interface circuit and has a suitable filter Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

14 DAC Characteristics Analog output of an n bit DAC is when input = decimal m, the input is m times the DAC resolution, when lowest output is V ref. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

15 DAC Characteristics Input bits b, b,., b1 and b0 n 1 n 2 are given by the equation: b w n 1 + b w n 2 +b n 1 0 n 2 0 n 3 w n b w 2 + b w 1 +b w = m = V / β integer out part, where w =2 and b, b,., 0 n 1 n 2 b 1 and b 0 are the input bits from the DAC when the output = V out Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

16 DAC Characteristics DAC conversion time a time in which the expected analog output change results from the digital input change. DAC drift due to circuit temperature defined in ppm/ C Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

17 DAC Characteristics Relationship between the DAC input in decimal vs. the output voltage in Volt or current in ma Linearity of DAC defined as the maximum deviation from a straightline relationship. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

18 DAC Characteristics Accuracy of DAC output is defined by the DAC quantization error. Quantized level means a discrete level. 10-bit DAC, peak output Voltage = 2.0V. Resolution is then equal to division of 2.0 V by 2 10 = 1.95 mv/bit = 1024 quantized levels Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

19 DAC Characteristics A digital multiplexer can receive the digital inputs from the various channels and can give the one output at a time from the DAC Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

20 DAC Characteristics DAC glitch example. Input to a DAC suddenly falls from 127 D to 63 D. Let resolution is 10 mv/bit. If DAC is of high speed and is of resolution 10 mv, then the output may first drop from 1.27 V to 0 V and then eventually settle at 630 mv. Appearance of transient in output signal is called glitch Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

21 DAC Characteristics DAC settling time is the time in which the expected output voltage is within 0.5 times of resolution, i.e., within 1/2 lsb. Consider that the input to a DAC suddenly falls from 1023 d to 511 d. Let DAC is of high speed and is of resolution 5 mv/bit. Let in 8µs, the expected output drop of 512 mv and the output settles between the ( ) and ( ) mv. The settling time = 8 µs. Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

22 Summary Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

23 DAC converts n-bit digital signal into analog output and input is between to Analog output signal between V + ref and V ref or i+ max and i mi Number of bits, Resolution, drift, settling time glitch and quantization error characteristics R-RR ladder network Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

24 End of Lesson 2 on DIGITAL TO ANALOG CONVERTER (DAC or D/A) Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,

25 THANK YOU Ch20L2-"Digital Principles and Design", Raj Kamal, Pearson Education,