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Computer Data Acquisition and Signal Conversion

Computer Data Acquisition and Signal Conversion. Chuck Kammin ABE 425 March 27, 2006. Overview. High Level DAQ System Description Quick Review of Digital Fundamentals Digital to Analog Conversion Analog to Digital Conversion.

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Computer Data Acquisition and Signal Conversion

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  1. Computer Data Acquisition and Signal Conversion Chuck Kammin ABE 425 March 27, 2006

  2. Overview • High Level DAQ System Description • Quick Review of Digital Fundamentals • Digital to Analog Conversion • Analog to Digital Conversion

  3. A data acquisition (DAQ) system is a collection of add-on hardware and software components that allow your computer to receive real-world information from sensors. It consists of Sensors I/O terminal panel(s) DAQ board(s) Software High Level System Description

  4. Digital Fundamentals • Binary Number System • Bits and Bytes • Binary / Decimal Conversion • 2’s compliment • Logic Gates • NOT, AND, NAND…. • Digital Devices • Decoder, Encoder

  5. Bits and Bytes • 8 Bits = 1 Byte • For n bits, highest decimal number = 2n - 1

  6. Decimal to Binary Conversion Whole Numbers: Repeated Division by 2 Convert 1210 to binary: 12/2 = 6 R0 LSB = 0 6/2 = 3 R0 = 0 3/2 = 1 R1 = 1 1/2 = 0 R1 MSB = 1 1210 = 11002

  7. Decimal to Binary Conversion Fractions: Repeated Multiplication by 2 • Convert 0.312510 to binary: • 0.3125 X 2 = 0.625 MSB = 0 • 0.625 X 2 = 1.25 = 1 • 0.25 X 2 = 0.5 = 0 • 0.5 X 2 = 1.0 LSB = 1 0.312510 = 0.01012

  8. Binary to Decimal Conversion Sum of Weights • Binary: 2n 2n -1 ……. 22 21 20 . 2-1 2-2 ……….. • Convert 1100.01012 to Decimal: (1 X 23) + (1 X 22 )+ (0 X 21) + (0 X 20) + (0 X 2-1) + (1 X 2-2) + (0 X 2-3) + (1 X 2-4) = 1100.01012 = 12.312510

  9. 2’s Compliment • Flip 0’s to 1 and 1’s to 0 and add 1. • Example: 2’s compliment of 00001001is 11110110 + 1 = 11110111 • Binary subtraction can be performed via addition using the 2’s compliment. • Example: 8 – 3 = 8 + (-3) = 5 00001000 + 11111101 (2’s compliment of -3) 1 00000101 carry over eliminated

  10. Truth Table NOT Gate (Inverter) A Q

  11. Truth Table AND Gate A Q B

  12. Truth Table NAND Gate A Q B

  13. Truth Table OR Gate A Q B

  14. Truth Table NOR Gate A Q B

  15. Truth Table EX-OR Gate A Q B

  16. Truth Table EX-NOR Gate A Q B

  17. y0 = a’b’c’ 3-to-8 Line Decoder y1 = a’b’c y2 = a’bc’ a y3 = a’bc b y4 = ab’c’ c y5 = ab’c y6 = abc’ y7 = abc Decoder Truth Table

  18. Encoder Truth Table y0 8 to 3 Encoder y1 y2 a y3 b y4 c y5 y6 y7

  19. A’ B’ I0 I0 4-to-1 MUX I1 Z I2 I3 Z A B’ I2 A B A B I3 A’ B I1 Multiplexer

  20. Digital to Analog Conversion • Converter Types • Binary Weighted Input • R/2R Ladder • Performance Characteristics • Resolution • Accuracy • Linearity • Monotonicity • Settling Time

  21. D/A Binary Weighted Input

  22. D/A R/2R Ladder Converter

  23. D/A Performance Characteristics • Resolution - reciprocal to number of discrete steps in D/A output.For n bit output, resolution = 1 / (2n - 1). • Accuracy - comparison of actual output with expected output expressed as a percentage of full scale. Ideal accuracy is at most  0.5 of LSB. For 8 bit converter LSB = 1/28 = 1/256 = 0.0039. Accuracy   0.2%

  24. D/A Performance Characteristics • Linearity - deviation from ideal straight-line output of D/A converter. Special case is offset error which is D/A converter output when input is all zeros. • Monotonicity - D/A converter is monotonic if it does not take any reverse steps when it is sequenced over entire range of input bits.

  25. D/A Performance Characteristics • Settling Time - the time it takes D/A converter to settle within  1/2 LSB of its final value when a change occurs in the input value.

  26. Analog to Digital Conversion • Converter Types • Flash • Stair Step Ramp • Tracking • Single and Dual Slope • Successive Approximation

  27. A/D Flash Short Conversion Time N-bit output requires 2N-1 comparators

  28. A/D Flash

  29. A/D Stair Step Ramp • Slower than Flash A/D • Converter • Worst case counter must • sequence through max number • of states before conversion made. • Conversion time dependant on • analog voltage.

  30. A/D Stair Step Ramp

  31. A/D Tracking • Counter tracks analog • input voltage. • Disadvantage - counter • oscillates between up and • down state if input voltage • constant.

  32. A/D Tracking

  33. A/D Single Slope

  34. A/D Single Slope

  35. A/D Dual Slope Analog Input Clock Counter A1 Switch A2 -V ref Ramp generator Comparator Control Logic Latches Binary Output

  36. A/D Successive Approximation • Most widely used A/D • converter • Faster than other methods • except for flash method • Fixed conversion time

  37. A/D Successive Approximations

  38. PBL 1 • Digital to Analog Conversion Determine the resolution expressed as a percentage, for each of the following D/A converters: • 3-bit • 10-bit • 18-bit

  39. PBL 1 Answer a) 3-bit converter, 1/(23-1) X 100 = 14.3% b) 10-bit converter, 1/(210-1) X 100 = 0.098% c) 18-bit converter, 1/(218-1) X 100 = 3.8E-4%

  40. PBL 2 • How many comparators are required for a 4-bit output flash A/D converter?

  41. PBL 2 Answer • 24-1 = 15 comparators required

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