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Chapter 15

Chapter 15. Interfacing to the Analog World. 1. Objectives. You should be able to: Perform the basic calculations involved in the analysis of operational amplifier circuits. Explain the operation of binary-weighted and R/2R digital-to-analog converters.

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Chapter 15

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  1. Chapter 15 Interfacing to the Analog World 1

  2. Objectives You should be able to: • Perform the basic calculations involved in the analysis of operational amplifier circuits. • Explain the operation of binary-weighted and R/2R digital-to-analog converters. • Make the external connections to a digital-to-analog IC to convert a numeric binary string into a proportional analog voltage. 2

  3. Objectives (Continued) • Discuss the meaning of the specifications for converter ICs as given in a manufacturer’s data manual. • Explain the operation of parallel-encoded counter-ramp, and successive-approximation analog-digital converters. 3

  4. Objectives (Continued) • Make the external connections to an analog-to-digital converters IC to convert an analog voltage to a corresponding binary string. • Discuss the operation of a typical data acquisition system. 4

  5. Digital and Analog Representations • An analog signal can be represented with digital values at some time interval. 5

  6. Digital and Analog Representations • Four binary positions = 4-bit resolution • 16 different representations • Eight binary positions = 8-bit resolution • 256 different representations 6

  7. Operational Amplifier Basics • Very high input impedance • Very high voltage gain • Very low output impedance 7

  8. Discussion Point • Determine Vout 8

  9. Binary-Weighted Digital-to-Analog Converters • Sum of the currents from the input resistors • Binary weighting factor 9

  10. Binary-Weighted Digital-to-Analog Converters • Accurate resistance over a wide range is difficult • Not practical for conversions greater than 4-bit 10

  11. R/2R Ladder Digital-to-Analog Converters • Only two resistor values • 8, 10, 12, 14, and 16 bit resolutions are common 11

  12. R/2R Ladder Digital-to-Analog Converters • Current division and analog output versus digital input 12

  13. R/2R Ladder Digital-to-Analog Converters • Current division and analog output versus digital input 13

  14. Integrated-Circuit Digital-to-Analog Converters • DAC0808 block diagram and pin configuration 14

  15. Integrated-Circuit Digital-to-Analog Converters • DAC0808 Application 14

  16. Integrated-Circuit Digital-to-Analog Converters • Testing the 256-step output of a DAC with an 8 bit counter 15

  17. Integrated-Circuit Digital-to-Analog Converters • Multisim DAC simulation 16

  18. IC Data Converter Specifications • Differential nonlinearity • Gain error • Missing codes 17

  19. IC Data Converter Specifications • Nonmonotonic, offset error, relative accuracy, settling time, and 3-bit ADC transfer characteristic 18

  20. Parallel-Encoded Analog-to-Digital Converters • Parallel encoding • Also called simultaneous, multiple comparator, or flash converting • Several comparators with different reference voltages drive a priority encoder 19

  21. Parallel-Encoded Analog-to-Digital Converters • Three-bit parallel encoded ADC • priority encoder • Analog range of 0-7 V • 3 bit (8 level) resolution 20

  22. Counter-Ramp Analog-to-Digital Converters • Counter used in conjunction with a D/A converter • To change for continuous conversions end-of-conversion line is tied back to clear input • Disadvantage is slow conversion time 21

  23. Counter-Ramp Analog-to-Digital Converters (Figure 15-12) 21

  24. Successive-Approximation Analog-to-Digital Conversion • Most used in modern ADC ICs • Converter circuit is similar to counter-ramp • Uses successive approximation register to quickly narrow in on the analog value • Result is a much faster conversion when compared to the counter-ramp method 23

  25. Successive-Approximation Analog-to-Digital Conversion • Simplified SAR A/D converter 24

  26. Integrated-Circuit Analog-to-Digital Converters • NE5034 – similar to the SAR ADC just presented but uses a three-state output buffer instead of a D flip-flop • Conversion speeds up to 17 s • Compatible with bus oriented microprocessors 25

  27. Integrated-Circuit Analog-to-Digital Converters • NE5034 block diagram and pin configuration 26

  28. Integrated-Circuit Analog-to-Digital Converters • ADC 0804 • Successive-approximation • Two analog inputs for differential measurements • Internal clock (determined by external R and C) • Operation similar to NE5034 • Analog and digital ground are both provided 27

  29. Integrated-Circuit Analog-to-Digital Converters • ADC 0804 block diagram and pin configuration 28

  30. Data Acquisition System Application • Data bus • Control bus • Analog Multiplexer Switch (AM3705) • Sample-and-Hold Circuit (LF198) • Programmable-Gain Instrumentation Amplifier (LH0084) • Analog-to-Digital Converter (ADC0804) 29

  31. Data Acquisition System Application 29

  32. Transducers and Signal Conditioning • Physical quantities to electrical quantities • Must be conditioned due to different output ranges and signals • Manufacturers specifications must be studied • Analog output of transducer is converted to binary by ADC • Data can then be manipulated by software 31

  33. Transducers and Signal Conditioning • Thermistor resistance is dependent on temperature and response is nonlinear 32

  34. Transducers and Signal Conditioning • Thermistors – Example conversion circuit 33

  35. Transducers and Signal Conditioning • Linear IC Temperature Sensors • Simplify process of converting a nonlinear response 34

  36. Transducers and Signal Conditioning • The Strain Gage • Resistance changes when stretched • Example of signal conditioning for a strain gage 35

  37. Summary • Any analog quantity can be represented by a binary number. Longer binary numbers provide higher resolution, which gives a more accurate representation of the analog quantity. • The binary-weighted D/A converter is the simplest to construct, but it has practical limitations in resolution (number of input bits). 36

  38. Summary • Operational amplifiers are important building blocks in analog-to-digital (A/D) and digital-to-analog (D/A) converters. They provide a means for summing currents at the input and converting a current to a voltage at the output of converter circuits. • The R/2R ladder D/A converter uses only two different resistor values, no matter how many binary input bits are included. This allows for very high resolution and ease of fabrication in integrated-circuit form. 37

  39. Summary • The DAC0808 (or MC1408) IC is an 8-bit D/A converter that uses the R/2R ladder method of conversion. It accepts 8 binary input bits and outputs an equivalent analog current. Having 8 input bits means that it can resolve up to 256 unique binary values into equivalent analog values. 38

  40. Summary • Applying an 8-bit counter to the input of an 8-bit D/A converter will produce a 256-step sawtooth waveform at its output. • The simplest way to build an analog-to-digital (A/D) converter is to use the parallel encoding method. The disadvantage is that it is practical only for low-resolution applications. 39

  41. Summary • The counter-ramp A/D converter employs a counter, a D/A converter, and a comparator to make its conversion. The counter counts from zero up to a value that causes the D/A output to exceed the analog input value slightly. That binary count is then output as the equivalent to the analog input. 40

  42. Summary • The method of A/D conversion used most often is called successive approximation. In this method, successive bits are tested to see if they contribute an equivalent analog value that is greater than the analog input to be converted. If they do, they are returned to zero. After all bits are tested, the ones that are left ON are used as the final digital equivalent to the analog input. 41

  43. Summary • The NE5034 and the ADC0802 are examples of A/D converter ICs. To make a conversion, the start-conversion pin is made LOW. When the conversion is completed the end-of-conversion pin goes LOW. Then to read the digital output, the output enable pin is made LOW. 42

  44. Summary • Data acquisition systems are used to read several different analog inputs, respond to the values read, store the results, and generate reports on the information gathered. • Transducers are devices that convert physical quantities such as heat, light, or force into electrical quantities. Those electrical quantities must then be conditioned (or modified) before they can be interpreted by a digital computer. 43

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