1. Interfacing to the Analog World
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 Application29

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