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Data-Converter Circuits A/D and D/A Chapter 9

Data-Converter Circuits A/D and D/A Chapter 9. 1. Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals Seat Belt- Thermocouple – temperature measuring device converts thermal energy to electric energy Voltmeters

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Data-Converter Circuits A/D and D/A Chapter 9

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  1. Data-Converter Circuits A/D and D/A Chapter 9 1

  2. Microphones - take your voice varying pressure waves in the air and convert them into varying electrical signals Seat Belt- Thermocouple – temperature measuring device converts thermal energy to electric energy Voltmeters Digital Multimeters ADSL Analog Signals every where Examples of A/D

  3. Most signals are analog Need to Sample an analog signal Then convert to digital by A/D converter Are sensor outputs Analog ? Eg. Seatbelt ? EEG, oil temp Figure 9.36 The process of periodically sampling an analog signal. (a) Sample-and-hold (S/H) circuit. The switch closes for a small part (t seconds) of every clock period (T). (b) Input signal waveform. (c) Sampling signal (control signal for the switch). (d) Output signal (to be fed to A/D converter).

  4. A/D converter and D/A Converters Analog to Digital Digital to Analog What parts of your iPhone operation are Analog ? / Digital Your internet access: Analog ? Digital ? Figure 9.37 The A/D and D/A converters as circuit blocks.

  5. A/D converterConverts analog signals into binary words

  6. D/A Conversion Normal Output from digital domain is staircase Filtered to produce smooth Analog output Figure 9.38 The analog samples at the output of a D/A converter are usually fed to a sample-and-hold circuit to obtain the staircase waveform shown. This waveform can then be filtered to obtain the smooth waveform, shown in color. The time delay usually introduced by the filter is not shown.

  7. Conversion accuracy: eg 2-bits Blue line ? • Analog is continuous • But digital is discrete • Limited by number of bits Red ?

  8. 3-bit conversion example Quantization levels Eg 5V divided into 8 levels – each 0.625 Each binary representation is a “range”

  9. D/A conversion implementation Weighted Summing Circuit in = ? i = ? vo = - [(Rf / R1) * v1 + (Rf / R2) * v2 +….+ (Rf / Rn) * vn] Figure 2.10 A weighted summer.

  10. N-bit D/A Converter Implementation Binary weighted resistive ladder digital values control switches S1 - Sn Figure 9.39 An N-bit D/A converter using a binary-weighted resistive ladder network.

  11. Quantizing - breaking down analog value to set of finite states Encoding - assigning a digital word or number to each state Analog  Digital Conversion 2-Step Process:

  12. Step 1: QuantizingExample: a 3 bit A/D , N=23=8 (no. of steps) 0-10V signals. Separated into discrete states with 1.25V increments. Analog quantization size: Q=(Vmax-Vmin)/N = (10V – 0V)/8 = 1.25V

  13. Encodinggive value to each state

  14. two ways to improve accuracy: Increase resolution: improves accuracy in measuring analog signal amplitude Increase sampling rate: increases max frequency that can be measured. Eg high pitch audio Accuracy of A/D Conversion

  15. Flash ADC Delta-Sigma ADC Dual Slope (integrating) ADC Successive Approximation ADC A/D Converter Types

  16. ADC Types Comparison

  17. Analog to Digital Converter Simple, Cheap but slow : (SAR) Increment counter  D/A  compare ANALOG DIGITAL Figure 9.43 A simple feedback-type A/D converter.

  18. PIC microcontroller A/D10-bit resolutioncontrolled by program. registers 8 Analog channels If 0-5V range What is pic resolution What is 3.65V In digital domain ?

  19. series of comparators, each one compares input to a unique reference voltage. comparator outputs connect to a priority encoder circuit  produces binary output Flash ADC

  20. Flash Analog to Digital Converter Fast – but more expensive : Single cycle - Uses many Comparators in parallel with different reference voltages Digital Analog • 2N-1 comparators for N-bits • Each reference voltage equivalent to a quantization level • Encoding logic produces word Figure 9.45 Parallel, simultaneous, or flash A/D conversion.

  21. As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs. How Flash Works

  22. 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 Lower resolution Expensive For each additional output bit, the number of comparators is doubled i.e. for 8 bits, 256 comparators needed Flash

  23. A / D Converter – CMOS Implementation Charge-redistribution A/D FYI Figure 9.46 Charge-redistribution A/D converter suitable for CMOS implementation: (a) sample phase, (b) hold phase, and (c) charge-redistribution phase.

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