DataConverter Circuits AD and DA Chapter 9 1

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

Analog Signals every where Examples of A/D • 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

Need to Sample an analog signal Most signals are analog 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). 3

A/D converter and D/A Converters Analog to Digital What parts of your i. Phone operation are Analog ? / Digital to Analog Your internet access: Analog ? Digital ? Figure 9. 37 The A/D and D/A converters as circuit blocks. 4

A/D converter Converts analog signals into binary words

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. 6

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

3 -bit conversion example Quantization levels Eg 5 V divided into 8 levels – each 0. 625 Each binary representation is a “range” 8

D/A conversion implementation Weighted Summing Circuit in = ? i =? vo = - [(Rf / R 1) * v 1 + (Rf / R 2) * v 2 +…. + (Rf / Rn) * vn] Figure 2. 10 A weighted summer. 9

N-bit D/A Converter Implementation Binary weighted resistive ladder digital values control switches S 1 - Sn Figure 9. 39 An N-bit D/A converter using a binary-weighted resistive ladder network. 10

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

Step 1: Quantizing Example: a 3 bit A/D , N=23=8 (no. of steps) 0 -10 V signals. Separated into discrete states with 1. 25 V increments. Analog quantization size: Q=(Vmax-Vmin)/N = (10 V – 0 V)/8 = 1. 25 V Output States Discrete Voltage Ranges (V) 0 0. 00 -1. 25 1 1. 25 -2. 50 2 2. 50 -3. 75 3 3. 75 -5. 00 4 5. 00 -6. 25 5 6. 25 -7. 50 6 7. 50 -8. 75 7 8. 75 -10. 0

Encoding give value to each state Output States Output Binary Equivalent 0 000 1 001 2 010 3 011 4 100 5 101 6 110 7 111

Accuracy of A/D Conversion 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

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

ADC Types Comparison Type Speed (relative) Cost (relative) Dual Slope Slow Med Flash Very Fast High Successive Appox Medium – Fast Low Sigma-Delta Slow Low

Analog to Digital Converter Simple, Cheap but slow : (SAR) Increment counter D/A compare A N A L O G D I G I T A L Figure 9. 43 A simple feedback-type A/D converter. 17

PIC microcontroller A/D 10 -bit resolution 8 Analog channels controlled by program. registers If 0 -5 V range What is pic resolution What is 3. 65 V In digital domain ? 18

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

Flash Analog to Digital Converter Fast – but more expensive : Single cycle - Uses many Comparators in parallel with different reference voltages Analog • 2 N-1 comparators for N-bits • Each reference voltage equivalent to a quantization level • Encoding logic produces word Digital Figure 9. 45 Parallel, simultaneous, or flash A/D conversion. 20

How Flash Works • 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.

Flash 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

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. 23