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

3 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 (  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). Need to Sample an analog signal Then convert to digital by A/D converter Most signals are analog Are sensor outputs Analog ? Eg. Seatbelt ? EEG, oil temp

4 Figure 9.37 The A/D and D/A converters as circuit blocks. 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 ?

A/D converter Converts analog signals into binary words

6 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. D/A Conversion Normal Output from digital domain is staircase Filtered to produce smooth Analog output

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

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

9 Figure 2.10 A weighted summer. D/A conversion implementation Weighted Summing Circuit vo = - [(R f / R 1 ) * v 1 + (R f / R 2 ) * v 2 +….+ (R f / R n ) * v n ] i n = ? i = ?

10 Figure 9.39 An N-bit D/A converter using a binary-weighted resistive ladder network. N-bit D/A Converter Implementation Binary weighted resistive ladder digital values control switches S 1 - S n

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=2 3 =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 Output States Discrete Voltage Ranges (V)

Encoding give value to each state Output States Output Binary Equivalent

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

TypeSpeed (relative)Cost (relative) Dual SlopeSlowMed FlashVery FastHigh Successive AppoxMedium – FastLow Sigma-DeltaSlowLow ADC Types Comparison

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

PIC microcontroller A/D 10-bit resolution controlled by program. registers 18 If 0-5V range What is pic resolution What is 3.65V In digital domain ? 8 Analog channels

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

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

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

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