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Analog to Digital & Digital to Analog Converters

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1 By K. SAI KRISHNA Assistant Professor Introduction: Most of information carrying signals such as voltage, current, temperature, pressure and time are available in the analog form. For processing, transmission and storage purposes, it is often more convenient to express such signals in the digital form. When expressed in the digital form, they provide better accuracy and reduce noise.

2 3 Basic DAC Techniques: 4 we shall discuss the following resistive techniques only: 1.Binary-Weighted resistor DAC 2.R-2R ladder 3.Inverted R-2R ladder 5 1.Binary-Weighted resistor DAC 6 Basic Ideas: Use a summing op-amp circuit Use transistors to switch between high and ground Assumptions: Ideal Op-Amp No Current into Op-Amp Virtual Ground at InvertingInput Vout = -IRf

Binary-Weighted resistor DAC 7 For ON-Switch ( i/p is 1), I= For OFF Switch ( i/p is 0), I=0 RVRBinary-Weighted resistor DAC 8 Advantages Simple Fast Disadvantages o Needs large range of resistor values (2000:1 for 12-bit) with high precision in low resistor values. o Needs very small switch resistances. Summary Use in fast, low-precision converter 2. R/2R Ladder Digital-to-Analog Converters Only two resistor values 9 Each bit corresponds to a switch: If the bit is high, the corresponding switch is connected to the inverting input of the op-amp. If the bit is low, the corresponding switch is connected to ground.R/2R Ladder Digital-to-Analog Converters Current division and analog output versus digital input 10 Output for R-2R network, Here, 11 |.|

\|+ + + =RVDRVDRVDRVD R V f16 8 4 2ref0ref1ref2ref3 outWhere b3 corresponds to bit 3,b2 to bit 2, etc. If bit n is set, bn=1,If bit n is clear, bn=0 |.|

\|+ + + =1618141210 1 2 3 ref outb b b b V VFor general n-Bit R-2R Ladder or Binary WeightedResister DACinii nb V V211ref out = =For a 4-Bit R-2R Ladder R R RS F2 = =R/2R Ladder Digital-to-Analog Converters Current division and analog output versus digital input 12 3. Inverted R-2R Ladder Digital-to-Analog Converters 13 R/2R Ladder Digital-to-Analog Converters 14 Advantages Only two resistor values Does not need the kind of precision as Binary weighted DACs Easy to manufacture Faster response time Disadvantages More confusing analysis Monolithic/Hybrid Integrated-Circuit Digital-to-Analog Converters IC 1408 DAC block diagram and pin configuration 15 Monolithic / Hybrid Integrated-Circuit Digital-to-Analog Converters IC 1408 DAC Application 16 Important Electrical Characteristics for IC 1408: 1. Reference Current: 2mA 2. Supply Voltage: +5v (Vcc) & -15v (-VEE) 3. Setting Time: 300ns 4. Full Scale Output Current: 1.992mA 5. Accuracy: 0.19% 17 Hybrid Integrated-Circuit Digital-to-Analog Converters 18 19 Analog to Digital Converters: 20 ADC Basic Principle: The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. Where d1 is the most significant bit and dn is the least significant bit. An ADC usually has two additional control lines: the START input to tell the ADC when to start the conversion and the EOC (end of conversion) output to announce when the conversion is complete. Depending upon the type of application, ADCs are designed for microprocessor interfacing or to directly drive LCD or LED displays. 21 Different Types Of A/D Converter: 22 Direct TypeIntegrating Type Counter type converter Trackingor servo converter Successive approximation type converter Parallel comparator (Flash) type converter Charge balancing ADC Dual slope ADC A/D Converter Direct types ADCs compare a given analog signal with the internally generated equivalent, signal. Integrating type ADCs perform conversion in an indirect manner by first changing the analog input signal to a linear function of time or frequency and then to a digital code. The most commonly used ADCS are successive approximation and the integrator type. The successive approximation ADCs are used in applications such as instrumentation where conversion speed is important. The flash (comparator) type is expensive for high degree of accuracy.The integrating type converter is used in applications such as digital meter and monitoring systems. 23 1. Parallel Comparator (Flash) Analog-to-Digital Converters Also called simultaneous, multiple comparator, or flash converting Several comparators with different reference voltages drive a priority encoder. This is the simplest possible A/D converter. It is at the same time, the fastest and most expensive technique. 24 I. DIRECT TYPE ADCs: Parallel Comparator Analog-to-Digital Converters Three-bit parallel encoded ADC. priority encoder. Analog range of0-7 V. 3 bit (8 level) resolution (3-bit priority encoder (8 to 3)). 25 Parallel Comparator A/D Converters 26 Parallel Comparator A/D Converters 27 The number of comparators required for n bit resolution is , no.of comparators=The maximum frequency for a sine wave Vin to be digitised within accuracy if is, Where Maximum input frequency. = Conversion Time. n= no.of bits. 28 1 2 nLSB ( )221maxnfTc[~f~ maxTcParallel Comparator A/D Converters Parallel Comparator A/D Converters 29 Advantages Very fast Disadvantages Needs many parts (255 comparators for 8-bit ADC) Lower resolution Expensive Large power consumption 2.Counter Type-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 30 31 The Counter Type A/D converter The Counter Type A/D converter Operation of the Counter: Initially, counter is reset then o/p is set to zero. By applying reset pulse the digital i/p to DAC. DAC is also zero then Vd is also zero. When analog i/p voltage Va is applied to ADC then Va>Vd. Then the comparator goes high value. When the comparator o/p is high then it allows the clock pulse through AND gate. Then the counter starts the counting clock pulse. The steps are continued till Vd is less than Va. Then comparator o/p goes low then Vd> Va. For a new value of analog input Va, a second reset pulse is applied to clear the counter. 32 Counter Type Analog-to-Digital Converters 21 Disadvantage is slow conversion time. Conversion time is not constant. 34 The Counter Type A/D converter 35 3.Tracking or servo A/D converter Tracking or servo A/D converter 36 Advantages Simple Disadvantages Time needed to stabilize as a new conversion value. 4. 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 method. 37 Successive-Approximation Analog-to-Digital Conversion 38 Successive-Approximation Analog-to-Digital Conversion 39 Successive-Approximation Analog-to-Digital Conversion Simplified SAR A/D converter 40 comparator Table 1 Voltage-level contributions by each successive approximation register bit. 41 Figure :Timing waveforms for a successive approximation A/D conversion. 42 Successive-Approximation Analog-to- Digital Conversion 43 Advantages Capable of high speed Medium accuracy compared to other ADC types Good tradeoff between speed and cost Disadvantages Higher resolution successive approximation ADCs will be slower Speed limited ~5Msps II. Integrating Type of ADCs 1. Charge Balancing ADC: The principle of charge balancing ADC is to first convert the input signal to a frequency using a voltage to frequency (V/F) converter. This frequency is then measured by a counter and converted to an output code proportional to the analog input. The main advantage of these converters is that it is possible to transmit frequency even in noisy environment or in isolated form. The limitation of the circuit is that the output of V/F converter depends upon an RC product whose value cannot be easily maintained with temperature and time. The drawback of the charge balancing ADC is eliminated by the dual slope conversion. 44 2. Dual Slope ADC (Ramp Generator): 45 46 The voltage v0 will be equal to v1 at the instant t2 and can be written as Dual Slope ADC Dual Slope ADC 47 Advantages Input signal is averaged Greater noise immunity than other ADC types High accuracy Disadvantages Slow High precision external components required to achieve accuracy Dual Slope ADC 48 Dual Slope ADC 49 DAC/ADC SPECIFICATIONS Both D/A and A/D converters are available with wide range of specifications: 50 Resolution Output Voltage Range Accuracy Setting Time or Conversion Time Linearity Stability Quantization Error For D/A Converters: 1.Resolution: (a) A DAC that can provide number of differentanalog output values is called resolution. For a DAC having n-bits, Resolution=2n (or) (b) A DAC is which the ratio of change in output voltage resulting from a change of LSB (i.e. 1 least significant bit) at the digital inputs is known as resolution. Resolution for n-bit DAC is given by: 51 1 2Resol uti onFS=nV52 Example: Calculate the resolution ofan 8-bit DAC.

Solution:Resolution = 8 bits Percentage resolution =% 391 . 0 % 1002551% 1001 218= = 1 2ResolutionFS=nVDigital to Analog Converters -Performance Specifications -Resolution 53 Better Resolution(3 bit) Poor Resolution(1 bit) Vout Desired Analog signal Approximate output 2 Volt. Levels Digital Input 0 0 1 Digital Input Vout Desired Analog signal Approximate output 8 Volt. Levels 000 001 010 011 100 101 110 111 110 101 100 011 010 001 000 54 2. Output Voltage Range:

Thisisthedifferencebetweenthemaximumand minimum output voltages expressed in volts.

Example:

Calculate the output voltage range of a 4-bit DAC if the outputvoltageis+4.5Vforaninputof0000and+7.5V for an input of 1111.

Solution:

Output vo

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