# Analog to Digital & Digital to Analog Converters

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### Text of Analog to Digital & Digital to Analog Converters

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 |.|

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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