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DIGITAL ANALOG CONVERSTERS

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DAC

Changes a digital value back into an analog voltage

Digital-to-analog conversion can be achieved

using a network of resistors called ³ladder 

network´.

 ADCObtains a digital value changes a digital value

back into an analog voltage

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Ladder Network

One Popular scheme uses a network resistors.

 Accepts inputs of binary values at, typically, 0V

or Vref , and provider an output voltage proportionalto the binary input value.

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 A ladder network with four input voltages, representing 4

bits of digital data and dc voltage output.

R R R R 

Vi(1)

 D1

Vi(2)

 D2

Vi(3)

 D3

Vi(0)

 D0

Digital input

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The output voltage is proportional to the digital

input values given by the relation:

Vo = Do x 20 + D1 x 21 + D2 x 22 + D3 x 23 Vref  

24

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In the above example the output voltage resultingshould be:

V0 = 0 x 1 + 1 x 2 + 1 x 4 + 0 x 8 (16 V) = 6 V

16

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Current Switches

R2R Ladder 

Reference CircuitVref 

Digital Inputs

Io

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DUAL SLOPE CONVERSION

Popular method for converting an analog

voltage into digital value

The analog voltage to be converted is applied

through an electronic switch to an integrator or 

ramp-generator circuit.

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Ladder Voltage

Count Interval

Start Stop Count

 Analog Voltage

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LADDER NETWORK CONVERSION

method for converting an analog voltage intodigital with counter and comparator circuits

 A digital counter advances from zero count while a ladder network driven by the counter

outputs a staircase voltage.

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Timer IC Unit Operation

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Timer IC

The figures above is the 555 timer, an analog-digital IC. As described inthe figures, it is made of linear comparators and digital flip-flops.

The one on the left is the internal of the IC and the one on the right isits external view.

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Ground-Negative Supply of the deviceTr igger -This pin is input w/c causes the output to go high & begin

the timing cycle.

Output-555 moves to a high level of 1.7 volts

Reset- A low logic level on this pin resets the timer and returns the

output to a low state.Normally connected to the + supply line if not use

Control-Allows changing the triggering and threshold voltages by applying

an external voltage.

Threshold-used to reset the latch and cause the output to go low

Dischar ge-An open collector output w/c is in phase w/ the main output on

pin 3 and has current sinking capability.

(V +)- Positive supply of the 555 timer IC.

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Multivibrators

Two-state devices called multivibrators

are used extensively in digital electronics.

The bistable multivibrators are called flip-flops and are the basic memory devices used

in sequential logic. Other two-state devices

include the astable multivibrator which serves

as an oscillator and the monostablemultivibrator ("one-shot" multivibrator) which

can serve as a pulse source.

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Operational Modes  Astable Multivibrator (Clock Circuit)

Simply an oscillator 

Generates a continuous stream of rectangular off-

on pulses that switch between two voltage levels

 ± One-Shot Mode (Monostable

Multivibator)

 ±Has a single stable state, which is off state

 ±Generates a single pulse of a fixed time

duration each time it receives an input trigger 

pulse

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M

onostable

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M

onostable monostable timer is a one-stable timer; that is, it

returns to its original (stable) state (off) after a

certain period of time. Monostable operation is

characterized by the clothes iron, which switches

itself off after being unattended for 10 minutes.

Each time the iron is moved, a small sensor resets

the timer to zero to begin another count. The iron

remains on as long as it is moved within the 10-minute limit.

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Astable Operation

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 AstableOne that is constantly pulsing, never staying

in one state or the other. It is constantly

switching between its two states. The flashing

light or pulsing buzzer that warns that the

seat belts are not fastened when you start a

car is an example of astable operation. In this

mode, the output pulse is fed back into the

chip to trigger another pulse, and the the

result is a continuous pulsed signal.

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Reported By:

Chris Pactolerin

BS-Computer Science 4