slides-06

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0 bit

1 bit

2 bit

3 bit

2

3 2 1 0

−

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2

0

2

1

2

1

2

1 ×

3

+

×

2

+

×

1

+

×

0

=

Example: Conversion for a decimal number 14

BINARY SYSTEM

MSB

(the most significant bit)

LSB

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ANALOG TO DIGITAL CONVERSION SIGNALS

Quantisation Levels for

(3-bit system)

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SAMPLING THEOREM

Different sampling rates for the

analogue signal can be used.

Nyquist Criterion

(Shannon’s Sampling Theorem) :

To get the right form of the original

(analogue) signal, sampling rate must

be at least twice that of the highest

frequency of the original signal.

Aliasing:

When the sampling rate is

less than twice the highest frequency,

the reconstruction represent some

other analogue signal and we obtain a

false image of the real signal.

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DIGITAL-TO-ANALOG CONVERTERS

Weighted Resistor Network:

The reference voltage is connected to a set of resistors

by means of electronic (solid state) switches which

respont to binary 1. The values of resistances depend

on which bit in theword a switch is responding to.

Hence the sum of the voltages a a weighted sum of the

digits in the word.

Resistor Network Types used in DAC’s

- Weighted Resistor

Limited to 4-bit conversions. Obtaining a wide variaty of different

resistors values in high resolution aplications is difficult.

-R-2R Ladder Resistor

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GEC PLESSEY ZN558D 8-bit

DIGITAL-TO-ANALOG CONVERTER

ENABLE= high Data is hold

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UNIPOLAR OPERATION

The output is required to be a voltage which varies between zero and

the referance voltage.

For output range + 5 V

, select R

₁

=8 kohm and R

₂

=8 kohm

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SPECIFICATIONS

Full Scale Output:

The output when the input word ids all 1’s. Typically it

is 2.55 V for the ZN558D.

Resolution:

8-bit DACS generally suitable for most microprocessor

control systems. The ZN558D is 8-bits.

Settling Time:

this is being the time taken by the DAC to reach within ½

LSB of its new voltage after a binary change. This is 800 ns for ZN558D.

Linearity:

This being the maximum deviation from the straignt line

through zero and the full range of th output. This is a maximum of 0.5

LSB for the ZN558D.

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ANALOG-TO-DIGITAL CONVERSION

Successive Approximation Method

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SINGLE-RAMP ANALOG-TO-DIGITAL

CONVERTER

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DUAL-RAMP ANALOG-TO-DIGITAL

CONVERTER

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TIME DIVISION MULTIPLEXING

not time

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