ACOE255 1

Microprocessors

Data Converters

• Analog to Digital Converters (ADC)– Convert an analog quantity (voltage, current) into

a digital code

• Digital to Analog Converters (DAC)– Convert a digital code into an analog quantity

(voltage, current)

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Video (Analog - Digital)

A/D

AmplifierFilters

Modulator

Image enhancement

and coding

Analog

Digital

Pre-amplifier

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Temperature Recording by a Digital System

Sampling & quantization

Coding

Temperature(ºC)

Time

Temperature(ºC)

Time

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Need for Data Converters

Digital processing and storage of physical quantities (sound, temperature, pressure

etc) exploits the advantages of digital electronics

– Better and cheaper technology compared to the analog

– More reliable in terms of storage, transfer and processing

• Not affected by noise

– Processing using programs (software)

• Easy to change or upgrade the system

– (e.g. Media Player 7 Media Player 8 ή Real Player)

• Integration of different functions

– (π.χ. Mobile = phone + watch + camera + games + email +

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Signals (Analog - Digital)

2

4

6

8

10

12

14

16

u(V)

1 2 73 4 5 6 8 9 t (S)

D3

D2

D1

D0

0

0

0

1

0

0

1

1

0

0

1

1 0

0

1

1 0

1

1

1

0

0

1

1 1

1

1

1

0

0

1

1

0

1

0

0

Analog Signal • can take infinity values • can change at any time

0100

1001

0110

0101

1010

11101111

1100

1000

Digital Signal • can take one of 2

values (0 or 1)• can change only

at distinct times

ADC

2

4

6

8

10

12

14

16

u(V)

1 2 73 4 5 6 8 9 t (S)

DAC

Reconstruction of an analog signal from a digital one

(Can take only predefined values)

1001

01100101

1010

11111110

1000

1100

0100

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QUANTIZATION ERROR• The difference between the true and quantized value of the analog signal• Inevitable occurrence due to the finite resolution of the ADC • The magnitude of the quantization error at each sampling instant is

between zero and half of one LSB.• Quantization error is modeled as noise (quantization noise)

2

4

6

8

10

12

14

16

u(V)

1 2 73 4 5 6 8 9 t (S)

2

4

6

8

10

12

14

16

u(V)

1 2 73 4 5 6 8 9 t (S)

Analog signal value at sampling time: 4.9 V

Quantized Analog signal value: 5.0 V

Quantization error: 5.0 - 4.9 = 0.1 V

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SAMPLING FREQUENCY (RATE)• The frequency at which digital values are sampled from the analog input of an

ADC• A low sampling rate (undersampling) may be insufficient to represent the analog

signal in digital form• A high sampling rate (oversampling) requires high bitrate and therefore storage

space and processing time• A signal can be reproduced from digital samples if the sampling rate is higher

than twice the highest frequency component of the signal (Nyquist-Shannon theorem)

• Examples of sampling rates – Telephone: 4 KHz (only adequate for speech, ess sounds like eff)

– Audio CD: 44.1 KHz

– Recording studio: 88.2 KHz

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Digital to Analog Converters

• The analog signal at the output of a D/A converter is linearly proportional to the binary code at the input of the converter.

– If the binary code at the input is 0001 and the output voltage is 5mV, then

– If the binary code at the input becomes 1001, the output voltage will become ......

• If a D/A converter has N digital inputs then the analog signal at the output can have one out of ……. values.

• If a D/A converter has 4 digital inputs then the analog signal at the output can have one out of …… values.

45mV

16

2Ν

D3 D2 D1 D0Vout(mV)

0 0 0 0 0

0 0 0 1 5

0 0 1 0 10

0 0 1 1 15

0 1 0 0 20

0 1 0 1 25

0 1 1 0 30

0 1 1 1 35

1 0 0 0 40

1 0 0 1 45

1 0 1 0 50

1 0 1 1 55

1 1 0 0 60

1 1 0 1 65

1 1 1 0 70

1 1 1 1 75

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Characteristics of Data Converters

1. Number of digital lines– The number bits at the input of a D/A (or output of an A/D) converter.

– Typical values: 8-bit, 10-bit, 12-bit and 16-bit

– Can be parallel or serial

2. Microprocessor Compatibility– Microprocessor compatible converters can be connected directly on the

microprocessor bus as standard I/O devices

– They must have signals like CS, RD, and WR

• Activating the WR signal on an A/D converter starts the conversion process.

3. Polarity– Polar: the analog signals can have only positive values

– Bipolar: the analog signals can have either a positive or a negative value

4. Full-scale output– The maximum analog signal (voltage or current)

– Corresponds to a binary code with all bits set to 1 (for polar converters)

– Set externally by adjusting a variable resistor that sets the Reference Voltage (or current)

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Characteristics of Data Converters (Cont…)

5. Resolution– The analog voltage (or current) that corresponds to a change of 1LSB in the binary

code

– It is affected by the number of bits of the converter and the Full Scale voltage (VFS)

– For example if the full-scale voltage of an 8-bit D/A converter is 2.55V the the resolution is:

VFS/(2N-1) = 2.55 /(28-1) 2.55/255 = 0.01 V/LSB = 10mV/LSB

6. Conversion Time– The time from the moment that a “Start of Conversion” signal is applied to an A/D

converter until the corresponding digital value appears on the data lines of the converter.

– For some types of A/D converters this time is predefined, while for others this time can vary according to the value of the analog signal.

0.1Vo

Vo

7. Settling Time – The time needed by the analog signal at the

output of a D/A converter to be within 10% of the nominal value.

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ADC RESPONSE TYPES• Linear

– Most common,

• Non-linear– Used in telecommunications, since human voice carries more energy in the low

frequencies than the high.

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ADC TYPES• Direct Conversion

– Fast

– Low resolution

• Successive approximation– Low-cost

– Slow

– Not constant conversion delay

• Sigma-delta– High resolution, low-cost, high accuracy

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Interfacing with Data Converters • Microprocessor compatible data converters are attached on the microprocessor’s

bus as standard I/O devices.

DAC

CS

Vout

D7

WR

D6

D5

D4

D3

D2

D1

D0 Vref

V(+)

V(-)

8088

Sys

tem

D7

D6

D5

D4

D3

D2

D1

D0

A19

A0

WR

IO/M'

RD

A11

A10

A9

A8

A7

A6

A5

A4

Vout

10K

10K

+5V

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Programming Example 1

Write a program to generate a positive ramp at the output of an 8-bit D/A converter with a 2V amplitude and a 1KHz frequency. Assume that the full scale voltage of the D/A converter is 2.55V. The D/A converter occupies the O/P address 0x6a0.

2V

0V

f = 1KHZ

main(){

do {

for (i=0;i<200;i++)

{

Out32(0x6a0,i);

delayu(5);

}} while (!_kbhit());}

200 stepsof 10 mV each==> 2V amplitude

200 steps of 5 us each==> 1ms period or 1KHz frequency

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D/A Converters example

0

4

3

2

1

V (volts)

t (msec )1 5 6 7 8

Write a program to generate the waveform, shown below, at the output of an 8-bit digital to analog converter. The period of the waveform should be approximately 8 ms. Assume that a time delay function with a 1 μs resolution is available. The full scale output of the converter is 5.12 V and the address of the DAC is 63H.

Co

Co

Co

Assuming that an 8-bit A/D converter is used to interface a temperature sensor measuring temperature values in the temperature range 0 - 51.2

, specify: The resolution in of the system in The digital output word for a temperature of 32.5 The temperature corresponding to a digital output word of 01001110