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)
Dr. Konstantinos Tatas and Dr. Costas Kyriacou

2. Video (Analog - Digital)
Modulator
Amplifier
Filters
Analog
Pre-
amplifier
A/D
Digital
Image
enhancement
and coding
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3. Temperature Recording by a Digital System
Time
Temperature
(ºC)
Time
Sampling &
quantization
Coding
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4. 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 + +
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5. Signals (Analog - Digital)2 4 6 8 10 12 14 16
u(V) 1 7 3 5 9
t (S)
Analog Signal
can take infinity values
can change at any time
1010
1110
1111
1100
1000
Digital Signal
can take one of 2 values (0 or 1)
can change only at distinct times
Reconstruction of an analog signal from a digital one (Can take only predefined values)
1001
0110
0101
0100 2 4 6 8 10 12 14 16
u(V) 1 7 3 5 9
t (S)
ADC
1001
0110
0101
1010
1111
1110
1000
1100
0100 D3 D2 D1 D0 1 1 1 1 1
DAC
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6. Microprocessors I - Frederick University
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)
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7. 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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8. 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 and the output voltage is 5mV, then
If the binary code at the input becomes 1001, the output voltage will become D3 D2 D1 D0
Vout
(mV) 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
45mV
If a D/A converter has 4 digital inputs then the analog signal at the output can have one out of …… values. 16
If a D/A converter has N digital inputs then the analog signal at the output can have one out of ……. values. 2Ν
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9. Characteristics of Data Converters
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
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.
Polarity
Polar: the analog signals can have only positive values
Bipolar: the analog signals can have either a positive or a negative value
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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10. Characteristics of Data Converters (Cont…)
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
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.
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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11. Microprocessors I - Frederick University
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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12. Microprocessors I - Frederick University
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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13. Interfacing with Data Converters
Microprocessor compatible data converters are attached on the microprocessor’s bus as standard I/O devices.
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14. Microprocessors I - Frederick University
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.
main() {
do {
for (i=0;i<200;i++)
Out32(0x6a0,i);
delayu(5); }
} while (!_kbhit());
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15. D/A Converters example
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.
Assuming that an 8-bit A/D converter is used to interface a temperature sensor measuring temperature values in the temperature range
, 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
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