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

Video (Analog - Digital) Modulator Amplifier Filters Analog Pre- amplifier A/D Digital Image enhancement and coding ACOE255 Microprocessors I - Frederick University

Temperature Recording by a Digital System Time Temperature (ºC) Time Sampling & quantization Coding ACOE255 Microprocessors I - Frederick University

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 + ACOE255 Microprocessors I - Frederick University

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 ACOE255 Microprocessors I - Frederick University

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) ACOE255 Microprocessors I - Frederick University

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 ACOE255 Microprocessors I - Frederick University

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 ...... 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 ACOE255 Microprocessors I - Frederick University

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) ACOE255 Microprocessors I - Frederick University

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. ACOE255 Microprocessors I - Frederick University

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. ACOE255 Microprocessors I - Frederick University

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 ACOE255 Microprocessors I - Frederick University

Interfacing with Data Converters Microprocessor compatible data converters are attached on the microprocessor’s bus as standard I/O devices. ACOE255 Microprocessors I - Frederick University

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()); ACOE255 Microprocessors I - Frederick University

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 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 ACOE255 Microprocessors I - Frederick University