ACOE2551 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)

ACOE2552 Video (Analog - Digital) A/D Amplifier Filters Modulator Image enhancement and coding AnalogDigital Pre- amplifier

ACOE2553 Temperature Recording by a Digital System Sampling & quantization Coding Temperature (ºC) Time Temperature (ºC) Time

ACOE2554 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 + +

ACOE2555 Signals (Analog - Digital) u(V) t (S) D3D3 D2D2 D1D1 D0D Analog Signal can take infinity values can change at any time Digital Signal can take one of 2 values (0 or 1) can change only at distinct times ADC u(V) t (S) DAC Reconstruction of an analog signal from a digital one (Can take only predefined values)

ACOE2556 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)

ACOE2557 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

ACOE2558 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 2Ν2Ν D3D3 D2D2 D1D1 D0D0 V out (mV)

ACOE2559 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)

ACOE25510 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 (V FS ) –For example if the full-scale voltage of an 8-bit D/A converter is 2.55V the the resolution is: V FS /(2 N -1) = 2.55 /(2 8 -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. 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.

ACOE25511 ADC RESPONSE TYPES Linear –Most common, Non-linear –Used in telecommunications, since human voice carries more energy in the low frequencies than the high.

ACOE25512 ADC TYPES Direct Conversion –Fast –Low resolution Successive approximation –Low-cost –Slow –Not constant conversion delay Sigma-delta –High resolution, low-cost, high accuracy

ACOE25513 Interfacing with Data Converters Microprocessor compatible data converters are attached on the microprocessor’s bus as standard I/O devices.

ACOE25514 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()); }

ACOE25515 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