The analog to digital (A/D) conversion is the reverse process of digital to analog (D/A) conversion. The A/D conversion is also called Quantization, in which the analog signal is represented binary data. The analog signals varies continuously and defined for any interval of time. The digital signals (or data) can take only finite values for discrete instant of time. If the digital data is represented by n-bit binary then it can have 2n different values. The given analog signal has to divided into steps of 2n values, and each step is represented by one of the 2n values.
The Analog to Digital Converters can be classified into two groups based on the technique involved for conversion.
The first group includes successive-approximation, counter and flash type converters. The technique involved in these devices is that the given analog signal is compared with internally generated analog signal.
The second group includes integrator converters and voltage to frequency converters. In the devices of second group, the given analog signal is converted to time or frequency and the new parameters (time or frequency) is compared with known values to produce digital signal.
The trade-off between the two techniques is based on Accuracy Vs Speed.
The successive approximation and flash type converters are accurate than the integrator and the voltage-to-frequency converters. Also, flash type is costlier. The successive-approximation type converters are used for high-speed conversion and the integrating type converters are used for high accuracy.
The resolution of the converter is the minimum analog value that can be represented by the digital data. If the ADC gives n-bit digital output and the full-scale analog input is X volts, then the resolution is 1/2n x X volts.
In ADC, another critical parameter is conversion time. The conversion time is defined as the total time required to convert an analog signal into its digital equivalent. It depends on the conversion technique and the propagation delay in various circuits.
Successive-Approximation ADC
A successive approximation ADC consist of D/A converter, successive approximation register and comparator. The figure below shows the functional blocks of a typical successive approximation A/D converter.
The conversion process is initiated by a start of conversion (SOC) signal from the processor to ADC. On receiving the SOC, the control unit of ADC will give a start command to successive approximation register and it starts generating digital signal by successive approximation method. The generated digital data is converted to analog signal by Dl A converter and then compared with given analog signal. When the analog signals are equal the comparator output informs the control unit to stop generation of digital signal. The digital data available at this instant is given as output through output register. Also the control unit generates a signal to indicate the End of Conversion (EOC) process to the processor.
In this method the MSD (Most Significant Digit) is first set to "1" and all other digits are reset to "0". The analog signal generated for this digital data is compared with given analog signal. (Initially the comparator output will be HIGH. After comparison the output of comparator remains in HIGH state, if the given analog signal is higher than generated analog signal. Otherwise, if the given signal is less than generated signal, then the output of comparator changes from HIGH to LOW state). If the output state of comparator changes then the MSD is reset to "0" otherwise it is retained as '1'. Then the above process is repeated by setting the next higher order bit to '1'. The process is continued for each bit starting from MSD to LSD. (During a process, the higher order bits are the bits determined in earlier steps and the lower order bits are reset to "0"). After one complete cycle through MSD to LSD, the data available on the successive approximation register will be the digital equivalent of the given analog signal.
ADC interfacing to 8085 microprocessor system
The ADC can be interfaced to 8085 microprocessor system through tri-state buffer or port devices such as 8255/8155. The interfacing of ADC0801 is presented in this section. The ADC0801 is a single channel, 8-bit successive approximation type A/D converter from National Semiconductor Corporation. It is a 20-pin IC available in DIP. The pin configuration of ADC080 1 is shown in figure below.
The ADC080 I has two analog inputs V IN(+) and V IN (-) .Both the analog inputs are used for differential mode of operation. When the analog signal is single ended positive, then V IN (+) is used as input and V IN(-) is grounded. When the analog signal is single ended negative, then V IN (-) is used as input and V IN (+) is grounded. The ADC requires an external clock in the
frequency range l00 kHz to 800 kHz or the clock can be generated by connecting a RC circuit between pin 4 and 19. Typically, the clock frequency is chosen as 640 kHz to provide a conversion time of l00 sec.
A typical circuit to interface ADC0801 to 8085 processor is shown in figure below.
The ADC is connected to system bus through tri-state buffer, 74LS245. The ADC can be either memory mapped or I/O mapped in the system. The chip select signal ( CS ) is obtained from the address decoder of the 8085 system. The conversion is initiated when both CS and WR are asserted LOW.
The write control signal (WR) is used to reset the successive approximation register (SAR) of ADC and to give start of conversion (SOC). The WR of ADC can be connected directly to WR of the 8085 processor. At the falling edge of WR, the SAR is resetted and at the rising edge of WR, the conversion starts.
The end of conversion is indicated by asserting INTR LOW and this signal can be inverted to interrupt the 8085 processor. The processor reads the digital data using RD and when the data is read, the DAC will set INTR HIGH.
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