Explain in detail about a/d and d/a interfacing with 8086.
Answers
Answer:
The function of an A/D converter is to produce a digital word which represents the magnitude of some analog voltage or current.
The specifications for an A/D converter are very similar to those for D/A converter:
· The resolution of an A/D converter refers to the number of bits in the output binary word. An 8-bit converter for example has a resolution of 1 part in 256.
· Accuracy and linearity specifications have the same meaning for an A/D converter as they do for a D/A converter.
· Another important specification for an ADC is its conversion time. - the time it takes the converter to produce a valid output binary code for an applied input voltage. When we refer to a converter as high speed, it has a short conversion time.
The analog to digital converter is treated as an input device by the microprocessor that sends an initialising signal to the ADC to start the analog to digital data conversation process.
The start of conversion signal is a pulse of a specific duration. The process of analog to digital conversion is a slow process, and the microprocessor has to wait for the digital data till the conversion is over.
After the conversion is over, the ADC sends end of conversion (EOC) signal to inform the microprocessor that the conversion is over and the result is ready at the output buffer of the ADC.
These tasks of issuing an SOC pulse to ADC, reading EOC signal from the ADC and reading the digital output of the ADC are carried out by the CPU using 8255 I/O ports. The time taken by the ADC from the active edge of SOC pulse (the edge at which the conversion process actually starts) till the active edge of
EOC signal is called as the conversion delay of the ADC- the time taken by the converter to calculate the equivalent digital data output from the instant of the start of conversion is called conversion delay. It may range anywhere from a few microseconds in case of fast ADCs to even a few hundred milliseconds in case of slow ADCs.
A number of ADCs are available in the market, the selection of ADC for a particular application is done, keeping in mind the required speed, resolution range of operation, power supply requirements, sample and hold device requirements and the cost factors are considered.
The available ADCs in the market use different conversion techniques for the conversion of analog signals to digital signals.
Parallel converter or flash converter, Successive approximation and
dual slope integration
A general algorithm for ADC interfacing contains the following steps.
1. Ensure the stability of analog input, applied to the ADC.
2. Issue start of conversion (SOC) pulse to ADC.
3. Read end of conversion (EOC) signal to mark the end of conversion process.
4. Read digital data output of the ADC as equivalent digital output.
It may be noted that analog input voltage must be constant at the input of the ADC right from the start of conversion till the end of conversion to get correct results. This may be ensured by a sample and hold
circuit which samples the analog signal and holds it constant for a specified time duration. The microprocessor may issue a hold signal to the sample and Hold circuit. If the
applied input changes before the complete conversion process is over, the digital equivalent of the analog input calculated by the ADC may not be correct. If the applied input changes before the complete conversion process is over, the digital equivalent of the analog input calculated by the ADC may not be correct.