Interfacing Analog to Digital Data Converters
• In most of the cases, the PPI 8255 is used for interfacing the analog to digital converters with microprocessor.
• The analog to digital converters is treaded as an input device by the microprocessor, that sends an initialising signal to the ADC to start the analogy to digital data conversation process. The start of conversation 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 till the active edge of EOC signal is called as the conversion delay of the ADC.
• It may range anywhere from a few microseconds in case of fast ADC to even a few hundred milliseconds in case of slow ADCs.
• The available ADC in the market use different conversion techniques for conversion of analog signal to digitals. Successive approximation techniques and dual slope integration techniques are the most popular techniques used in the integrated ADC chip.
• 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 pulse to ADC
3. Read end of conversion signal to mark the end of conversion processes.
4. Read digital data output of the ADC as equivalent digital output.
5. Analog input voltage must be constant at the input of the ADC right from the start of conversion till the end of the 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 specific time duration. The microprocessor may issue a hold signal to the sample and hold circuit.
6. 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.
• The analog to digital converter chips 0808 and 0809 are 8-bit CMOS, successive
approximation converters. This technique is one of the fast techniques for analog to digital conversion. The conversion delay is 100μs at a clock frequency of 640 KHz, which is quite
low as compared to other converters. These converters do not need any external zero or full scale adjustments as they are already taken care of by internal circuits.
· These converters internally have a 3:8 analog multiplexer so that at a time eight different analog conversion by using address lines - ADD A, ADD B, ADD C. Using these address inputs, multichannel data acquisition system can be designed using a single ADC. The CPU may drive these lines using output port lines in case of multichannel applications. In case of single input applications, these may be hardwired to select the proper input.
• There are unipolar analog to digital converters, i.e. they are able to convert only positive analog input voltage to their digital equivalent. These chips do no contain any internal sample and hold circuit. If one needs a sample and hold circuit for the conversion of fast signal into equivalent digital quantities, it has to be externally connected at each of the analog inputs.
Interfacing Digital to Analog Converters: The digital to analog converters convert binary number into their equivalent voltages. The DAC find applications in areas like digitally controlled gains, motors speed controls, programmable gain amplifiers etc.
AD 7523 8-bit Multiplying DAC: This is a 16 pin DIP, multiplying digital to analog converter, containing R- 2R ladder for D-A conversion along with single pole double thrown NMOS switches to connect the digital inputs to the ladder.