Interfacing Analog to DigitalData Converters

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In most of the cases, the PPI 8255 is used forinterfacing the analog to digital converterswith microprocessor.The analog to digital converters is treaded as aninput device by the microprocessor, that sendsan initialising signal to the ADC to start theanalogy to digital data conversation process. Thestart of conversation signal is a pulse of aspecific duration.

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• The process of analog to digital conversion is aslow process, and the microprocessor has to waitfor the digital data till the conversion is over.

• After the conversion is over, the ADC sends end ofconversion EOC signal to inform themicroprocessor that the conversion is over andthe result is ready at the output buffer of theADC.

• These tasks of issuing an SOC pulse to ADC,reading EOC signal from the ADC and reading thedigital output of the ADC are carried out by theCPU using 8255 I/O ports.

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• The time taken by the ADC from the active edgeof SOC pulse till the active edge of EOC signal iscalled as the conversion delay of the ADC.

• It may range anywhere from a few microsecondsin case of fast ADC to even a few hundredmilliseconds in case of slow ADCs.

• The available ADC in the market use differentconversion techniques for conversion of analogsignal to digitals.

• Successive approximation techniques and dualslope integration techniques are the mostpopular techniques used in the integrated ADCchip.

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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 ADC3.Read end of conversion signal to mark the end ofconversion processes.4. Read digital data output of the ADC as equivalentdigital output.5. Analog input voltage must be constant at the input ofthe ADC right from the start of conversion till the end ofthe conversion to get correct results. This may beensured by a sample and hold circuit which samples theanalog signal and holds it constant for a specific timeduration. The microprocessor may issue a hold signal tothe sample and hold circuit.

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6. If the applied input changes before thecomplete conversion process is over, the digitalequivalent of the analog input calculated by theADC may not be correct.

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Signals / PINS

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Analog input IN0 to IN7Address Lines A B CDigital output bits D0 to D7SC - Start conversion; input pin; a low to highpulse is givenEOC -End of conversion; output pin; goes lowwhen the conversion is overALE - Input pin; low to high pulse is required tolatch in the addressClock input - Clock input; to provide externalclock

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Interfacing of ADC with 8255

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Digital to Analog Converter

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Digital to Analog Converter• The digital-to-analog converter (DAC) is a device widely used

to convert digital pulses to analog signals. two methods ofcreating a DAC: binary weighted and R/2R ladder.

• The first criterion for judging a DAC is its resolution, which is afunction of the number of binary inputs. The common onesare 8, 10, and 12 bits. The number of data bit inputs decidesthe resolution of the DAC since the number of analog outputlevels is equal to 2″, where n is the number of data bit inputs.

• Therefore, an 8-input DAC such as the DAC0808 provides 256discrete voltage (or current) levels of output. Similarly, the 12-bit DAC provides 4096 discrete voltage levels

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MC1408 DAC (or DAC0808)

In the MC1408 (DAC0808), the digital inputs areconverted to current (Iout), and by connecting aresistor to the Ioutpin, we convert the result to

voltage.

The total current provided by the Iout pin is afunction of the binary numbers at the DO – D7inputs of the DAC0808 and the reference current(Iref), and is as follows:

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where DO is the LSB, D7 is the MSB for theinputs, and Iref is the input current that mustbe applied to pin 14. The Iref current isgenerally set to 2.0 mA.

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