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ADC

ADC. ADC. Perform a function which is exactly opposite to that of a DAC. Input to ADC is the analog voltage V A and at the output we get an “n” bit digital word. In n bit digital output d 1 represents the MSB and d n is the LSB. Analog voltage= or.

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ADC

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  1. ADC

  2. ADC • Perform a function which is exactly opposite to that of a DAC. • Input to ADC is the analog voltage VA and at the output we get an “n” bit digital word. • In n bit digital output d1 represents the MSB and dn is the LSB. • Analog voltage= or

  3. Basic block diagram Reference voltage VR MSB ADC Analog voltage VA N-bit digital output . . . Conversion time LSB SOC EOC SOC EOC Control lines

  4. Characteristics of ADC • Resolution:-defined as the ratio of the change in the value of the input analog voltage required to change the digital output by 1 LSB. • Conversion time:-total time required to convert the analog input signal into a corresponding digital output

  5. Characteristics of ADC 3. Quantization error:- digital output is not always the accurate representation of the analog input. This approximation process is called as quantization and the error due to the quantization process is called as quantization error. Maximum value of quantization error is ½ LSB It should be as small as possible.

  6. Types • Flash type/simultaneous ADC/ parallel comparator • Successive approximation A to D converter • Single slope A to D converter • Dual slope A to D converter(integrator type)

  7. Parallel comparator/ Flash ADC Uses a number of Op-amp comparators The numbers of comparators =

  8. Parallel comparator/ Flash ADC Series of op-amp as comparators Encoder converts comparator outputs into n-bit binary number Analog Input (Va) Resistive network provides voltage reference levels

  9. Parallel comparator/ Flash ADC

  10. Advantages and disadvantages Advantages:- • High speed of conversion hence this is the fastest ADC Disadvantages:- • Large number of comparators are required to be used. • With the increased number of bits , the priority encoder becomes increasingly complex.

  11. Successive approximation A to D converter Va>Vd=1 Va<Vd=0

  12. Advantages and disadvantages Advantages:- • Conversion time is very short . • Conversion time is constant and independent of the amplitude of analog signal VA Disadvantages:- • Circuit is complex • Conversion time is more as compared to flash type ADC.

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