Successive approximation adc converter

For a 4-bit ADC, the resolution will be Though ADC is implemented using various techniques these days, this article will focus on the Successive Approximation method. It is the most frequently used ADC technique for general applications. The ADC comprises a comparator, digital to analog converter, register, and a control circuit.

The schematic is shown below:. At the point when the new conversion begins, the sample and hold circuit samples the input voltage and then this sampled signal is compared with the output signal of the digital to analog converter. To grasp the concept, consider a 4-bit ADC with a sampling rate i. We take the comparator reference voltage as16 Volts.

Whenever the new transformation begins, the successive approximation register sets the most significant bit to 1 and all others to zero. So the output voltage of the DAC corresponding to the stated digital code and reference voltage of 16 turns out to be. This is the threshold voltage to which the input voltage will be compared.

Thus, the output voltage of the comparator will change the output value of the successive register. If the output voltage of DAC is lesser than the input voltage then the most significant bit remains intact and the next bit will be changed to 1 for new comparison. On the other hand, if the output DAC voltage is greater than the input voltage then the MSB is transformed to zero but the next bit is pulled high for the new comparison.

Now, the code has become So, the output of DAC corresponding to this binary code is 12 Volts measured using the same output voltage formula. This is the new DAC voltage set to be compared with. Once more, the input voltage is compared with the DAC voltage. If again, the latter is lesser, then the second bit remains the same while the third bit is made 1 for the new comparison.

But if vice versa happens then the second bit is changed to zero and the third bit to 1 for the next comparison. It means that the current input code for the DAC is We will deduce the output voltage which is updated to 10 Volts. Repeat the same process again. If Vin is less than 10 Volts then the third bit is kept as it is and the least significant bit is made high.

For vice versa, the third bit is turned to 0 and LSB alters to 1 to compare with the input voltage. The input is for sure greater than the Vdac so, the next input code is The corresponding output voltage becomes 11 Volts. It is again compared and the final code is Below is the tree hierarchy that depicts all the possibilities that occur during the iterations.

Hence, based on the comparison, only one code is selected in the end. It is the time needed by analog to digital converter to completely convert continuous signals to digital signals. The conversion time is based on the number of bits because the N number of bits takes N number of clock cycles. Each bit iteration takes one cycle. So, the general conversion time formula is. We can see that the conversion time is independent of the input voltage which is not the case in the majority of the other ADCs.

The speed with which the conversion of the signal takes place is called conversion speed. Talking about the resolution, it is the number of bits utilized by the analog to digital converter to discrete the analog inputs. The typical resolution of the successive approximation analog to digital converter is in a wide range starting from 8-bits to bits. The digital equivalent voltage is compared with the unknown analog input voltage.

Comparison is made as given in step 1 to decide whether to retain or reset the second MSB. The above steps are more accurately illustrated with the help of an example. The functional block diagram of successive approximation type of ADC is shown below. The equivalent analog output voltage of DAC, VD is applied to the non-inverting input of the comparator.

The second input to the comparator is the unknown analog input voltage VA. The output of the comparator is used to activate the successive approximation logic of SAR. When the start command is applied, the SAR sets the MSB to logic 1 and other bits are made logic 0, so that the trial code becomes Advantages: 1 Conversion time is very small. Disadvantages: 1 Circuit is complex. Ac dc power converters single phase full wave controlled rectifier single phase half wave controlled rectifier three phase full wave controlled rectifier three phase half controlled rectifier.

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