Today in this paper we will be understanding how an successive approximation ADC works. We are learning this basic working principle because sooner we will be dealing on how to interface an analog to digital converter with AT89s52 microcontroller. And because most of analog to digital converter uses successive approximation analog to digital conversion method for conversion of analog signal to digital signal it is important to understand successive approximation method of digital conversion before interfacing an analog to digital converter with any microcontrollers.
A successive approximation ADC is a type of ADC that coverts a real time analog signal into a discrete time digital signal using binary search algorithm.Successive approximation ADC is most generally and dominantly used method of analog to digital conversion in many of available IC's because of its high performance and low power consumption.
Block Diagram:-
As shown in figure alongside,it consists of a sample/hold circuitry, a comparator, a DAC, and an successive approximation register.A sample/hold circuitry is used to perform sampling of the analog input and hold the sampled value while the binary search is being performed. A DAC is used to convert the digital value received from the SAR into the analog value by utilizing the reference voltage. The SAR is used to supply an approximate digital signal of input analog voltage to the DAC.
How does successive approximation ADC works?
In the beginning of this paper, we were made aware about the fact that successive approximation ADC uses binary search algorithm to perform the conversion from analog signal to digital signal. Now, we are a step behind from knowing the working principle of successive approximation ADC. The step by step process of this conversion method using binary search is described below with an suitable example.
1) First the MSB of N-bit successive approximation register is set to 1.
2) Then, compute the output of internal DAC (Vdac) utilizing the reference voltage.
5) Perform step 2 to 4 for every bit of successive approximation register.
Thus for N-bit successive approximation ADC, N-comparison is performed by the internal circuitry of ADC before the digital equivalent of the analog input signal is obtained.
Example:
Convert 1.80 V into 4-bit digital equivalent signal using successive approximation method. Use reference voltage 2.5V.
Solution:
First, set the MSB of successive approximation register to 1 i,e 1000. For this digital value the output of internal DAC is 2.5/(2^4)*8=1.25 which is equal to Vref/2. Since Vin>Vdac, the MSB of successive approximation register remains set.Thus the value of successive approximation register after this comparison is 1000.
Now, next bit (MSB-1) of successive approximation register is set to 1 i,e 1100. For this digital value the output of internal DAC is 2.5/(2^4)*12=1.875V. Since Vin < Vdac, this bit is cleared to 0. Thus the value of successive approximation register is 1000.
Now, next bit (MSB-2) of successive approximation register is set to 1 i,e 1010. For this digital value the output of internal DAC is 2.5/(2^4)*10=1.5625V. Since Vin > Vdac, this bit is set to 0. Thus the value of successive approximation register is 1010.
Now, the LSB of successive approximation register is set to 1 i,e 1011. For this digital value the output of internal DAC is 2.5/(2^4)*11=1.71875V. Since Vin > Vdac, this bit is set to 1. Thus the value of successive approximation register is 1011.
Thus, the digital equivalent signal is 1011. Also, from above example we have seen that in a 4-bit successive approximation ADC, four comparison are made before we obtain the digital equivalent.
A successive approximation ADC is a type of ADC that coverts a real time analog signal into a discrete time digital signal using binary search algorithm.Successive approximation ADC is most generally and dominantly used method of analog to digital conversion in many of available IC's because of its high performance and low power consumption.
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| Fig:-Block diagram of Successive approximation ADC |
As shown in figure alongside,it consists of a sample/hold circuitry, a comparator, a DAC, and an successive approximation register.A sample/hold circuitry is used to perform sampling of the analog input and hold the sampled value while the binary search is being performed. A DAC is used to convert the digital value received from the SAR into the analog value by utilizing the reference voltage. The SAR is used to supply an approximate digital signal of input analog voltage to the DAC.
How does successive approximation ADC works?
In the beginning of this paper, we were made aware about the fact that successive approximation ADC uses binary search algorithm to perform the conversion from analog signal to digital signal. Now, we are a step behind from knowing the working principle of successive approximation ADC. The step by step process of this conversion method using binary search is described below with an suitable example.
1) First the MSB of N-bit successive approximation register is set to 1.
2) Then, compute the output of internal DAC (Vdac) utilizing the reference voltage.
3) If Vin > Vdac then the output of comparator is 1.As a result, the bit of SAR remains at 1 and if Vin<Vdac the output of comparator is 0 which in turn clears the bit of SAR to 0.
4) Now set the next bit (MSB-i) to 1 and go to step 2.5) Perform step 2 to 4 for every bit of successive approximation register.
Thus for N-bit successive approximation ADC, N-comparison is performed by the internal circuitry of ADC before the digital equivalent of the analog input signal is obtained.
Example:
Convert 1.80 V into 4-bit digital equivalent signal using successive approximation method. Use reference voltage 2.5V.
Solution:
First, set the MSB of successive approximation register to 1 i,e 1000. For this digital value the output of internal DAC is 2.5/(2^4)*8=1.25 which is equal to Vref/2. Since Vin>Vdac, the MSB of successive approximation register remains set.Thus the value of successive approximation register after this comparison is 1000.
Now, next bit (MSB-1) of successive approximation register is set to 1 i,e 1100. For this digital value the output of internal DAC is 2.5/(2^4)*12=1.875V. Since Vin < Vdac, this bit is cleared to 0. Thus the value of successive approximation register is 1000.
Now, next bit (MSB-2) of successive approximation register is set to 1 i,e 1010. For this digital value the output of internal DAC is 2.5/(2^4)*10=1.5625V. Since Vin > Vdac, this bit is set to 0. Thus the value of successive approximation register is 1010.
Now, the LSB of successive approximation register is set to 1 i,e 1011. For this digital value the output of internal DAC is 2.5/(2^4)*11=1.71875V. Since Vin > Vdac, this bit is set to 1. Thus the value of successive approximation register is 1011.
Thus, the digital equivalent signal is 1011. Also, from above example we have seen that in a 4-bit successive approximation ADC, four comparison are made before we obtain the digital equivalent.
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