Successive Approx ADC MCQ Quiz - Objective Question with Answer for Successive Approx ADC - Download Free PDF

Explanation:

Successive Approximation ADC (Analog-to-Digital Converter):

Definition: A successive approximation ADC is a type of analog-to-digital converter that uses a binary search algorithm to convert an analog signal into its corresponding digital representation. It achieves this by comparing the input signal to a series of reference voltages generated by a DAC (Digital-to-Analog Converter) in conjunction with a comparator and a control circuit.

Working Principle:

The successive approximation ADC works by iteratively refining the digital output to approximate the analog input signal. The process involves:

• Using an internal DAC to generate reference voltages based on the current digital approximation.
• Comparing the input analog signal with the reference voltage using a comparator.
• Adjusting the digital output bit-by-bit to minimize the difference between the reference voltage and the input signal, eventually converging on the closest digital representation.

Correct Option Analysis:

The correct option is:

Option 2: N clock pulses for conversion, a binary counter, and a comparator.

This option correctly describes the operation of a successive approximation ADC. Here’s why:

• N Clock Pulses: The successive approximation ADC requires precisely N clock pulses for conversion, where N is the number of bits in the digital output. During each clock pulse, one bit of the digital output is determined.
• Binary Counter: A binary counter is used to control the successive approximation process. It iteratively refines the digital output by setting or clearing individual bits, starting with the most significant bit (MSB) and moving to the least significant bit (LSB).
• Comparator: The comparator compares the input analog signal with the reference voltage generated by the DAC. Based on this comparison, the binary counter adjusts the digital output to improve accuracy.

Hence, option 2 correctly outlines the essential components and process required for the operation of a successive approximation ADC.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 2N clock pulses for conversion, an up-down counter, and a DAC.

This option is incorrect because:

• The successive approximation ADC does not require 2N clock pulses for conversion. Instead, it requires only N clock pulses, as each bit is determined sequentially in N steps.
• An up-down counter is not used in a successive approximation ADC. Instead, a binary counter is employed to refine the digital output systematically.

Option 3: 2N clock pulses for conversion and a binary counter only.

This option is partially correct but ultimately flawed:

• While the binary counter is an essential component of the successive approximation ADC, 2N clock pulses are not required. The conversion process is completed in N clock pulses.
• The absence of a comparator in this option makes it invalid, as the comparator is a critical component for comparing the input signal with the reference voltage.

Option 4: N clock pulses for conversion, an up-down counter, and a DAC.

This option is incorrect because:

• While the N clock pulses are correct, the use of an up-down counter is not appropriate for a successive approximation ADC. It relies on a binary counter for bit-wise refinement of the output.
• The DAC is correctly mentioned, but the inclusion of an up-down counter makes this option invalid.

Conclusion:

The successive approximation ADC operates using N clock pulses for conversion, a binary counter to iteratively refine the digital output, and a comparator to compare the analog input signal with the reference voltage generated by the DAC. This combination ensures accurate and efficient conversion of the analog signal to a digital representation.

Understanding the essential components and processes of a successive approximation ADC is crucial for identifying its operational characteristics. The correct option (Option 2) accurately captures the requirements and functionality of this type of ADC, making it the right choice among the given options.

Explanation:

Types of A/D Converters: There are several types of A/D converters, each with its own characteristics, advantages, and disadvantages. The most common types include:

• Single-slope integrating A/D converters
• Dual-slope integrating A/D converters
• Successive approximation A/D converters
• Flash A/D converters

Dual-Slope Integrating A/D Converter:

Working Principle: The dual-slope integrating A/D converter operates in two phases: the integration phase and the de-integration phase. During the integration phase, the input analog signal is integrated over a fixed period, resulting in a ramp signal. In the de-integration phase, a known reference voltage of opposite polarity is applied to the integrator, and the time required for the ramp signal to return to zero is measured. This time is proportional to the input voltage.

The correct option is: Option 3: A dual-slope integrating-type A/D converter has a higher accuracy than a single-slope integrating-type A/D converter.

Successive Approximation type DVM:

• The successive approximation type digital voltmeter works on the principle of balancing the weights in a simple balance.
• To understand the concept clearly, let us consider whether we want to
• measure the weight of some unknown quantity of sugar.
• What do we do? First, we approximate the weight of sugar to some known weight,
• If the weight of sugar is more than the known weight, then we add some more weight to the known weight.
• If it is less, then we replace the weight with a lesser value.
• This process is repeated until the pointer balances the two weights.
• The successive approximation type DVM uses the same principle.
• A simple block diagram of SADVM is given below

An elaborated block diagram of SADVM is given below

• Consists of an input attenuator for selecting the desired range of input voltage and also to attenuate any noise in the given voltage.
• Selected input is applied to the comparator through a sample and holds circuit.
• The successive approximation register (SAR) receives its 8-bit input from the ring counter after each clock pulse.
• This input is applied to the Digital to Analog Converter (DAC) which converts the digital data into an analog voltage.
• This voltage is applied as a second input to the comparator.
• The output of the AND gate goes high when there is a positive o/p at the comparator.
• Finally, the digital output is taken out from the successive approximation register with input voltages other than dc; the input level changes during digitization, and decisions made during conversion are not consistent.
• To avoid this error, a sample and hold circuit is used and placed in the input directly following the input attenuator.
• This digital voltmeter is capable of 1000 readings per second.

Concept:

The Quantization nose power is given by:

\(\frac{A^2}{12} = X\)

Analysis:

\(\rm A = \frac{V}{2^N - 1} \) = step size in N - bit

\(\rm X = \left( \frac{V}{2^N - 1 } \right) . \frac{1}{12}\)

\(\rm X = \frac{V^2}{12(2^N - 1)^2 } \)

Concept:

A successive-approximation ADC is a type of analog-to-digital converter that converts a continuous analog waveform into a discrete digital representation using a binary search through all possible quantization levels before finally converging upon a digital output for each analog voltage conversion.

For an N-bit successive approximation ADC, the conversion time is

= N T

Where T is the time period of the clock pulse.

∴ The conversion time does not depend on the magnitude of the input voltage.

Analysis:

As the conversion time does not change with the change in input.

The conversion time for V_A = 4 V is: 50 μs 

Concept:

A successive-approximation ADC is a type of analog-to-digital converter that converts a continuous analog waveform into a discrete digital representation using a binary search through all possible quantization levels before finally converging upon a digital output for each analog voltage conversion

If V_in  ≥ V_DAC ⇒ maintain a loaded bit

V_in ≤  V_DAC⇒ clear the loaded bit

Calculation:

Given,

Reference voltage V_R = 5 V, number of bits n is 8

Resolution \(= \frac{5}{{{2^8} - 1}} \approx 20\ mv\)

Clock pulse 1:

1 st clock = 10000000

Output voltage V₀ = 20 mV × 2⁷ = 2.5 V

As 3.5 V > 2.5 V it maintains the loaded bit

Clock pulse 2:

2 nd clock pulse = 11000000

Output voltage V₀ = 20 mV × (2⁷ + 2⁶) = 3.75 V

As 3.5V < 3.75 V it clears the loaded bit

Clock pulse 3:

So at the end of the 3^rd clock pulse output is = (10100000)₂

The conversion time of successive approximation type ADC is n T_clk.

Where n is the number of bits and T_clk is the clock period.

1 T_clk is used for the start of the conversion signal and 1 T_clk is used for the end of the conversion signal.

Here n = 12

So total conversion time

\(\begin{array}{*{20}{c}} { = 12 \times 1\;\mu sec + \;}\\ {} \end{array}\begin{array}{*{20}{c}} {1\mu \sec + \;}\\ {\left( {EOC} \right)} \end{array}\begin{array}{*{20}{c}} {1\;\mu sec}\\ {\left( {SOC} \right)} \end{array}\) 

= 14 μsec

Successive Approximation type DVM:

• The successive approximation type digital voltmeter works on the principle of balancing the weights in a simple balance.
• To understand the concept clearly, let us consider whether we want to
• measure the weight of some unknown quantity of sugar.
• What do we do? First, we approximate the weight of sugar to some known weight,
• If the weight of sugar is more than the known weight, then we add some more weight to the known weight.
• If it is less, then we replace the weight with a lesser value.
• This process is repeated until the pointer balances the two weights.
• The successive approximation type DVM uses the same principle.
• A simple block diagram of SADVM is given below

An elaborated block diagram of SADVM is given below

• Consists of an input attenuator for selecting the desired range of input voltage and also to attenuate any noise in the given voltage.
• Selected input is applied to the comparator through a sample and holds circuit.
• The successive approximation register (SAR) receives its 8-bit input from the ring counter after each clock pulse.
• This input is applied to the Digital to Analog Converter (DAC) which converts the digital data into an analog voltage.
• This voltage is applied as a second input to the comparator.
• The output of the AND gate goes high when there is a positive o/p at the comparator.
• Finally, the digital output is taken out from the successive approximation register with input voltages other than dc; the input level changes during digitization, and decisions made during conversion are not consistent.
• To avoid this error, a sample and hold circuit is used and placed in the input directly following the input attenuator.
• This digital voltmeter is capable of 1000 readings per second.

Concept:

The Quantization nose power is given by:

\(\frac{A^2}{12} = X\)

Analysis:

\(\rm A = \frac{V}{2^N - 1} \) = step size in N - bit

\(\rm X = \left( \frac{V}{2^N - 1 } \right) . \frac{1}{12}\)

\(\rm X = \frac{V^2}{12(2^N - 1)^2 } \)

Explanation:

Successive Approximation ADC (Analog-to-Digital Converter):

Definition: A successive approximation ADC is a type of analog-to-digital converter that uses a binary search algorithm to convert an analog signal into its corresponding digital representation. It achieves this by comparing the input signal to a series of reference voltages generated by a DAC (Digital-to-Analog Converter) in conjunction with a comparator and a control circuit.

Working Principle:

The successive approximation ADC works by iteratively refining the digital output to approximate the analog input signal. The process involves:

• Using an internal DAC to generate reference voltages based on the current digital approximation.
• Comparing the input analog signal with the reference voltage using a comparator.
• Adjusting the digital output bit-by-bit to minimize the difference between the reference voltage and the input signal, eventually converging on the closest digital representation.

Correct Option Analysis:

The correct option is:

Option 2: N clock pulses for conversion, a binary counter, and a comparator.

This option correctly describes the operation of a successive approximation ADC. Here’s why:

• N Clock Pulses: The successive approximation ADC requires precisely N clock pulses for conversion, where N is the number of bits in the digital output. During each clock pulse, one bit of the digital output is determined.
• Binary Counter: A binary counter is used to control the successive approximation process. It iteratively refines the digital output by setting or clearing individual bits, starting with the most significant bit (MSB) and moving to the least significant bit (LSB).
• Comparator: The comparator compares the input analog signal with the reference voltage generated by the DAC. Based on this comparison, the binary counter adjusts the digital output to improve accuracy.

Hence, option 2 correctly outlines the essential components and process required for the operation of a successive approximation ADC.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 2N clock pulses for conversion, an up-down counter, and a DAC.

This option is incorrect because:

• The successive approximation ADC does not require 2N clock pulses for conversion. Instead, it requires only N clock pulses, as each bit is determined sequentially in N steps.
• An up-down counter is not used in a successive approximation ADC. Instead, a binary counter is employed to refine the digital output systematically.

Option 3: 2N clock pulses for conversion and a binary counter only.

This option is partially correct but ultimately flawed:

• While the binary counter is an essential component of the successive approximation ADC, 2N clock pulses are not required. The conversion process is completed in N clock pulses.
• The absence of a comparator in this option makes it invalid, as the comparator is a critical component for comparing the input signal with the reference voltage.

Option 4: N clock pulses for conversion, an up-down counter, and a DAC.

This option is incorrect because:

• While the N clock pulses are correct, the use of an up-down counter is not appropriate for a successive approximation ADC. It relies on a binary counter for bit-wise refinement of the output.
• The DAC is correctly mentioned, but the inclusion of an up-down counter makes this option invalid.

Conclusion:

The successive approximation ADC operates using N clock pulses for conversion, a binary counter to iteratively refine the digital output, and a comparator to compare the analog input signal with the reference voltage generated by the DAC. This combination ensures accurate and efficient conversion of the analog signal to a digital representation.

Understanding the essential components and processes of a successive approximation ADC is crucial for identifying its operational characteristics. The correct option (Option 2) accurately captures the requirements and functionality of this type of ADC, making it the right choice among the given options.

Concept:

A successive-approximation ADC is a type of analog-to-digital converter that converts a continuous analog waveform into a discrete digital representation using a binary search through all possible quantization levels before finally converging upon a digital output for each analog voltage conversion

If V_in  ≥ V_DAC ⇒ maintain a loaded bit

V_in ≤  V_DAC⇒ clear the loaded bit

Calculation:

Given,

Reference voltage V_R = 5 V, number of bits n is 8

Resolution \(= \frac{5}{{{2^8} - 1}} \approx 20\ mv\)

Clock pulse 1:

1 st clock = 10000000

Output voltage V₀ = 20 mV × 2⁷ = 2.5 V

As 3.5 V > 2.5 V it maintains the loaded bit

Clock pulse 2:

2 nd clock pulse = 11000000

Output voltage V₀ = 20 mV × (2⁷ + 2⁶) = 3.75 V

As 3.5V < 3.75 V it clears the loaded bit

Clock pulse 3:

So at the end of the 3^rd clock pulse output is = (10100000)₂

Concept:

Successive approximation ADC is the advanced version of Digital ramp type ADC which is designed to reduce the conversion and to increase the speed of operation.

The SAR ADC will use widely data acquisition techniques at sampling rates higher than 10 kHz.

Advantages:

• Speed is high compared to counter type ADC.
• Good ratio of speed to power.
• Compact design compared to Flash Type and it is inexpensive.

Disadvantage

• Cost is high because of SAR
• Complexity in design.

Conversion time: n × T_CLK

Where 

n = no of bit

T_CLK = Clock time

f = Frequency

Explanation:

Given 

f = 2 MHz

T_clk = 0.5 × 10^-6 sec

​Conversion time: n × Tclk 

= 8 × 0.5 × 10^-6

= 4 μs

Successive approximate Analog to Digital converter is as given below

It contains a Comparator, a DAC and a successive approximation register.

Successive approximation has shorter conversion time of the order of micro seconds and depends upon the number of bits only. For n bit ADC, it requires n clock cycles.

It requires both digital to analog converter (DAC) and comparator. The same is explained in the given block diagram.

Initially all the bits are zero

Clock pulse 1: Let b₅ = 1,

\({V_1} = \frac{1}{2}\left( {{V_{ref}}} \right) = 0.5\;V\) 

V_in = 0.67 V, V₁ < V_in

⇒ b₅ = 1

Clock pulse 2:

Let b₅ = 1, b₄ = 1

\({V_2} = \left( {\frac{1}{2} + \frac{1}{{{2^2}}}} \right)\;{V_{ref}} = 0.75\;V\) 

V_in = 0.67 V, V₂ > V_in

⇒ b₄ = 0

Clock pulse 3:

Let b₅ = 1, b₃ = 1

\({V_3} = \left( {\frac{1}{2} + \frac{1}{{{2^3}}}} \right)\;{V_{ref}} = 0.625\;V\) 

V_in = 0.67 V

V₃ < V_in

⇒ b₃ = 1