Test: Analog to Digital Converters - Electronics and Communication Engineering (ECE) MCQ

• Analog-to-Digital Converters (ADCs) transform an analog voltage to a binary number (a series of 1’s and 0’s).
• Then eventually a digital number (base 10) for reading on a meter, monitor, or chart.
• The ADC resolution depends upon the number of bits used to represent the digit number
• As the number of bits increases the resolution of an Analog to Digital Converter improves and the quantization error decreases.
• The resolution of DAC is a change in analog voltage corresponding to the LSB bit increment at the input. The resolution (R) is calculated as:
  

where
N is the number of bits
V_Fs is the full scale deflection 
Given
V_FS = 5 V
N = 6
R = 
= 78 mV

• SNR (Signal-to-Noise Ratio) of an ADC (Analog-to-Digital Converter) is a measure of the quality of the ADC's output signal.
• It represents the ratio of the amplitude of the input signal to the amplitude of the noise present in the output signal. In other words, it is a measure of how much the signal level exceeds the noise level in the output of the ADC.
• SNR is a calculated value that represents the ratio of RMS signal to RMS noise.
• If we multiply the log10 of this ratio by 20 to derive SNR in decibels.
• An ADC’s ideal SNR equals 6.02N + 1.76 dB, where N is the number of bits.

Calculation:
SNR of an 10 bit ADC = 6.02 × 10 + 1.76 = 61.96 dB
Hence the correct answer is option 3.

The ADC resolution is defined as the smallest incremental voltage that can be recognized and thus causes a change in the digital output.
The resolution is given by:

where, R = Resolution
V_ref = Maximum analog voltage
n = No. of bits
Calculation
Given, V_ref = 5 V
n = 12

R = 1.22 mV

• DA (Digital to analog) converter is a part of the proximity sensor.
• A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.
• A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation and looks for changes in the field or return signal. 
• The sensor can also be used to detect a wide variety of non-metallic and metallic objects and typically operate over a range of 3 to 30 mm.
• A capacitive proximity sensor is suitable for a plastic target an inductive proximity sensor always requires a metal target.

The advantage of using a dual-slope ADC in a digital voltmeter is that its accuracy is high. 
Dual slope ADC:

1. Dual slope integration type ADC is the most accurate type of ADC because of non-dependency on variation in component values caused by noise. 
2. Dual slope ADC is the slowest ADC among all other ADC's but the advantage of dual-slope ADC is its accuracy. 
3. These are used in the precision application. 

Advantage of Dual Slope ADC:

1. High resolution can be achieved by using an accurate count.
2. Component value variations will have no effect on accuracy.
3. For instance, the capacitance may change due to temperature variation. But since the charging and discharging processes are done through the same capacitor, the net effect of this capacitance variation is negligible.

Disadvantages:

• Dual slope ADC is the slowest ADC

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.

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.

Digital ramp ADC conversion time is given as:

Given:
n = 10
f = 500 kHz
Analysis:

= 2 μs
Conversion time = (2^N – 1) T_clk
= (1024 - 1) × 2 μs = 2046 μs

• Analog-to-Digital Converters (ADCs) transform an analog voltage to a binary number (a series of 1’s and 0’s).
• Then eventually to a digital number (base 10) for reading on a meter, monitor, or chart.
• The ADC resolution depends upon the number of bits used to represent the digit number. 
• As the number of bits increases the resolution of an Analog to Digital Converter improves and the quantization error decreases.

The resolution of DAC is a change in analog voltage corresponding to the LSB bit increment at the input.
The resolution (R) is calculated as:

No. of levels = 2^N – 1
V_r is reference voltage 'or' Full-scale voltage
Resolution for n – bit A/D converter in percentage will be:

Given: n = 10, VFS = 10 V

= 9.77 mV

The conversion time of the countertype A/D counter is (2n−1)Tclk

From above when bit increases, conversion doubles, So the answer is counter type A/D converter.

• Counter type ADC and successive approximate ADC uses DAC
• Counter type ADC uses linear search and successive approximation type ADC uses binary search
• Ring counter is used in successive approximation type ADC
• Flash type ADC is fastest ADC
• Flash type ADC requires no counter
• For an n-bit ADC, flash type ADC requires (2ⁿ – 1) comparators
• Dual slope ADC is most accurate