Test: A/D Convertors - Electrical Engineering (EE) MCQ

Flash type (or) Parallel type (or) Simultaneous ADC:
The following figure shows a 3-bit flash ADC circuit.

• It is formed of a series of comparators, each one comparing the input signal to a unique reference voltage.
• The comparator outputs connect to the inputs of a priority encoder circuit, which then produces a binary output.
• Vref is a stable reference voltage provided by a precision voltage regulator as part of the converter circuit.
• As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state.
• The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs.
• Flash type ADC is fastest ADC
• Flash type ADC requires no counter
• For an n-bit ADC, flash type ADC requires (2ⁿ – 1) comparators
• Conversion time: T_clk

Calculation:
Given that, n = 3
Number of required comparators, N = 2³ – 1 = 7

Concept:

The common Analog to digital converters are:

Ramp-type:

• The principle of Ramp-type DVM is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage (or) to decrease from the level of the input voltage to zero.
• This type of Analogue to Digital Converter is very slow (but cheap and simple).
• It is ideal for data that changes fairly slowly such as vehicle or aircraft control systems.
• Audio signals are slow enough to be converted.

Dual-slope converter:

• In the dual-slope technique, an integrator is used to integrate an accurate voltage reference for a fixed period of time. The same integrator is then used to integrate with the reverse slope, the input voltage, and the time required to return to the starting voltage is measured.
• The automatic zero correction function is performed before each conversion so that changes in the offset voltages & current will be compensated.

Successive Approximation:

• The basic principle is binary regression, in which analog input is compared with DAC reference voltage which is repeatedly divided in half.
• A successive approximation A/D converter consists of a comparator, a successive approximation register (SAR), output latches, and a D/A converter.
• It is capable of high speed and is reliable.

Important Points

R-2R ladder is used for Digital to Analog Converter:
It uses a summing amplifier with an R-2R ladder network as shown below.

For n-bit DAC, it requires only 2 different values of resistors i.e. R and 2R.

Concept:

• 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.

For n-bit conversion, the conversion time for different ADC are:
Counter type ADC: (2ⁿ – 1) T_clk
Successive approx. time ADC: n T_clk
Flash type ADC: T_clk
(Flash Type ADC is also known as Parallel comparator type)
Dual slope ADC: (2ⁿ⁺¹ – 1) T_clk
The fastest type of Analog to Digital converter is the Flash type / Parallel comparator type.

Important points:

• 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 the fastest ADC
• Flash type ADC requires no counter
• For an n-bit ADC, flash type ADC requires (2ⁿ – 1) comparators
• Dual slope ADC is the most accurate.

Concept:

Digital ramp ADC conversion time is given as:

Given:
n = 10
f = 500 kHz

Analysis:

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

Important Points

The conversion time of different types of n-bit ADC is shown :

Concept:
Flash Type ADC:
1) It is the fastest ADC among all the ADC types.
2) An n-bit flash type ADC requires: 2ⁿ -1 comparators, 2n resistors, and one 2ⁿ × n priority encoder.
Analysis: Number of bits(n) = 4
Number of comparators required = 2⁴ -1 = 15

Integrating type ADC is the slowest.

The conversion time of different ADC is shown below.

From the above table, the fastest ADC is Flash Type ADC whose conversion time is independent of the number of bits.

The resolution of the dual-slope ADC is determined by the length of the run-down period and by the time measurement resolution (i.e., the frequency of the controller's clock).
The resolution (in the number of bits) is the minimum length of the run-down period for a full-scale input(V_in = - V_ref).

Where t_d is the run-down period,
r is resolution
f_clk is the frequency of the clock.

Important Point:

There are limits to the maximum resolution of the dual-slope integrating ADC. It is not possible to increase the resolution of the basic dual-slope ADC to arbitrarily high values by using longer measurement times or faster clocks. 

Concept:

Digital ramp ADC conversion time is given as:

Important Points
The conversion time of different types of n-bit ADC is shown:

Digital multimeter necessarily employs an analog-to-digital converter (ADC).

• Voltmeter: While some digital voltmeters use ADCs, there are also analog voltmeters that use a moving needle mechanism to display voltage.
• Wattmeter: Similarly, some digital wattmeters use ADCs for measuring current and voltage, but analog wattmeters exist that rely on electromagnets to measure power.
• Energy meter: Similar to wattmeters, both digital and analog energy meters exist, with digital ones utilizing ADCs and analog ones employing rotating disks or induction coils.
• Digital multimeter: This multi-function instrument combines the functionalities of measuring voltage, current, resistance, and sometimes other electrical properties. All digital multimeters require an ADC to convert the analog electrical signals from these measurements into digital readings displayed on the screen.

Therefore, while ADCs can be found in some digital versions of the other instruments, it's a necessary component only in digital multimeters.

Additionally, digital multimeters typically employ multiple ADCs, each tailored for specific measurement ranges and types (e.g., high-resolution ADC for voltage, high-speed ADC for current). This allows for accurate and versatile measurements across various electrical parameters.