Analog To Digital Conversion

An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts a continuous quantity to a discrete digital number. Or A device that converts continuously varying analog signals from instruments and sensors that monitor conditions, such as sound, movement and temperature into binary code for the computer. The A/D converter may be contained on a single chip or can be one circuit within a chip.

Analog to Digital Converter (ADC) is an electronic integrated circuit used to convert the analog signals such as voltages to digital or binary form consisting of 1s and 0s. Most of the ADCs take a voltage input as 0 to 10V, -5V to +5V, etc., and correspondingly produces digital output as some sort of a binary number.

Analog: continuously valued signal, such as temperature or speed, with infinite possible values in between

Digital: discretely valued signal, such as integers, encoded in binary

Analog-to-digital converter: ADC, A/D, A2D; converts an analog signal to a digital signal

Analog signals

directly measurable quantities in terms of some other quantity

Examples

Thermometer – mercury height rises as temperature rises

Car Speedometer – Needle moves farther right as you accelerate

Digital Signals

have only two states. For digital computers, we refer to binary states, 0 and 1. “1” can be on, “0” can be off.

Examples

Light switch can be either on or off

Door to a room is either open or closed

What is Analog to Digital Converter?

A converter that is used to change the analog signal to digital is known as an analog to digital converter or ADC converter. This converter is one kind of integrated circuit or IC that converts the signal directly from continuous form to discrete form. This converter can be expressed in A/D, ADC, A to D. The inverse function of DAC is nothing but ADC. The analog to digital converter symbol is shown below.

The process of converting an analog signal to digital can be done in several ways. There are different types of ADC chips available in the market from different manufacturers like the ADC08xx series. So, a simple ADC can be designed with the help of discrete components.

The main features of ADC are sample rate and bit resolution.

The sample rate of an ADC is nothing but how fast an ADC can convert the signal from analog to digital.

Bit resolution is nothing but how much accuracy can an analog to digital converter can convert the signal from analog to digital.

One of the major benefits of ADC converter is the high data acquisition rate even at multiplexed inputs. With the invention of a wide variety of ADC integrated circuits (IC’s), data acquisition from various sensors becomes more accurate and faster. Dynamic characteristics of the high-performance ADCs are improved measurement repeatability, low power consumption, precise throughput, high linearity, excellent Signal-to-Noise Ratio (SNR), and so on.

A variety of applications of the ADCs are

Measurement and control systems,

Industrial instrumentation,

Communication systems,

And all other sensory-based systems. Classification of ADCs based on factors like performance, bit rates, power, cost, etc.

ADC Block Diagram

The block diagram of ADC is shown above which includes sample, hold, quantize, and encoder. The process of ADC can be done like the following.

First, the analog signal is applied to the first block namely a sample wherever it can be sampled at an exact sampling frequency. The amplitude value of the sample like an analog value can be maintained as well as held within the second block like Hold. The hold sample can be quantized into discrete value through the third block like quantize. Finally, the last block like encoder changes the discrete amplitude into a binary number.

In ADC, the conversion of the signal from analog to digital can be explained through the above block diagram.

Sample

In the sample block, the analog signal can be sampled at an exact interval of time. The samples are used in continuous amplitude and hold real value however they are discrete with respect to time. While converting the signal, the sampling frequency plays an essential role. So it can be maintained at a precise rate. Based on the system requirement, the sampling rate can be fixed.

Hold

In ADC, HOLD is the second block and it doesn’t have any function because it simply holds the sample amplitude till the next sample is taken. So the value of hold doesn’t change until the next sample.

Quantize

In ADC, this is the third block which is mainly used for quantization. The main function of this is to convert the amplitude from continuous (analog) into discrete. The value of continuous amplitude within hold block moves throughout quantize block to turn into discrete in amplitude. Now, the signal will be in digital form because it includes discrete amplitude as well as time.

Encoder

The final block in ADC is an encoder that converts the signal from digital form to binary. We know that a digital device works by using binary signals. So it is required to change the signal from digital to binary with the help of an encoder. So this is the entire method to change an analog signal to digital using an ADC. The time taken for the entire conversion can be done within a microsecond.

Analog to Digital Conversion Process

There are many methods to convert analog signals to digital signals. These converters find more applications as an intermediate device to convert the signals from analog to digital form, display output on LCD through a microcontroller. The objective of an A/D converter is to determine the output signal word corresponding to an analog signal. Now we are going to see an ADC of 0804. It is an 8-bit converter with a 5V power supply. It can take only one analog signal as input.

The digital output varies from 0-255. ADC needs a clock to operate. The time taken to convert the analog to digital value depends on the clock source. An external clock can be given to CLK IN pin no.4. A suitable RC circuit is connected between the clock IN and clock R pins to use the internal clock. Pin2 is the input pin – High to low pulse brings the data from the internal register to the output pins after conversion. Pin3 is a Write – Low to high pulse is given to the external clock. Pin11 to 18 are data pins from MSB to LSB.

Analog to Digital Converter samples the analog signal on each falling or rising edge of the sample clock. In each cycle, the ADC gets the analog signal, measures it, and converts it into a digital value. The ADC converts the output data into a series of digital values by approximates the signal with fixed precision.

In ADCs, two factors determine the accuracy of the digital value that captures the original analog signal. These are quantization level or bit rate and sampling rate. The below figure depicts how analog to digital conversion takes place. Bit rate decides the resolution of digitized output and you can observe in the below figure where 3-bit ADC is used for converting the analog signal.

Assume that one-volt signal has to be converted from digital by using 3-bit ADC as shown below. Therefore, a total of 2^3=8 divisions are available for producing 1V output. This results 1/8=0.125V is called as minimum change or quantization level represented for each division as 000 for 0V, 001 for 0.125, and likewise upto 111 for 1V. If we increase the bit rates like 6, 8, 12, 14, 16, etc. we will get a better precision of the signal. Thus, bit rate or quantization gives the smallest output change in the analog signal value that results from a change in the digital representation.

Suppose if the signal is about 0-5V and we have used 8-bit ADC then the binary output of 5V is 256. And for 3V it is 133

There is an absolute chance of misrepresenting the input signal on the output side if it is sampled at a different frequency than the desired one. Therefore, another important consideration of the ADC is the sampling rate. The Nyquist theorem states that the acquired signal reconstruction introduces distortion unless it is sampled at (minimum) twice the rate of the largest frequency content of the signal as you can observe in the diagram. But this rate is 5-10 times the maximum frequency of the signal in practice.

Factors

The ADC performance can be evaluated through its performance based on different factors. From that, the following two main factors are

SNR (Signal-to-Noise Ratio)
Bandwidth

SNR (Signal-to-Noise Ratio)

The SNR reflects the average number of bits without noise in any particular sample.

Bandwidth

The bandwidth of an ADC can be determined by estimating the sampling rate. The analog source can be sampled per second to produce discrete values.

Types of Analog to Digital Converters

Dual Slope A/D Converter
Flash A/D Converter
Successive Approximation A/D Converter
Semi-flash ADC
Sigma-Delta ADC
Pipelined ADC

Dual Slope A/D Converter

In this type of ADC converter, comparison voltage is generated by using an integrator circuit which is formed by a resistor, capacitor, and operational amplifier combination. By the set value of Vref, this integrator generates a sawtooth waveform on its output from zero to the value Vref. When the integrator waveform is started correspondingly counter starts counting from 0 to 2^n-1 where n is the number of bits of ADC.

When the input voltage Vin equal to the voltage of the waveform, then the control circuit captures the counter value which is the digital value of the corresponding analog input value. This Dual slope ADC is a relatively medium cost and slow speed device.

Flash A/D Converter

This ADC converter IC is also called parallel ADC, which is the most widely used efficient ADC in terms of its speed. This flash analog to digital converter circuit consists of a series of comparators where each one compares the input signal with a unique reference voltage. At each comparator, the output will be a high state when the analog input voltage exceeds the reference voltage. This output is further given to the priority encoder for generating binary code based on higher-order input activity by ignoring other active inputs. This flash type is a high-cost and high-speed device.

Successive Approximation A/D Converter

The SAR ADC a most modern ADC IC and much faster than dual slope and flash ADCs since it uses a digital logic that converges the analog input voltage to the closest value. This circuit consists of a comparator, output latches, successive approximation register (SAR), and D/A converter. At the start, SAR is reset and as the LOW to HIGH transition is introduced, the MSB of the SAR is set. Then this output is given to the D/A converter that produces an analog equivalent of the MSB, further it is compared with the analog input Vin. If comparator output is LOW, then MSB will be cleared by the SAR, otherwise, the MSB will be set to the next position. This process continues till all the bits are tried and after Q0, the SAR makes the parallel output lines to contain valid data.

Semi-flash ADC

These types of analog to digital converts mainly works approximately their limitation size through two separate flash converters, where each converter resolution is half of the bits for the semi-flush device. The capacity of a single flash converter is, it handles the MSBs (most significant bits) whereas the other handles the LSB (least significant bits).

Sigma-Delta ADC

Sigma Delta ADC (ΣΔ) is fairly a recent design. These are extremely slow as compared to other kinds of designs however they offer the maximum resolution for all kinds of ADC. Thus, they are extremely compatible with high-fidelity based audio applications, however, they are normally not utilizable wherever high BW (bandwidth) is required.

Pipelined ADC

Pipelined ADCs are also known as sub ranging quantizers which are related in concept to successive approximations, even though more sophisticated. While successive approximations grow through every step by going to the next MSB, this ADC uses the following process.

It is used for a coarse conversion. After that, it evaluates that change toward the input signal.
This converter acts as a better conversion by allowing for a temporary conversion with a range of bits.
Usually, pipelined designs offer a center ground among SARs as well as flash analog to digital converters by balancing its size, speed & high resolution.

Advantages of ADC

Flash ADCs are the fastest compared to the other Analog to Digital Converter.
Compared to other converters, Sigma Delta ADCs offer high resolution at low-cost.
Successive Approximation ADCs operate at high speed and are more reliable.
Sigma-Delta ADCs have higher noise shaping capability and also offer high resolution.
Pipelined ADCs also offer high resolution at high speed.

Disadvantages of ADC

Circuit Complexity increases with the increase in the use of Comparators in Flash ADCs.
Flash ADCs are expensive.
Converting non-periodic signals using Pipeline ADCs can be difficult as it typically runs at a periodic rate.
Pipeline ADCs are sensitive to board layout.
Pipeline latency of the input signal occurs in Pipeline ADCs which causes non-linearities in the parameters like Offset and Gain.
Successive Approximation converters used for higher resolution will be slower.

Applications of Analog to Digital Converter

At present, the usage of digital devices is increasing. These devices work based on the digital signal. An analog to digital converter plays a key role in such kind of devices to convert the signal from analog to digital. The applications of analog to digital converters are limitless which are discussed below.
AC (air conditioner) includes temperature sensors to maintain the temperature within the room. So this conversion of temperature can be done from analog to digital with the help of ADC.
It is also used in a digital oscilloscope to convert the signal from analog to digital to display.
ADC is used to convert the analog voice signal to digital in mobile phones because mobile phones use digital voice signals but actually, the voice signal is in the form of analog. So ADC is used to convert the signal before sending the signal toward the transmitter of the cell phone.
ADC is used in medical devices like MRI and X-Ray to convert the images from analog to digital before alteration.
The camera in the mobile mainly used for capturing images as well as videos. These are stored in the digital device, so these are converted to digital form using ADC.
The cassette music can also be changed into a digital like CDS & thumb drives use ADC.
At present ADC is used in every device because almost all devices available in the market are in digital version. So these devices use ADC.

What is Analog To Digital Conversion and it's types, advantages, disadvantages and applications