Analog to Digital Converter | What is ADC, its Types, specification and Application

Analog to Digital Converter performs a function which is exactly performed to that of a Digital to Analog Converter.

The input to an A to D converter is the analog voltage V_A and at the output, we get an “n” bit digital word.

Block Diagram of ADC (Analog to Digital Converter)

In the N-bit digital output d₁ represents the most significant bit (MSB) and d_n is the least significant bit (LSB). The analog input voltage V_A produces an output digital word, having functional value “D” is given by,

D= d_1 2^{-1}\,+\,d_2 2^{-2}\,+......+\,d_n 2^{-n}\,

In addition to the analog input voltage V_A, the ADC block has a reference voltage V_R input and two control lines SOC and EOC.

The start of conversion (SOC) input is used to start the A to D converter whereas the end of conversion (EOC) output goes high to indicate that the conversion is complete.

The relation between analog input voltage V_i and the digital output D is given by,

D= d_1 2^{-1}\,+\,d_2 2^{-2}\,+......+\,d_n 2^{-n}\,=\frac{V_i}{K V_R}=\frac{V_i}{V_{FS}}

where,

• K=Constant,
• V_R = reference voltage,
• V_FS = Full scale input voltage,
• V_i= Analog input voltage

The time difference between the SCO signal and EOC signal is called as “Conversion Time“. The Conversion time should be as small as possible. Practically t can have values from a few hundreds of µs to a few msec.

Types of ADC (Analog to Digital Converter)

There are various circuits are available for the A to D converter conversion. They are different principles of operation for conversion. Some of them are:

• Flash typed or simultaneous ADC
• Tracking or Servo type
• Single slope A to D converter
• Counter type ADC
• Successive approximation A to D converter
• Dual slope A to D converter (Integrator type)

Out of them the most commonly used ADCs are successive approximation type and integrator type ADC.

All the circuits mentioned above have theory advantages and dendrites and they are proffered specific application. 

You can learn each type of Analog to digital converter in detailed by clicking their names.

Guidelines to selection ADC

In this section we are going to discuss the general guidelines about the selecting an ADC. While selecting the ADC the following points should be considered.

1. The number of bits:

• Depending on the requirement of resolution we can select an 8-bit, 12-bit, or 16-bit ADC. Higher the number of bits better the resolution. 

2. The required accuracy:

• The accuracy required for different applications is going to be different, so the selection of ADC should be done accordingly.

3. Speed or conversion time:

• The speed of an ADC should be large enough or conversion time should be small in order to convert the fast-changing analog signal successfully into digital form.

4. Range of the input signal:

• The ADC full-scale input range should match with the complete range of the analog input signal.

5. Cost budget:

• Cheaper ADCs do not provide high performance and high-performance ADCs are costly. So selection of an ADC is a compromise between performance and cost.

Characteristics of ADC (Analog to Digital converter) 

Some of the important characteristics of ADC are:

1. Resolution:

• Resolution is defined as the maximum number of digital output codes. This is the same as that of a DAC.

Resolution=2^n

• Alternatively, resolution can be defined as the ratio of the change in the value of the input analog voltage V_A, required to change the digital output by 1 LSB.

\boxed{Resolution=\frac{V_{FS}}{2^n - 1}} 

2. Conversion Time:

• It is the total time required to convert the analog signal into a corresponding digital output.
• As we know, the conversion time depends on the conversion technique used for an ADC.
• The conversion time also depends on the propagation delay introduced by the circuit components.
• Conversion time should ideally be zero and practically as small as possible.

3. Quantization Error:

• As shown in the figure, the digital output is not always an accurate representation of the analog input.
• For example, any input voltage between 1/8 and 2/8 of full scale will be converted to a digital word of “001”. This approximation process is called quantization and the error due to quantization is called quantization error.
• The maximum value of quantization error is ±1/2 LSB.
• The quantization error should be as small as possible. It can be reduced by increasing the number of bits. The increase in the number of bits will also improve resolution.

Application of ADC (Analog to Digital Converter)

Some of the important general applications of ADC is as follows:

• In the digital instruments such as digital voltmeter, frequency counter, etc.
• In the data acquisition system
• In the digital tachometers for speed measurements and feedback
• In digital recording and reproduction
• In computerized instrument systems
• NC and CNC machines