ADC : Analog to Digital Converter | How ADC works?

 Analog to Digital Converter (ADC)

In this blog we will discuss about analog to digital converter in very detailed manner. Digital meters are measuring instruments that convert analog signal into digital and display on the front panel. Using digital meters we can measure current, voltage, power frequency, phase etc. 

What is ADC?

ADC is the heart of digital meters that converts measured analog signal into digital form. Basically, it converts analog physical variables into digital signals for processing. It uses less power and has high conversion efficiency.  

There are several ways to convert an analog signal to digital. Different manufacturers produce different types of ADC chips, such as the ADC08xx series. Thus, discrete components can be used to design an ADC.

How ADC works?

Mainly there are two steps for conversion of analog signals into digital form:

1. S/H: Sampling and holding

2. Q/E: Quantizing and Encoding

Fig. Circuit symbol for ADC

i. Analog input is V_A which produces an output in digital form.
ii. V_R is reference voltage.
iii. Two control lines are present.
• Soc – Start of Conversion
• Eoc – End of conversion
• Soc – starts A to D conversion
• Eoc – ends A to D conversion
iv. n bit digital output with d₁ as MSB and d_n as LSB.

Analog to digital conversion is a quantizing process where an analog signal is converted into equivalent binary word.  

ADC process

Fig. ADC process

Sampling and Holding

In order to measure an analog signal, we must hold it steady for a short period of time. The signal can be measured repeatedly and very quickly, and then the right time scale can be determined. Alternatively, we could measure the signal at different times and average them together. We can also digitize the signal after holding it for a specific amount of time and then sample the value after digitizing it. This is done by a sample and hold circuit.

Quantization and Encoding

In order to understand quantizing, we need to first understand what the term Resolution means in ADC. It is the smallest variation in analog signal that produces a change in digital output. It actually represents the error in the quantization process.

V → Range of reference voltage

2^N → Number of states

N → Number of bits in digital output  

In quantizing, the reference signal is divided into discrete quanta, and the input signal is matched to the correct quantum.

During encoding, each quantum will receive a unique digital code that will be assigned to the input signal. 

The performance parameters of ADC are as follows:

1. Resolution
2. Quantization error and Accuracy
3. Conversion Time

Types of ADC

a. Dual Slope ADC

b. Successive approximation ADC

c. Digital Ramp ADC

d. Flash Type ADC

Applications of ADC

1) ADC is used in microprocessor based data acquisition system.

2) ADC is used in digital voltmeter.

3) ADC is used in tachometer for speed measurement.

4) ADC is used in DMM.

5) ADC is used in DSO.

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