Analog to Digital (A/D) Converter

 

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.

ADC Process

There are mainly two steps involves in the process of conversion. They are:

• Sampling and Holding
• Quantizing and Encoding

Sampling and Holding

In the process of Sample and hold (S/H), the continuous signal will gets sampled and freeze (hold) the value at a steady level for a particular least period of time. It is done to remove variations in input signal which can alter the conversion process and thereby increases the accuracy. The minimum sampling rate has to be two times the maximum data frequency of the input signal.

Quantizing and Encoding

For understanding quantizing, we can first go through the term Resolution used in ADC. It is the smallest variation in analog signal that will result in a variation in the digital output. This actually represents the quantization error.

V → Reference voltage range

2N → Number of states

N → Number of bits in digital output

• Quantizing: It is the process in which the reference signal is partitioned into several discrete quanta and then the input signal is matched with the correct quantum.
• Encoding: Here; for each quantum, a unique digital code will be assigned and after that the input signal is allocated with this digital code. 

Types of A/D Converters

Some of the types of analog to digital converters  include:

• Dual Slope A/D Converter
• Flash A/D Converter
• Successive Approximation A/D Converter

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 control circuit captures the counter value which is the digital value of corresponding analog input value. This Dual slope ADC is relatively medium cost and slow speed device.

Flash A/D Converter

This ADC converter IC is also called as parallel ADC, which is a 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 high state when the analog input voltage exceeds the reference voltage. This output is further given to 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.

Application of ADC

• Used with the transducer.
• Convert the analog signal to digital signal in computers and cell phones.
• Microcontrollers.
• Digital signal processing.
• Digital storage oscilloscopes.
• Used in scientific instruments.
• Music reproduction technology