Introduction to ADC Classification and Characteristics

    With the rapid development of integrated circuits, new design ideas and manufacturing technologies for A/D converters are emerging one after another. A/D converter with different structures and performance has emerged to meet various detection and control needs.

    Below we will discuss the basic principles and classification of A/D converters.

    According to the principle of A/D converters, they can be divided into two categories. One type is a direct A/D converter, which converts the input voltage signal directly into digital code without any intermediate variables; Another type is indirect A/D converters, which convert the input voltage into some intermediate variable (time, frequency, pulse width, etc.), and then convert this intermediate variable into a digital code output.

    Although there are many types of A/D converters, there are currently three widely used types: successive approximation A/D converters, double integral A/D converters, and V/F conversion A/D converters. In addition, in recent years, there has been a new type of Σ-Δ Type A/D converters have emerged and been widely used in instruments.

    The basic principle of Sequential Approximation (SAR) A/D Converter (SAR) is to compare the analog input signal to be converted with a speculative signal, and decide whether to increase or decrease the input signal based on the size of the two, in order to push forward towards the analog input signal. It is speculated that the signal is obtained from the output of the D/A converter. When the two are equal, the digital signal input to the D/A converter corresponds to the digital quantity of the analog input. This type of A/D converter is generally fast, but its accuracy is generally not high. Commonly used ones include ADC0801, ADC0802, AD570, etc.

    The basic principle of a dual integral A/D converter is to first integrate the input analog voltage for a fixed time, then convert it to inverse integration of the standard voltage, until the integration input returns to the initial value. The length of these two integration times is proportional to the magnitude of the two, and the corresponding digital quantity of the analog voltage can be obtained. This type of A/D converter has a slower conversion speed but higher accuracy. The development from double integral to multiple methods such as quadruple integral and quintuple integral has improved conversion speed while ensuring conversion accuracy. Commonly used ones include ICL7135, ICL7109, etc.

    Σ-Δ The type AD consists of an integrator, comparator, 1-bit D/A converter, and digital filter. In principle, it is similar to an integral type, which converts the input voltage into a time (pulse width) signal and processes it with a digital filter to obtain a digital value. The digital part of the circuit is basically easy to be monolithic, so it is easy to achieve high resolution. Mainly used for audio and measurement. This type of converter has extremely high conversion accuracy, reaching 16 to 24 bit conversion accuracy, low price, but its weakness is that the conversion speed is relatively slow, making it more suitable for inspection equipment that requires high detection accuracy but not too high speed requirements. Commonly used ones include AD7705, AD7714, etc.

    The V/F converter converts voltage signals into frequency signals with good accuracy and linearity, and has a simple circuit, strong adaptability to the environment, and low cost. Suitable for A/D conversion process of non fast long-distance signals. Commonly used ones include LM311, AD650, etc.