The Knowledge of AD/DA Converter

- Sep 04, 2018-

1. What is A/D, D/A conversion?

With the rapid development and popularization of digital technology, especially information technology, in the fields of modern control, communication and detection, in order to improve the performance index of the system, digital computer technology is widely used for signal processing. Since the actual objects of the system are often analog (such as temperature, pressure, displacement, image, etc.), in order for the computer or digital instrument to recognize and process these signals, these analog signals must first be converted into digital signals; The digital quantity output after analysis and processing also needs to be converted into the corresponding analog signal to be accepted by the actuator. Thus, there is a need for a circuit that can bridge the relationship between analog and digital signals - analog to digital and digital to analog converters.

A circuit that converts an analog signal into a digital signal is called an analog-to-digital converter (A/D converter or ADC, Analog to Digital Converter); a circuit that converts a digital signal into an analog signal is called a digital-to-analog converter (referred to as D/A converter or DAC, Digital to Analog Converter); A/D converter and D/A converter has become an indispensable interface circuit in information systems.

In order to ensure the accuracy of the system processing results, the A/D converter and D/A converter must have sufficient conversion accuracy; if real-time control and detection of fast-changing signals is to be realized, A/D and D/A converters are also required. Has a higher conversion speed. Conversion accuracy and conversion speed are important technical indicators for measuring A/D and D/A converters. With the development of integration technology, many single-chip and hybrid integrated A/D and D/A converters have been developed and produced, which have increasingly advanced technical specifications.

2. D/A and A/D converter related performance parameters

The D/A converter is a linear circuit device that converts digital quantities into analog quantities and has been made into an integrated chip. Since the principle of implementing such a conversion differs from the circuit structure and process technology, various D/A converters have appeared. There are hundreds of products on the market, and they all have their own characteristics in terms of conversion speed, conversion accuracy, and resolution and use value.

The main parameters of the D / A converter:

 (1) Resolution

It refers to the number of digits of the binary number that the D/A converter can convert, and the higher the multi-resolution of the bit number.

(2) Conversion time

Refers to the time required for the digital input to complete the conversion and the output reaches the final value and stabilizes. Current mode D/A conversion is faster, typically between a few ns and a few hundred ns. The voltage type D/A conversion is slower depending on the response time of the op amp.

(3) Accuracy

It refers to the error between the actual output voltage of the D/A converter and the theoretical value. Generally, the least significant digit of the digital quantity is used as a unit of measurement.

(4) Linearity

When the digital quantity changes, the analog quantity output by the D/A converter changes proportionally. The ideal D/A converter is linear, but it is actually error-prone. The maximum value of the analog output deviating from the ideal output is called linearity error.

The function of the A/D converter is to convert the analog quantity into a digital quantity. Since the implementation of this conversion is different from the process technology, a wide variety of A/D conversion chips are produced. The A/D converter is divided into 4 bits, 6 bits, 8 bits, 10 bits, 14 bits, 16 bits, and 31/2 bits, 51/2 bits, and the like of the BCD code by resolution.

According to the conversion speed, it can be divided into ultra-high speed (conversion time 330μS), sub-high speed (330~3.3μS), high speed (conversion time 3.3~333μS), low speed (transition time>330μS).

The A/D converter can be divided into a direct A/D converter and an indirect A/D converter according to the conversion principle.

The so-called direct A/D converter is to convert the analog signal directly into a digital signal, such as successive approximation type, parallel comparison type and so on. Among them, successive approximation A/D converters are easy to implement with integrated process, and can achieve higher resolution and speed. Therefore, the current integrated A/D chip adopts successive approximation type; indirect A/D converter is first the analog quantity is converted into an intermediate quantity and then converted into a digital quantity, such as a voltage/time conversion type (integral type), a voltage/frequency conversion type, a voltage/pulse width conversion type, and the like.

The integral type A/D converter circuit is simple, has strong anti-interference ability, and can be cut to high resolution, but the conversion speed is slow. Some converters also integrate multiplex switches, reference voltage sources, clock circuits, decoders and conversion circuits in a single chip, which is beyond the simple A/D conversion function and is very convenient to use.



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