An analog-to-digital converter, or A/D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. The usual analog-to-digital converter converts an input voltage signal into an output digital signal. Since the digital signal itself has no practical significance, it only represents a relative size. Therefore, any analog-to-digital converter requires a reference analog quantity as the conversion standard. The most common reference standard is the maximum convertible signal size. The output digital quantity indicates the size of the input signal relative to the reference signal.
A circuit that converts an analog signal to a digital signal is called an analog-to-digital converter (A/D converter or ADC, Analog to Digital Converter). The role of A/D conversion is to convert the analog signal with continuous time and continuous amplitude. It is converted into a digital signal with discrete time and discrete amplitude. Therefore, A/D conversion generally goes through four processes: sampling, holding, quantization and encoding. In the actual circuit, some of these processes are combined, for example, sampling and holding, quantization and encoding are often achieved simultaneously in the conversion process.
The basic principle of this converter is to sample the input analog signal at a specified time interval and compare it with a series of standard digital signals. The digital signals converge successively until the two signals are equal. Then it displays the binary number representing this signal. There are many kinds of analog-to-digital converters, such as direct, indirect, high-speed, high-precision, and ultra-high-speed. Each has many forms. The function opposite to the analog-to-digital converter is called "digital-to-analog converter", also known as "decoder". It is a device that converts digital quantities into continuously changing analog quantities. There are also many types and many forms.
There are many types of analog-to-digital converters, which can be divided into indirect ADC and direct ADC according to different working principles.
The indirect ADC is to convert the input analog voltage into time or frequency, and then convert these intermediate quantities into digital quantities. The commonly used intermediate quantity is time double integral ADC.
Parallel comparison ADC: Since the parallel comparison ADC uses various magnitudes for parallel comparison at the same time, each output code is also generated in parallel at the same time, so the fast conversion speed is its outstanding advantage, and the conversion speed is independent of the number of output code bits. The disadvantages of parallel comparison ADCs are high cost and high power consumption. Because the ADC with n-bit output requires 2n resistors, (2n-1) comparators and D flip-flops, and a complex coding network, the number of components increases with the number of digits and rises in geometric series. Therefore, this ADC is suitable for occasions requiring high speed and low resolution.
Successive-approximation ADC: Successive-approximation ADC is another direct ADC. It also generates a series of comparison voltages VR, but unlike parallel comparison ADCs, it generates comparison voltages one by one and compares them with the input voltage one by one to gradually approach Analog-to-digital conversion. The successive approximation ADC requires bit-by-bit comparison for each conversion, and requires (n+1) beat pulses to complete, so it is slower than the parallel comparison ADC and much faster than the double-product ADC. Fast ADC device. When there are more digits, it requires much less components than the parallel comparison type, so it is one of the more widely used integrated ADCs.
Double integration ADC: It is an indirect ADC. It integrates the input sample voltage and the reference voltage twice to obtain a time interval proportional to the average value of the sample voltage. At this time, the counter clocks the standard clock pulse. (CP) counting, the counting result output by the counter is the corresponding digital quantity. The advantages of the double integral ADC are strong anti-interference ability; good stability; can realize high-precision analog-to-digital conversion. The main disadvantage is that the conversion speed is low, so this kind of converter is mostly used in instruments and meters that require high accuracy and low conversion speed, such as multi-digit high-precision digital DC voltmeters.
Conversion accuracy
1. Resolution
The resolution of the A/D converter is expressed by the number of output binary (or decimal) digits. It shows the resolution ability of the A/D converter to the input signal. In theory, the n-bit output A/D converter can distinguish 2n different levels of input analog voltage, and the minimum value of the input voltage is 1/2n of full-scale input. When the maximum input voltage is fixed, the more output digits, the higher the resolution. For example, the output of the A/D converter is an 8-bit binary number, and the maximum value of the input signal is 5V, then this converter should be able to distinguish the minimum voltage of the input signal is 19.53mV.
2. Conversion error
The conversion error is usually given in the form of the maximum value of the output error. It represents the difference between the digital quantity actually output by the A/D converter and the theoretical output digital quantity. Commonly used multiples of the least significant digit. For example, if the relative error is not greater than ±LSB/2, it means that the error between the actual output digital quantity and the theoretically output digital quantity should be less than the lowest half of the word.
Conversion time
The conversion time refers to the time that the A/D converter starts from the arrival of the conversion control signal and gets a stable digital signal at the output.
Different types of converters have very different conversion speeds. Among them, the parallel conversion A/D converter has the highest conversion speed. The conversion time of the single-chip integrated A/D converter with 8-bit binary output can reach less than 50ns, followed by the successive comparison A/D converters, and most of them have conversion times of 10 Within -50μs. The speed of the indirect A/D converter is the slowest. For example, the conversion time of the double integral A/D converter is mostly between tens of milliseconds and hundreds of milliseconds. In practical applications, the selection of the A/D converter should be considered in terms of the total number of system data, accuracy requirements, the range of the input analog signal, and the polarity of the input signal.
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