Analog to digital converterA/D converter, or ADC for short, usually refers to aanalog signalAn electronic component that is converted into a digital signal.GeneralAnalog-to-digital converterIt is a digital signal that converts an input voltage signal into an output signal.Since digital signal itself has no practical significance, it only represents a relative size.Therefore, any analog-to-digital converter needs a reference analog quantity as the conversion standard, and the common reference standard is the maximum convertible signal size.The output digital quantity indicates that the input signal is relative to thereference signal Size of[1]。
The circuit that converts analog signals into digital signals is called analog-to-digital converter (A/D converter or ADC for short),Analog to Digital Converter), the function of A/D conversion is to convert analog signals with continuous time and amplitude into digital signals with discrete time and amplitude,A/D conversion generally goes through four processes: sampling, holding, quantification and coding.In the actual circuit, some of these processes are combined. For example, sampling and holding, quantization and coding are often realized simultaneously in the conversion process[2]。
Fundamentals
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The basic principle of this converter is to sample the input analog signal at a specified time interval, compare it with a series of standard digital signals, and the digital signal will converge gradually until the two signals are equal.Then the binary number representing the signal is displayed. There are many kinds of analog-to-digital converters, such as direct, indirect, high-speed, high-precision, ultra high-speed, etc.Each has many forms.Opposite to the function of analog digital converter, it is called "digital analog converter", also called "decoder". It is a device that converts digital quantity into continuously changing analog quantity. There are also many kinds and forms[3]。
Steps of analog-to-digital conversion
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Analog to digital conversion usually goes through the steps of sampling, quantization and coding[4]。
Sampling refers to replacing the original continuous signal in time with a signal sample value sequence at a certain interval, that is, discretizing the analog signal in time[4]。
Quantization is to use a limited number of amplitude values to approximate the original continuously changing amplitude values, and change the continuous amplitude of the analog signal into a limited number of discrete values with a certain interval[4]。
Coding is to use the quantized value according to certain rulesBinary digitRepresents, and then converts to a binary or multivalued digital signal stream.The digital signals thus obtained can be transmitted through digital lines such as cables, microwave trunks, satellite channels, etc[4]。
classification
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Analog-to-digital converterThere are many kinds of ADCs, which can be divided into indirect ADC and direct ADC according to different working principles[5]。
Indirect ADC is to convert the input analog voltage into time or frequency first, and then convert these intermediate quantities into digital quantities. The commonly used dual integral ADC has the intermediate quantity of time[5]。
Parallel comparison ADC: Since parallel comparison ADC uses parallel comparison of all orders of magnitude at the same time, and each output code is generated in parallel at the same time, 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 ADC are high cost and high power consumption.Because of the n-bit output ADC, 2 is requirednResistance, (2n- 1) comparators and D flip-flop, as well as complex coding network, the number of elements increases geometrically with the number of bits.So this kind of ADC is suitable for occasions requiring high speed and low resolution[5]。
Successive approximation ADC: Successive approximation ADC is another kind of direct ADC, which also generates a series of comparison voltage VR, but unlike parallel comparison ADC, it generates comparison voltage one by one, compares with input voltage one by one, and performs analog-to-digital conversion in a gradual approximation manner.Successive approximation ADC needs bit by bit comparison for each conversion, which requires (n+1) beat pulses to complete. Therefore, it is slower than parallel comparison ADC in conversion speed and much faster than double integration ADC. It is a medium speed ADC device.When there are many other digits, it needs much less components than the parallel comparison type, so it is a widely used type of integrated ADC[5]。
Dual integration ADC: it is an indirect ADC. It first integrates the input sampling voltage and reference voltage twice to obtain a time interval proportional to the average value of the sampling voltage. At the same time, during this time interval, the counter counts the standard clock pulse (CP), and the counter output count result is the corresponding digital quantity.The advantage of double integral ADC is strong anti-interference ability;Good stability;It 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 precision but not high conversion speed, such as multi bit high-precision digitalDC voltmeterin[5]。
technical parameter
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Conversion precision
1. Resolution
A/D converterThe resolution of is expressed by the number of digits of the output binary (or decimal) number.It shows the resolution of the A/D converter to the input signal.Theoretically,The A/D converter with n-bit output can distinguish 2nInput analog voltage of different levels, the minimum value that can distinguish the input voltage is 1/2 of the full range inputn。When the maximum input voltage is fixed, the more output bits, the higher the resolution.For example, if the output of the A/D converter is an 8-bit binary number and the maximum value of the input signal is 5V, the converter should be able to distinguish the minimum voltage of the input signal as 19.53mV[6]。
2. Conversion error
The conversion error is usually given in the form of the maximum output error.It represents the difference between the actual output digital quantity of the A/D converter and the theoretical output digital quantity.CommonLSBA multiple of.For example, if the relative error is not more than ± LSB/2, it means that the error between the actual output digital quantity and the output digital quantity theoretically should be less than the lowest half word[6]。
Conversion time
The conversion time refers to the time from the arrival of the conversion control signal to the stable digital signal at the output end of the A/D converter[7]。
The conversion speed of different types of converters is quite different.Among them, the parallel comparison A/D converter has the highest conversion speed. The conversion time of the monolithic integrated A/D converter with 8-bit binary output can reach within 50ns, followed by the sequential comparison A/D converter. Most of their conversion time is within 10-50 μ s.The speed of indirect A/D converter is the slowest. For example, the conversion time of double integral A/D converter is mostly between tens of milliseconds and hundreds of milliseconds.In practical application, the selection of A/D converter should be comprehensively considered from the aspects of the total number of bits of system data, accuracy requirements, the range of input analog signals and the polarity of input signals[7]。
