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An analog-to-digital converter, or A/D converter, or ADC for short, is usually an electronic component that converts an analog signal into a digital signal. A typical analog-to-digital converter converts an input voltage signal into an output digital signal.
Analog signal refers to the information expressed by continuously changing physical quantities, such as temperature, humidity, pressure, length, current, voltage, etc. We usually call the analog signal a continuous signal, which can have infinitely many different values in a certain time range. The digital signal is a discrete and discontinuous signal in terms of values.
The circuit that converts an analog signal into a digital signal is called an analog-to-digital converter (referred to as A/D converter or ADC). Therefore, generally, there are 4 processes during A/D conversion: sampling, holding, and Quantization, and Coding. In the actual circuit, some of these processes are combined, for example, sampling, and holding. Quantizing and Encoding are often achieved simultaneously in the conversion process.
Now the software, radio, digital image acquisition are required to have high-speed A/D sampling to ensure effectiveness and accuracy, general measurement, and control systems also want to make a breakthrough in accuracy. The wave of digitization of mankind has driven the continuous transformation of the A/D converter, and the A/D converter is the pioneer of mankind to achieve digitalization. A/D converter has undergone many technological innovations since more than 30 years ago from flash ADC, SAR ADC, integrating ADC, and the newly developed sigma-delta ADC and flow line ADC in recent years. They have their own advantages and disadvantages and can meet the use of different applications.
ADC (analog to digital converter) conversion process
ADC conversion process
The basic conversion principle of the ADC is divided into four processes.
(1) Anti-aliasing, which can be understood as a low-pass filter.
(2) Sampling and holding circuit.
(3) Quantizing
(4) Encoding
Sampling and Holding
Sampling is the process of replacing the original signal that was continuous in time with a sequence of signal sample values at regular intervals, i.e., discretizing the analog signal in time. The results of the sampling are stored until the next sampling, and this process called holding.
Quantizing and Encoding
Quantizing is to convert the continuous amplitude of the analog signal into a finite number of discrete values with a certain interval using a finite number of amplitude values that approximate the original continuous changing amplitude value. Encoding is in accordance with certain rules, to represent the quantized value with binary numbers and then convert it into a binary or multi-valued digital signal stream. The digital signals thus obtained can be transmitted over digital lines such as cables, microwave trunks, satellite channels, etc.
This process of an analog signal converted into a digital signal through the ADC is called quantizing. Due to the quantization of the output digital signal is limited to a number of bits, the output digital signal and the analog signal you sample will have an error, known as quantization error. For an N-bit ADC, assuming that its full-scale voltage Vref, Vref ADC is divided into 2N intervals, the width of the interval with LSB ( (last significant bit) indicates LSB=Vref/2N.
For example, Vref=8V, ADC is 3 bits, LSB=1, so each interval is 1V. The resolution of this ADC is 1V.
000 means voltage 0 ≤ V < 1
001 means voltage 1 ≤ V < 2
010 means voltage 2 ≤ V < 3
011 means voltage 3 ≤ V < 4
100 means voltage 4 ≤ V < 5
101 means voltage 5 ≤ V < 6
110 represents voltage 6 ≤ V < 7
111 represents voltage 7 ≤ V < 8
ADC output and error
There are many types of analog-to-digital converters, which can be divided into indirect ADCs and direct ADCs according to the different working principles.
Indirect ADC is to first convert the input analog voltage into time or frequency, and then convert these intermediate quantities into digital quantities, the commonly used indirect ADC is Dual Slope ADC whose intermediate quantity is the time.
As a result of the flash ADC using the magnitude of the parallel comparison, the output code is also generated in parallel at the same time, so the conversion speed is its outstanding advantage, while the conversion speed and the number of output code bits independent. Flash ADC’s disadvantages are high cost and high power consumption. Because of the n-bit output ADC, which requires 2n resistors, (2n-1) comparators and D flip-flops, and a complex coding network, the number of its components increases geometrically with the number of bits. Therefore, this kind of ADC is suitable for situations where high speed and low resolution are required.
Circuit diagram of flash ADC
The successive approximation ADC is another kind of direct ADC, which also generates a series of comparative voltages VR. Unlike the flash ADC, it generates comparative voltages one by one, and compares them with the input voltages one by one, and performs the analog-to-digital conversion in the way of gradual approximation. The successive approximation ADC is bit by bit comparison for each conversion, and needs (n + 1) beat pulse to complete, so it is slower than the conversion speed of the parallel comparison ADC, much faster than the double fractional product ADC, which belongs to the medium-speed ADC device. In addition, it needs to use much fewer components than the flash type, so it is one of the more widely used integrated ADC.
Circuit diagram of successive approximation ADC
Dual Slope ADC belongs to indirect ADC, which integrates the input sampling voltage and reference voltage twice to get a time interval proportional to the average value of the sampling voltage. At the same time, it counts the standard clock pulse (CP) with a counter in this time interval, and the counter output result is the corresponding digital quantity. The advantages of double-integral ADC are strong anti-interference ability, good stability, and can realize high-precision analog-to-digital conversion. The main disadvantage is the low conversion speed, so this type of converter is mostly used in instrumentation which requires high precision but not high conversion speed, such as multi-bit high-precision digital DC voltmeter.
Dual Slope ADC schematic diagram
(1) Resolution
The resolution of an A/D converter is expressed in bits of the output binary (or decimal) number. It indicates the ability of the A/D converter to distinguish between input signals. Theoretically speaking, the n-bit output of the A/D converter can distinguish between 2n different levels of input analog voltage, and the minimum value that can distinguish between input voltages is 1/2n of the full-scale input. For example, if the output of the A/D converter is 8 binary digits and the maximum value of the input signal is 5V, then the 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 actual digital output of the A/D converter and the theoretical output of digital quantity. It is commonly expressed as a multiple of the lowest effective bit. For example, the relative error is given as no greater than ±LSB/2, which indicates that the error between the actual output digital quantity and the theoretically desirable output digital quantity is less than half a word of the lowest effective bit.
The conversion time is the time it takes for an A/D converter to convert the control signal from the time it arrives at the time it gets a stable digital signal at the output.
The conversion speed varies considerably between different types of converters. Parallel comparative A/D converters have the highest conversion speed. Single-chip integrated A/D converters with 8-bit binary output have a conversion time of less than 50ns, followed by successive approximation A/D converters, most of which have a conversion time of less than 10-50μs. Indirect A/D converters are the slowest, such as dual slope A/D converters with conversion times mostly in the tens of milliseconds to hundreds of milliseconds. In practical applications, the selection of A/D converters should be considered from the total number of bits of system data, accuracy requirements, the range of the input analog signal, and the polarity of the input signal.
Sampling accuracy - i.e. resolution, typically 8, 10, 12, 16 bits, etc..
Conversion time - i.e., the time required for each sample, characterizing the ADC's conversion speed, which is related to the ADC's clock frequency, sampling period, and conversion period.
Data output methods - e.g. parallel output, serial output.
ADC types - as mentioned above, there are many types of ADCs, and different types have different performance limits.
Operating voltage - attention needs to be paid to the operating voltage range of the ADC, the ability to measure negative voltages directly, etc.
Chip packaging - whether the chip package meets product design requirements.
Value for money - controlling costs.
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