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One, the basic concept of AD
AD conversion is to convert time continuous and amplitude continuous analog quantity into time discrete, amplitude also discrete digital quantity. The output digital quantity is proportional to the input analog quantity.
The AD conversion process has four stages, namely sampling, holding, quantizing and encoding.
Sampling is the process of turning a continuous-time signal into a discrete-time signal. After sampling, the time-continuous and numerically continuous analog signal becomes a time-discrete and numerically continuous signal, which is called a sampled signal. The sampling circuit is equivalent to an analog switch, and the analog switch works periodically. Theoretically, within each cycle, the closing time of the analog switch approaches zero. At the moment when the analog switch is closed (sampling moment), we "take" a "sample" of the analog signal.
Quantization is the process of turning a continuous numerical signal into a discrete numerical signal. Theoretically, after quantization, we can transform a time-discrete and numerically continuous sampling signal into a time-discrete and numerically discrete digital signal.
We know that in circuits, digital quantities are usually represented by binary codes. Therefore, there is an encoding circuit behind the quantization circuit, which converts the value of the digital signal into a binary code.
However, quantization and coding always take a certain amount of time to complete, so there is also a hold circuit in front of the quantization circuit. Hold is the process of changing a time-discrete and numerically continuous signal into a time-continuous and numerically discrete signal. During quantization and encoding, the hold circuit is equivalent to a constant voltage source, which “holds” the signal voltage at the sampling moment at the input of the quantizer. Although logically the retainer is an independent unit, the retainer is always built together with the sampler in engineering. The two are collectively called the sample holder.
2. The main technical parameters of ADC
1. Resolution
For the ADC, the resolution represents the amount of change in the input analog voltage required for the output digital to change an adjacent digital. It is usually defined as the ratio of the full-scale voltage to 2 n , where n is the number of bits of the ADC. For example, an ADC with 12-bit resolution can resolve 1/12 12 (0.0244%) of full scale .
Sometimes the resolution is also represented by the number of bits of the A/D converter. For example, the resolution of ADC0809 is 8 bits, and the resolution of AD574 is 12 bits.
2. Quantization error
The quantization error is the error caused by the limited resolution of the ADC, which is the inherent error of the continuous analog signal after integer quantization. For the rounding quantization method, the quantization error is between ±1/2LSB.
3. Absolute accuracy
Absolute accuracy refers to the difference between the actual required analog input value represented by the given digital code generated at the output and the theoretically required analog input value.
4. Relative accuracy
It is similar to absolute accuracy, except that the deviation is expressed as a percentage of the full-scale analog voltage.
5. Conversion time
The conversion time is the time required for the ADC to complete a conversion, that is, the time required to obtain a stable digital output from the start of the start signal to the end of the conversion, usually in the order of microseconds.
6. Range
Range refers to the input voltage range that can be converted.
Third, the general steps of AD conversion
Sampling and holding are usually completed in the sample and hold circuit, and quantization and coding are usually completed in the AD conversion circuit.
Since AD conversion requires a certain amount of time, after each sampling, the sampling voltage needs to be maintained for a period of time.
During the valid period of s(t), VT is turned on, u charges to C, u 0 (=u c ) changes with the change of u I ;
during the invalid period of s(t) , VT is turned off, u 0 (=u c ) remains inactive Change until the next sampling.
Since the integrated operational amplifier A has a high input impedance, the charge stored on the capacitor C is not easy to discharge during the holding phase.
Fourth, the general classification of AD converters
AD converter can be divided into direct AD converter and indirect AD converter according to the conversion principle.
The so-called direct AD converter directly converts analog signals into digital signals, such as successive approximation type, parallel comparison type and so on. Among them, the successive approximation AD converter is easy to implement with integrated technology and can achieve higher resolution and speed. Therefore, the current integrated AD chip adopts the successive approximation type; the
indirect AD converter first converts the analog quantity into the intermediate Quantities, and then converted into digital quantities, such as voltage/time conversion type (integral type), voltage/frequency conversion type, voltage/pulse width conversion type, etc. Among them, the integral A/D converter has simple circuit, strong anti-interference ability, and can achieve high resolution, but the conversion speed is slow. Some converters also integrate multiplexers, reference voltage sources, clock circuits, decoders and conversion circuits into one chip, which has gone beyond the pure A/D conversion function and is very convenient to use.
Five, the general characteristics of ADC
- Analog input, which can be single-channel or multi-channel analog input;
- Reference input voltage, which can be provided externally or generated inside the ADC;
- Frequency input, usually provided by the outside, used to determine the conversion rate of the ADC;
- Power input, usually with analog and digital power pins;
- Digital output, ADC can provide parallel or serial digital output.
Under the requirements of more output bits (the better the resolution) and the faster the conversion time, the more expensive the manufacturing cost and unit price.