Integral type
The integral type AD works by converting the input voltage into time (pulse width signal) or frequency (pulse frequency), and then the digital value is obtained by the timer/counter.
Successive comparison
The successive comparison type AD is constituted by a comparator and a DA converter by successive comparison logics, starting from the MSB, sequentially comparing the input voltage to the output of the built-in DA converter for each bit, and outputting the digital value after n comparisons. Its circuit scale is medium.
Parallel comparison type/string parallel comparison type
Parallel comparison type AD uses multiple comparators to perform conversion only for one comparison, also known as FLash (fast) type. Since the slew rate is extremely high, the n-bit conversion requires 2n-1 comparators.
Serial parallel comparison
Half flash (semi-fast) type: It consists of two n/2-bit parallel AD converters combined with a DA converter, which performs conversion with two comparisons.
The three-step or multi-step AD conversion is called grading (MulTIstep/Subrangling) type AD, and from the conversion timing angle, it can also be called Pipelined type AD. The modern grading type AD also adds the result of multiple conversions. Digital calculations and correction of features.
Σ-Δ modulation
The Σ-Δ type AD is composed of an integrator, a comparator, a 1-bit DA converter, a digital filter, and the like. In principle, it approximates the integral type, converting the input voltage into a time (pulse width) signal, and processing it with a digital filter to obtain a digital value.
Frequency conversion type
The frequency-frequency conversion type realizes analog-to-digital conversion by indirect conversion. The input analog signal is converted to a frequency, and then the counter is used to convert the frequency into a digital quantity.
Advantages and disadvantages analysis:
When choosing a model, we generally need to consider the following parameters:
Determining the accuracy of the A/D converter: Accuracy is the physical quantity that reflects the accuracy of the converter's actual output close to the ideal output.
ResoluTIon refers to the amount of change in the analog signal when the digital quantity changes by a minimum amount, defined as the ratio of full scale to 2n. The resolution, also known as precision, is usually expressed in terms of the number of bits of the digital signal.
Quantuming Error The error caused by the finite resolution of AD, that is, the maximum deviation between the stepwise transfer characteristic curve of the finite resolution AD and the transfer characteristic curve (straight line) of the infinite resolution AD (ideal AD). Usually one or half of the smallest digital analog variation, expressed as 1LSB, 1/2LSB.
During the conversion process, accuracy is lost due to quantization errors and systematic errors. The influence of the quantization error on the accuracy is computable, and it is mainly determined by the number of bits of the A/D conversion device.
Generally, an A/D converter of 8 bits or less is called a low resolution ADC, 9 to 12 bits are called a medium resolution ADC, and 13 or more bits are high resolution. The higher the number of bits in the A/D device, the higher the resolution, the smaller the quantization error, and the higher the accuracy that can be achieved.
Select the conversion rate of the A/D converter
The Conversion Rate is the reciprocal of the time required to complete an AD conversion from analog to digital. Sampling time is another concept that refers to the interval between two conversions. In order to ensure the correct completion of the conversion, the Sample Rate must be less than or equal to the conversion rate. Commonly used units are kpss and Msps, which means sampling thousands/million times per second.
Choose the right range
The dynamic range of the analog signal is large, and sometimes a negative voltage may occur. When selected, the dynamic range of the signal under test is preferably within the range of the A/D device.
Select the output interface of the A/D device
There are many types of A/D device interfaces, including parallel bus interfaces, and serial bus interfaces such as SPI, I2C, and 1-Wire. They are identical in principle and precision, but the control method and interface circuit can vary greatly.
Select the channel number and package of the A/D device
This is related to the system, and the number of channels must meet the needs of the entire acquisition system. The package determines the size of the PCB layout and also affects the distribution parameters of the connection during high-speed applications.
Select A/D device temperature range
This is only relevant for some harsh environments, noting that each AD has a fixed application temperature range.
Common selection table :
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