A/D devices and chips are commonly used peripheral devices for data acquisition of single-chip microcomputers. A variety of A/D converters have different performances. When designing a data acquisition system, the first thing that comes to be encountered is how to choose the right A/D converter to meet the system design requirements. The choice of A/D converter device needs to consider the quality of the device itself and the application requirements. Basically, an A/D device can be selected according to the following aspects.

(1) A/D converter digits The determination of the number of A/D converter bits should be considered from the two aspects of static accuracy and dynamic smoothness of the data acquisition system. From the aspect of static accuracy, it is necessary to consider the error caused by the original error of the input signal being transmitted to the output, which is the main part of the error caused by the digitization of the analog signal. The quantization error is related to the number of A/D converter bits. Generally, an A/D converter of 8 bits or less is classified as a low resolution A/D converter, a 9 to 12 bit is called a medium resolution converter, and 13 or more bits is called a high resolution converter. The 10-bit A/D chip has the following errors, and 11 or more bits do not contribute much to the error reduction, but the requirements for the A/D converter are too high. Therefore, taking 10 or 11 bits is appropriate. Since the analog signal passes through the measuring device and is processed by the A/D converter, the total error is composed of the measurement error and the quantization error. The accuracy of the A/D converter should match the accuracy of the measuring device. That is to say, on the one hand, it is required that the quantization error accounts for a small proportion in the total error, so that it does not significantly increase the measurement error; on the other hand, the number of bits of the A/D converter must be based on the accuracy level of the current measuring device. Make the appropriate requirements.
At present, the accuracy value of most measuring devices is not less than 0.1%~0.5%, so the accuracy of the A/D converter can be 0.05%~0.1%, and the corresponding binary code is 10~11 bits, plus the sign bit, ie It is 11~12. When there are special applications, the A/D converter requires more bits, and a double-precision conversion scheme is often used.
(2) A/D converter conversion rate The A/D converter converts from the start to the end of the conversion, and outputs a stable digital quantity, which requires a certain conversion time. The reciprocal of the conversion time is the number of conversions that can be completed per second, called the conversion rate.
When determining the slew rate of an A/D converter, the sampling rate of the system should be considered. For example, if an A/D converter with a conversion time of 100us ($1.6606) is used, its conversion rate is 10KHz. According to the sampling theorem and actual needs, a waveform of one cycle needs to take 10 samples, then such an A/D converter can only process an analog signal with a frequency of 1KHz. By reducing the conversion time, the signal frequency can be increased. For a general-purpose microcontroller, it is difficult to perform operations other than A/D conversion within the sampling time, such as reading data, restarting, storing data, and loop counting.
(3) Sample/holder The sample holder can be used to acquire DC and very slow analog signals. For other analog signals, a sample holder is usually added. If the signal frequency is not high, the conversion time of the A/D converter is short, that is, when the high-speed A/D is sampled, the sample/holder can also be omitted.
(4) A/D converter range A/D conversion requires bipolarity and sometimes unipolarity. The minimum value of the input signal starts from zero and also starts from non-zero. Some converters provide different ranges of pins, and only the correct use can guarantee the conversion accuracy. In use, factors affecting the A/D converter range are: range conversion and bipolar bias; dual reference voltage; proper use of the comparator input at the A/D converter.
(5) Full scale error The difference between the corresponding input signal and the ideal input signal value at full output.
(6) Linearity The maximum offset of the transfer function of the actual converter from the ideal straight line.

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