A/D devices and chips are commonly used peripheral devices to realize data acquisition of single-chip microcomputers. There are many kinds of A/D converters with different performances. When designing a data acquisition system, the first thing to encounter is how to choose a suitable A/D converter to meet the system design requirements. When choosing an A/D conversion device, you need to consider the quality of the device itself and the requirements of the application. Basically, you can choose an A/D device according to the following indicators.
(1) A/D converter bits
The determination of the number of bits of the A/D converter should be considered from the static accuracy and dynamic smoothness of the data acquisition system. In terms of static accuracy, it is necessary to consider the error caused by the transmission of the original error of the input signal to the output, which is the main part of the error generated when the analog signal is digitized. The quantization error is related to the number of bits in the A/D converter. Generally, A/D converters with less than 8 bits are classified as low-resolution A/D converters, those with 9 to 12 bits are called medium-resolution converters, and those with more than 13 bits are called high-resolution converters. The error below the 10-bit A/D chip is relatively large, and more than 11-bit does not contribute much to reducing the error, but the requirements for the A/D converter are too high. Therefore, taking 10 or 11 bits is appropriate. Since the analog signal first passes through the measuring device and then is converted by the A/D converter before processing, 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, the proportion of quantization error in the total error is required to be small, so that it does not significantly expand the measurement error; Make appropriate requests.
At present, the accuracy 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%, the corresponding binary code is 10~11 bits, plus the sign bit, that is 11~12 bits. When there are special applications, the A/D converter requires more bits, and a double-precision conversion scheme can often be used at this time.
(2) Conversion rate of A/D converter
The A/D converter needs a certain conversion time to output a stable digital quantity from the start of the conversion to the end of the conversion. The inverse of the conversion time is the number of conversions that can be done per second, called the conversion rate.
When determining the conversion rate of the 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 is used, its conversion rate is 10KHz. According to the sampling theorem and actual needs, a cycle of waveforms needs to take 10 samples, so such an A/D converter can only process analog signals with a frequency of 1KHz at the highest. By reducing the conversion time, the signal frequency can be increased. For general single-chip microcomputers, it is difficult to complete the work other than A/D conversion within the sampling time, such as reading data, restarting, storing data, and circulating count.
(3) Sample/Hold
The sample-and-hold is not required for the acquisition of DC and very slowly changing analog signals. For other analog signals, a sample-and-hold device is generally required. If the signal frequency is not high, the conversion time of the A/D converter is short, that is, when sampling high-speed A/D, the sample/hold device is not required.
(4) A/D converter range
A/D conversion requires bipolar, and sometimes unipolar. The minimum value of the input signal is either zero-based or non-zero-based. Some converters provide pins with different ranges, and only when used correctly can the conversion accuracy be guaranteed. In use, the factors that affect the range of the A/D converter are range conversion and bipolar bias; dual reference voltages; the correct use of the internal comparator input of the A/D converter.
(5) Full-scale error
The difference between the corresponding input signal and the ideal input signal value at the full-scale output.
(6) Linearity
The maximum deviation of the transfer function of the actual converter from the ideal straight line.
AD7705 is a typical A/D converter. You can learn more information about it.
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