How to choose the A / D chip Industry News Silver Wing

A / D devices and chips are common peripherals that implement single-chip data acquisition. A / D converter variety, performance varies, in the design of data acquisition system, the first encounter is how to choose the appropriate A / D converter to meet the system design requirements. Select the A / D converter device needs to consider the quality of the device itself and the application requirements, basically, according to the following aspects of the index to select an A / D device.

(1) A / D converter bits
A / D converter to determine the number of bits should be from the data acquisition system static accuracy and dynamic smoothness of these two aspects to consider. From the static accuracy, it is necessary to consider the input signal of the original error transmitted to the output of the error, it is analog signal digitization error occurs when the main part. The quantization error is related to the A / D converter bits. Generally, the A / D converter with 8 bits or less is classified as a low resolution A / D converter. The 9 ~ 12 bits are called the medium resolution converter and the 13 bits or more are called high resolution converters. 10-bit A / D chip the following error is greater, more than 11 to reduce the error is not much contribution, but the A / D converter requirements are raised too high. Therefore, taking 10 or 11 bits is appropriate. As the analog signal first through the measurement device, and then converted by the A / D converter before processing, therefore, the total error is measured by the error and quantization error together. The accuracy of the A / D converter should match the accuracy of the measuring device. In other words, on the one hand, the quantization error is required to be smaller than the proportion of the total error, so that it does not significantly increase the measurement error; on the other hand, according to the accuracy level of the current measuring device, the A / D converter Make the appropriate request.
At present, most of the measurement device accuracy is not less than 0.1% to 0.5%, so the accuracy of A / D converter can take 0.05% to 0.1%, the corresponding binary code for 10 to 11, plus the sign bit, that is, For 11 to 12 bits. When there is a special application, A / D converter requires more bits, then often can be used double conversion program.

(2) conversion rate of A / D converter
A / D converter from the start to the end of conversion, the output of stable digital, need 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 the A / D converter, consider the sampling rate of the system. For example, if the conversion time is 100us A / D converter, the conversion rate is 10KHz. According to the sampling theorem and the actual needs of a cycle of waveforms need to take 10 samples, then the highest A / D converter only deal with the frequency of 1KHz analog signal. The conversion time is reduced, the signal frequency can be improved. For the general microcontroller, to be completed within the sampling time A / D conversion work, such as reading data, restart, save the data, the cycle count has been more difficult.

(3) Sampling / Holders
It is not necessary to sample and hold the acquisition of DC and very slow analog signals. For other analog signals are generally added sample holder. If the signal frequency is not high, A / D converter conversion time is short, that is, sampling high-speed A / D, you can not use the sample / holder.

(4) A / D converter range
A / D conversion requires bipolar, sometimes unipolar. The minimum value of the input signal starts at zero and also Amotech starts from non-zero. Some converters provide different range of pins, only the correct use, in order to ensure conversion accuracy. In use, the factors that affect the A / D converter range are: range conversion and bipolar bias; dual reference voltage; A / D converter internal comparator input correct use.

(5) full scale error
The difference between the corresponding input signal and the ideal input signal value when the full scale is output.

(6) linearity
The actual converter's transfer function is the maximum offset of the ideal straight line.

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How to choose the A / D chip

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