By PURPLELEC | 10 November 2023 | 0 Comments
How to choose the right data acquisition card
1. Clarify application requirements
Before choosing a data acquisition card, you must analyze the application requirements, fully understand the characteristics of various data acquisition products, types of supported development platforms, running operating system environments, and development difficulty, etc., and then make reasonable choices based on relevant indicators.
2. Considerations when choosing a bus
The widely used bus structures include PCI, PXI, USB, ISA and other bus structures. Different buses have different transmission speeds, electrical characteristics, structural dimensions, configuration processes, etc. Users should choose according to the characteristics they need.
3. Considerations when choosing sampling rate
The high sampling rate of the system depends on the rate at which the ADC chip converts the analog signal, usually the unit is SPS (sampling points/s). According to the Nyquist sampling theory, the sampling frequency must be more than twice the high effective frequency in the signal, otherwise aliasing signal distortion will occur, commonly known as "false frequency". For many users, they may need to carefully observe and analyze the details of the signal, which often requires a higher sampling rate. It is usually recommended to use an acquisition card with a maximum sampling rate that is 5 to 10 times greater than the highest frequency component of the signal.
4. Considerations in selecting resolution and measuring range
The first is resolution. The higher the resolution, the higher the degree of subdivision of the input signal, and the smaller the amount of signal changes that can be identified. For example: a sine wave signal with a resolution of 8 bits. The digital result obtained by A/D conversion is equivalent to subdividing the input range into 256 parts. Some small details will be lost during the A/D conversion process. This is caused by quantization noise in the restored data due to insufficient resolution. If the resolution is 16 bits, the subdivision value of A/D conversion can be increased from 256 to 65536. The quantization signal-to-noise ratio is SNR (dB) = (6.02 × bit) + 1.76. It can be seen that the more quantization bits, the more signal-to-noise. The higher the ratio.
When the A/D conversion resolution is determined, a suitable range should be selected in accordance with the principle of ensuring low signal quantization noise and high signal-to-noise ratio, and then the collected signal should be adjusted to this reasonable range through signal conditioning. Inside.
5. Choose the appropriate product model
there are technical issues such as input impedance, output impedance, number of channels, number of signal lines, and isolation that need to be considered. These are closely related to sensors and signal conditioning and must be considered together. After the above indicators are selected, users can select a capture card that meets the requirements and find a suitable capture card model based on the principle of "functionality available". Don’t blindly buy expensive, powerful devices. Generally speaking, when the accuracy requirements are not very high and the sampling frequency is low, PCI and USB bus data acquisition cards can meet the requirements; if the working environment is harsh, an industrial computer can be used; when the collection accuracy requirements are high and the sampling frequency is very High, when the working environment is harsh, PXI type data acquisition products are preferred.
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