While some types of noise result from imperfections in ICs or environmental factors, such as temperature, the resolution of the board can also create noise. This is known as quantization error. To minimize this type of error, NI 12-bit E Series boards can improve resolution beyond specification with a hardware technique called dithering.
NI driver software allows you to enable dithering through software. When you enable the software, it adds approximately 0.5 LSBrms of Gaussian white noise to the input signal. This noise is added to the signal before the input to the ADC. As a result, a signal that might fall somewhere in the smallest voltage difference that the board can detect (known as code width and defined by the formula ) now randomly bounces above and below the boundaries of that code. When sampled, points now appear on both the top and bottom boundaries, and the number of points on either the top or bottom of the code width are weighted based on the location of the actual signal. You can then use averaging to essentially zoom in past the specified resolution of the board, providing more accurate measurements that are less influenced by wide band noise. For instance, a 12-bit board can perform with 14-bit resolution with dithering enabled. You can also disable dithering for high-speed applications that do not use averaging.
Figure 2. You can decrease quantization error on 12-bit devices using dithering
NI 16-bit E Series boards do not require dithering due to the significant decrease in code width. However, you can still use oversampling and averaging to decrease the effect of wide band noise on the accuracy of your measurements.
These techniques not only reduce noise caused by nonideal components on the measurement device, but they also help reduce noise originating from other components of the measurement system. In addition, noise at the system level can result from the sensor, so you should choose a high-quality sensor to help insure a lower noise floor for your overall system. The environment, long wires, nearby electromagnetic fields, and other sources can also induce noise on the system. To reduce noise from external sources, you should ground your system properly and use shielded cables. For more information on these topics, please see the related links below.
Field Wiring and Noise Considerations for Analog Signals
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