Chapter 3 Hardware OverviewNational Instruments Corporation3-11AT-MIO/AI E Series User ManualDitherWhen you enable dither, you add approximately 0.5 LSB rms of whiteGaussian noise to the signal to be converted by the ADC. This additionis useful for applications involving averaging to increase the resolutionof your AT E Series board, as in calibration or spectral analysis. Insuch applications, noise modulation is decreased and differentiallinearity is improved by the addition of the dither. When taking DCmeasurements, such as when checking the board calibration, you shouldenable dither and average about 1,000 points to take a single reading.This process removes the effects of quantization and reducesmeasurement noise, resulting in improved resolution. For high-speedapplications not involving averaging or spectral analysis, you may wantto disable the dither to reduce noise. You enable and disable the dithercircuitry through software.Figure 3-7 illustrates the effect of dither on signal acquisition.Figure 3-7a shows a small (±4 LSB) sine wave acquired with dither off.The quantization of the ADC is clearly visible. Figure 3-7b shows whathappens when 50 such acquisitions are averaged together; quantizationis still plainly visible. In Figure 3-7c, the sine wave is acquired withdither on. There is a considerable amount of noise visible. Butaveraging about 50 such acquisitions, as shown in Figure 3-7d,eliminates both the added noise and the effects of quantization. Ditherhas the effect of forcing quantization noise to become a zero-meanrandom variable rather than a deterministic function of the input signal.
